Tecnologia SMT

Tecnologia SMT from Marcelo Chirai

Solder Joint Acceptance

Solder Joint Acceptance from Marcelo Chirai

SMT - Feasibility of Low or No Silver Alloy Solder Paste Materials

SMT - Feasibility of Low or No Silver Alloy Solder Paste Materials from Marcelo Chirai

SMT - Good Practice Guide to Reduce Copper Dissolution in Lead-Free Assembly

SMT - Good Practice Guide to Reduce Copper Dissolution in Lead-Free Assembly from Marcelo Chirai

Análise de falhas no processo de Pick and Place

Análise de falhas no processo de Pick and Place from Marcelo Chirai

A Generic Reflow Profile

We have a wide mix of PCB assembly types. Some folks here are proposing that we use the same reflow profile setting regardless of the PCB thickness, thermal mass or component mix? It has always been my understanding that different PCB types should use different profile settings. Is there any justification for using one generic reflow profile?

G.S.

Experts Comments

Using a generic profile in production will bring on more grief and scrap boards than you are willing to produce.   Each board assembly has a unique profile because of the variations not only in board thickness, but also the thermal mass and distribution of the attached components.   To ensure proper solder joints and successful assemblies, you need a profile for each assembly you intend to run.

Kris Roberson Manager of Assembly Technology IPC Kris Roberson has experience as a machine operator, machine and engineering technician and process engineer for companies including Motorola, and US Robotics. Kris is certified as an Master Instructor in IPC-7711 / 7721, IPC A-610 and IPC J-STD 001.

In general, no there is not. Through careful testing, you may find that you can narrow the number of required profiles to just a few, however (the number depends on your product mix). The emphasis here is on the testing. Every board has unique thermal requirements. The thickness of the PWB, amount and location of copper in the PWB, the component locations and masses, presence of "back-to-back" components on the assembly, and other variables all contribute to the thermal requirements. And don't forget that most ovens respond to load slowly enough that the second board may see slightly different temperatures than the first, and that temperatures very near the rails are almost always cooler. There is no substitute for really understanding the thermal requirements of each assembly part number. Once you do, it may well be possible to group them according to common profiles. Further, if you have a basic profile, and you need a slightly warmer or cooler profile, it may be possible to achieve this by small changes in the conveyor speed, as long as the timing of preheat, time over liquidus, and cool-down are not affected too much. The advantage of doing this is that profile changes become nearly instant, since no temperature settings are changed.

Fritz Byle Process Engineer Astronautics Fritz's career in electronics manufacturing has included diverse engineering roles including PWB fabrication, thick film print & fire, SMT and wave/selective solder process engineering, and electronics materials development and marketing. Fritz's educational background is in mechanical engineering with an emphasis on materials science. Design of Experiments (DoE) techniques have been an area of independent study. Fritz has published over a dozen papers at various industry conferences.

The only way that a single profile could be used for all board designs would be if the rate of rise and cool down temperatures are not in violation of the chip suppliers and that the time above Liquidus does not become excessive with a resultant heavy IMC being formed that would have a negative impact on reliability. In addition, you would have to verify that voiding would not become an issue with a single profile. In other words you typically cannot use a single reflow profile for significantly different boards, i.e. thickness, copper weight, density etc.

Gerard O'Brien President S T and S Testing and Analysis Gerald O'Brien is Chairman of ANSI J-STD 003, and Co Chairman of IPC 4-14 Surface Finish Plating Committee. He is a key member of ANSI J-STD 002 and 311 G Committees Expert in Surface finish, Solderability issues and Failure analysis in the PWA, PWB and component fields.

You are correct in your thinking that different board types, thicknesses and component densities may demand different oven recipe settings (profile). Whether a different assembly needs new oven recipe settings or not can only be shown by performing a thermal profile of that new assembly. This will allow you to see for sure of the current oven recipe settings meet the thermal requirements of the components and solder paste. Given the thermal profile of the new assembly, you can determine if the oven recipe settings need to be changed to prevent component damage and or meet the thermal profile needs of the solder paste. Without the thermal profile of the new assembly, you will only be guessing and you will have no proof the recipe settings are sufficient.

Paul Austen Senior Project Engineer Electronic Controls Design Inc Paul been with Electronic Controls Design Inc. (ECD) in Milwaukie, Oregon for over 34 years as a Senior Project Engineer. He has seen and worked with the electronic manufacturing industry from many points of view, including: technician, designer, manufacture, and customer. His focus has been the design and application of thermal process measurement tools used to improve manufacturing processes like: mass reflow and wave soldering, bread baking, paint and powder curing, metal heat treatment and more.

Each and every board should have its own profile. Granted there may be some boards so similar that a new reflow profile is not needed; e.g. a revised board where a few discrete components are added or deleted. There have been attempts to "profile" by board weight, or grouped by type, etc. I have never seen this work effectively and in fact have seen lots of production material sacrificed because of lack of adequate profiling. Take the needed time and materials to develop an adequate profile.    The following is only a partial list of things that have the potential to alter reflow profile of two boards that "look" alike: PCB layer count Differing copper plane thicknesses  (e.g., 1 oz. vs. 2 oz.) Copper trace dimensions Overall thickness of board Thermal conductivity of board dielectric Number of components Types of components Size of components Layout density of components (how closely devices are spaced to one another) Number of component connections to inner planes Etc.   Many times I have seen boards from a production run only to find that they were under-reflowed as evidenced by cold solder joints especially under a BGA or other area array component. Why?  For two reasons: guessed profile and not validating the result of the profile before reflowing boards en masse. To avoid loss of material, loss of time and needless rework prepare and validate a proper profile for each and every board.  Like everything else in life, check your work before proceeding to the next step. Each side of the board needs a profile too. The bottom (secondary) side should be profiled with only bottom side components on it. The top (primary) side should be profiles with all top components and all bottom components.      Don't think only in terms of inadequate reflow/cold solder joints.  Each part used on the assembly comes with a thermal specification; generally a JEDEC specified maximum temperature. Exceed that temperature and it may either ruin the device or lessen its reliability. Do the work to ensure that the profile is suitable in all respects. Use profile data review along with visual inspection, x-ray and even cross-sections if warranted before committing your company's money or your customer's faith in a production run. Temperature in the middle of a BGA may be significantly different than at periphery of the device.  Don't guess! Measure and inspect.   How a board is instrumented with thermocouples can make a difference too.  Know how to attach, where to measure and how many thermocouples are needed. There are many publications on this topic. With today's profile tools (thermal data recorders) and profile prediction software that comes with such profile trackers, profiling is not as time consuming as it once was. Take the extra time to get it right. By the way, the same logic applies to wave soldering and automated methods of rework such as hot gas repair.

Gary Freedman President Colab Engineering A thirty year veteran of electronics assembly with major OEMs including Digital Equipment Corp., Compaq and Hewlett-Packard. President of Colab Engineering, LLC; a consulting agency specializing in electronics manufacturing, root-cause analysis and manufacturing improvement. Holder of six U.S. process patents. Authored several sections and chapters on circuit assembly for industry handbooks. Wrote a treatise on laser soldering for Laser Institute of America's LIA Handbook of Laser Materials Processing. Diverse background includes significant stints and contributions in electrochemistry, photovoltaics, silicon crystal growth and laser processing prior to entering the world of PCAs. Member of SMTA. Member of the Technical Journal Committee of the Surface Mount Technology Association.

The short term  justification is that it saves time (change over and recipe generation), the same way as not performing maintenance does. The reality is that unless you're using vapor phase condensation reflow, varying masses will produce different thermal profiles, and should have a unique recipe that maximizes your solders ability to solder. This difference is related to your ovens capability / capacity to heat these varying masses in a given process time.  How much difference can only be determined by profiling, and that is why we at least check the profile once on every assembly. If your products have similar masses, similar board thickness, and similar foot prints, you can expect a similar profile will be achieved (although you have no proof unless profiled). If there is a change in any of these parameters, there will be differences in the profile, and the only way to prove that those differences won't violate any reflow specification is to profile it. If you prove one recipe will work by profiling all your assembly's, then you can use that recipe with confidence. Can you get away with using the same recipe on every board, YES for a while, until you get a board with enough difference to cause soldering defects.

Mark Waterman Engineer / Trainer Electronic Controls Design, Inc. (ECD) Mark Waterman is a trainer and field engineer with 17 years experience in service and applications specialties. Intimate knowledge of soldering processes and measurement systems. Six sigma and statistical process control generalist.

The main objective of the reflow oven is to process all PCBs within the process limits set by the components, solder paste and substrate vendors. Not only do the different PCBs types tend to have different process windows but, as you point out, their different thermal mass may produce a different thermal profile when using the same oven settings.   In markets such as the US and Europe, most electronic assemblers tend to operate in a high mix / low to medium volume environment. It may be expensive and unproductive for them to make numerous oven setup changes every day as it can take the reflow oven a long time to stabilize on the new settings.  Fortunately, there are setup optimization systems on the market today (my company KIC offers this as well), that enable you to identify one or a select few oven setups that can process all or at least a wide range of your PCBs types in spec. There are even systems that can do most of this work for you without profiling, and instead simply entering the process window and the PCB length, width, and weight.   Once you start running your reflow ovens at optimized settings you may also experience additional benefits such as faster throughput or lower electricity use.

Bjorn Dahle President KIC Bjorn Dahle is the President of KIC. He has 20 years experience in the electronic manufacturing industry with various manufacturing equipment companies covering pick & place, screen printers and thermal process management.

No there is no justification for using one profile unless you really want to produce potential scrap PCB's. I usually recommend three profiles, TO START with and then fine tune by profiling the actual PCB, populated ideally. If not populated then add 10C to each zone to allow for components. So set up a small, medium and Large profiles but use them only as a starting point. You may in the future get confident that you will know which Circuit will fall under which profile in the future so you don't have to profile each and every board that comes through your factory. BUT until that time get profiling would be my advice

Greg York Technical Sales Manager BLT Circuit Services Ltd Greg York has twenty two years of service in Electronics industry. York has installed over 350 Lead Free Lines in Europe with Solder and flux systems as well as Technical Support on SMT lines and trouble shooting.

Being able to use a single reflow oven recipe for all products would be nice but it will certainly get you in trouble if your boards are significantly different. In some cases it is possible to group boards into categories that share the same recipe only if the boards are similar.    In 2009 BTU did a study that described what happens to the thermal profile of a board when various reflow parameters are changed. One of those changes was using the same oven setting with boards with different mass. The difference in peak temperature of over 5 degrees C was seen when the PCB weight changed from 100 grams to 250 gram PCB without changing the oven recipe.   There are two papers that can be downloaded from the Knowledge Center on the BTU web site under the "About US" tab that will explain why the thermal profiles cannot be generic. One is "Thermal Profiles - Why Getting Them Right is Important." and the other is "Oven Adjustment Effects on a Solder Reflow Profile."

Fred Dimock Manager, Process Technology BTU International Mr. Dimock is the manager of Process Technology at BTU International. His extensive experience in thermal processing includes positions at Corning, GE, and Sylvania. He has authored numerous articles on lead free processing and process control, taught classes at SMTAI, and participated in the IPC Reflow Oven Process Control Standard committee.

Based on the board diversity detailed in the question, it is unlikely the submitter can use just one profile for all their boards. The only justification for single profile reflowing would be a product mix that lends itself to that choice. Depending upon the heating capabilities of an oven, one may be able to use a few "generic" profiles as opposed to many PCB specific ones. We recommend that a customer assesses their product mix, looking for groupings of PCBs that lend themselves to use of a single profile for the group. Most of our customers that use this method end up with two to four profiles that serve the vast majority of their products and a handful of specialized profiles for their more challenging PCBs.

John Vivari Application Engineering Supervisor Nordson EFD Mr. Vivari has more than 15 years of electronic engineering design and assembly experience. His expertise in fluid dispensing and solder paste technology assists others in identifying the most cost effective method for assembling products.

I would definitely not recommend a generic profile. There might be "families" of boards that can use the same settings for reflow but with so many variables involved, a generic profile will not help. The mass of the board, surface, copper plane thickness and especially the population makes a huge difference on the results. We are running a high mix, low to medium volume product and every single assembly has its own reflow profile.

Georgian Simion Engineering and Operations Management Independent Consultant Georgian Simion is an independent consultant with 20+ years in electronics manufacturing engineering and operations. Contact me at georgiansimion@yahoo.com.

Reader Comment A generic "Reflow Profile" is a great idea but the problem is hitting it with different board designs. There is a lot you can do with a quick visual of the board and parts. What I did was to generate a small array of "Generic Profiles. For example: Light Double Sided, Heavy Double Sided, Light Layer, Heavy Multi-layer. I suspect you are doing this to save time on the various incoming jobs. Spend a little time with qualifying a couple of test case boards and make sure you run on the hot side. Going a little low in temp is typically more of a problem than a little hot.  If you plan to do this with lead free it is more difficult. I would seriously suggest a vapor phase oven. Maybe not for all boards but just for the board of the edges of your board type definitions.  Bob Kondner, Index Designs, USA

Reader Comment Years ago the reflow oven couldn't achieve a good delta between boards with different mass, fortunately now the reflow ovens can stabilize the delta under the process window in a group of boards with similar mass and component density, so I recommend selecting a group of boards or families and run a profile in order to know the delta between boards, if the delta is under the process window, you can do a generic profile, only for a selected group of boards. Sergio Ilescas, Arris, Mexico

Wave Soldering System Nitrogen Use

How can I calculate the amount of nitrogen used in my wave soldering system?

R.D.

Experts Comments

The best way to determine you N2 consumption is to hook up a mass flow meter to the incoming N2 source line.  These meters will provide you with an accurate consumption.

John Norton Eastern Manager Vitronics Soltec John Norton started his soldering career in 1983 for Hollis Engineering. He has also worked with Electrovert as a technical training manager and Vitronics Soltec for the last ten years. He has held various technical development and sales positions.

You'll need some type of flow meter somewhere. The best solution is an electronic, totalizing flowmeter installed directly upstream of the wave solder system. The meter will record usage and give you a running total of the usage. A simpler strategy would be to install a mechanical flowmeter, such as a "rotameter" type meter, again preferably installed directly ahead of the wave machine. The latter will give you the instantaneous flow, and you'll need to calculate usage based on duty cycle and machine up-time.    If the only flow meter available is up-line far enough to encompass more than one usage point, you can still work with this by taking the difference in the flow with the wave on and off. This assumes you can control the state of the other devices on the line, and that the flowmeter has enough resolution to distinguish.    Finally, if it is not possible to tap into the line to the equipment to install a flow meter, there are non-invasive techniques on the market, e.g. ultrasonic flow measurement. These can be expensive, however.    With any of the above techniques, you need to compensate for the temperature and pressure at which you are measuring the flow. You will have to convert flows measured at the line pressure and temperature to flow at "standard" pressure and temperature, typically in SCFH (Standard Cubic Feet per Hour). Some electronic meters can perform this conversion because they have temperature/pressure sensing incorporated into them.

Fritz Byle Process Engineer Astronautics Fritz's career in electronics manufacturing has included diverse engineering roles including PWB fabrication, thick film print & fire, SMT and wave/selective solder process engineering, and electronics materials development and marketing. Fritz's educational background is in mechanical engineering with an emphasis on materials science. Design of Experiments (DoE) techniques have been an area of independent study. Fritz has published over a dozen papers at various industry conferences.

Many wavbe soldering machines come equipped with flowmeters for air & nitrogen. These flowmeters usually measure Cubic Feet per Minute or Liters per Minute.  In a steady-state system all you need to do is to log the flowmeter reading vs. time the equipment is using Nitrogen.

Edithel Marietti Senior Manufacturing Engineer iDirect Edithel is a chemical engineer with 20 year experience in manufacturing & process development for electronic contract manufacturers in US as well as some major OEM's. Involved in SMT, Reflow, Wave and other assembly operations entailing conformal coating and robotics.

A simple answer to your Q is put in a flow meter and measure it. However, I sense that you do not have a flow meter, hence, the calculation request. Air velocity formula is as follows: Air Velocity in a Pipe Using the equation and typical values of V, D and L explained to the right approximate values of P are computed as follows: V = air velocity in feet per second D = pipe inside diameter in inches L = length of pipe in feet P = pressure loss due to air friction in ounces/square inch formula from B.F. Sturtevant Company Whether you have all these data inputs is unknown. However, some online examples of this formula show that velocity is about 30 fps. Hence, for a good estimate for your wave solder machine usage is to  multiply the cross-sectional area of the pipe or tubing that you're using by 30 to get cubic feet, and then convert ft3 of N2 to lbs if that Is needed.

Lee Wilmot Director, EHS TTM Technologies Lee Wilmot has 20+ years doing EHS work in the PCB/PCBA industries, including environmental compliance, OSHA compliance, workers compensation, material content declarations, RoHS & REACH compliance. Active on IPC EHS committee and c-chaired committees on IPC-1331, J-STD-609A on labeling & marking, IPC-1758 on packaging and others.

If your wave soldering system came with a nitrogen inerting option, it should have a flowmeter somewhere on the nitrogen line going in to the machine, or built into the controls or display of the machine. Now, because nitrogen, like all gases, is compressible and changes volume when placed under pressure, or is heated or cooled, a given volume of nitrogen under one set of pressure and temperature conditions is not equivalent to the same gas under different conditions. Keep this in mind when attempting to measure volumes of gas.   A manual display flowmeter looks like this: http://www.amazon.com/ESAB-639749-L-32-Flowmeter-Inlet/dp/B007PJIBKA/ref=sr_1_5?ie=UTF8&qid=1428950749&sr=8-5&keywords=nitrogen+flowmeter. These can be configured as either a display-only flowmeter, or as a controlling valve, ie, they may be installed in series on the nitrogen gas line to both control the nitrogen gas rate to the chamber AND see the gas flow rate. The gas flow rate is usually measured in gallons per minute (GPM), liters per minute (LPM), cubic feet per minute (CFPM) or SCCM (standard cubic centimeters per minute) or whatever other foreign, obscure, and generally-misunderstood flow rate you wish to choose from. I recommend you look at Davis Instruments or just Google up "flowmeter, gas" and you will see a huge selection. There are even non-intrusive Ultra-Sonic Doppler flow meters meant for tracking the amount of gas flow inside a pipe. Or you may have a really fancy technogeek style digital electronic flowmeter like this: http://www.amazon.com/Digiflo-1000-770-Digital-Flowmeter-sccm/dp/B00NI4G7IO/ref=sr_1_5?ie=UTF8&qid=1428951246&sr=8-5&keywords=davis+instruments+flowmeter; programmable in all of the above-mentioned measurement rates and some others you understand even less, such as acm/h (actual cubic meters per hour), sm3/sec (standard cubic meters per second), kscm/h (thousand standard cubic meters per hour), LFM (linear feet per minute), or MMSCFD (million standard cubic feet per day). Anyway... that is how the flow rate is measured. So, with the chamber operating at a desired oxygen level in parts per million (50 ppm or less is a good nitrogen blanket for a wave solder machine)... what do you mean you don't know what the oxygen level is? Well, how do you know how much nitrogen is needed to get the oxygen level below 50 ppm if you don't have an oxygen monitor like this one installed? http://aoi-corp.com/oxygen-analyzers/percent/series-2520. Ok, now that you are able to monitor the oxygen level in the wave solder chamber, and you have finally adjusted the in-line nitrogen flow rate so the oxygen monitor shows around 50 PPM or less, you can simply multiply the nitrogen rate times the number of hours of wave solder operation to come up with the total usage. For example, if your required nitrogen flow rate for your machine is around 6 liters per minute, and the machine is run 8 hours per day, your total usage would then be approximately 6 liters/min times 60 minutes times 8 for a total of 3,840 liters per 8 hour shift. What does that mean to you? Well, think of it as 3,840 of those biggie-sized bottles of Diet Coke, but instead of Coke they were filled with nitrogen. That is how much you would use every 8 hours of machine operation time. So, if you ran two shifts, or 7,680 Coke bottles per day, times 5 days per week that would be 38,400 Coke bottles of nitrogen used every week, times 51 weeks per year (after holidays are subtracted) then you are looking at 1,958,400 Coke bottles of nitrogen used every year. That's almost two million Coke bottles of gas every year! How much does that cost?  Well, I can buy 80 liters of 99.995% nitrogen for about $5. So, if I divided 1,958,400 by 80 I get 24,480 tanks, times $5 per tank, I get $122,400 per year, plus installation and maintenance costs. Nitrogen tanks require special handling, read the OSHA rules. One OSHA fine for not having the tanks handled and secured properly can be another $50,000! And that answers your question. Have fun with that!

Richard D. Stadem Advanced Engineer/Scientist General Dynamics Richard D. Stadem is an advanced engineer/scientist for General Dynamics and is also a consulting engineer for other companies. He has 38 years of engineering experience having worked for Honeywell, ADC, Pemstar (now Benchmark), Analog Technologies, and General Dynamics.

To calculate this can be quite complex. Typically the manufacture will have this in the machine specification sheet. If you are worried that you are over consuming or under consuming nitrogen then I suggest that you fit a air flow sensor which can track the flow rate of the nitrogen. This sensor data will also show you the total amount lost due to planned and unplanned maintenance cycles.

Kishan Sarjoo Process Engineering Manager - Electronics Altech UEC, South Africa Currently with Altech UEC and responsible for technology road map in PCBA electronic manufacturing and technical support for PCBA electronic manufacturing for Altech UEC and its JDM's. Over 7 years in SMT, Radial Insertion, Wave solder & Test Applications.

http://www.circuitnet.com/experts/87610.shtml

Common Mistakes of BGA Rework

 

Ball Grid Array rework is one of the most challenging procedures performed at assembly facilities and repair depots around the world. Doing it right depends in large part on the skills and knowledge of the rework technician. That's why we say that BGA rework is mostly science, with a big slice of art thrown in!

Procedures for BGA rework are well-defined and long established, but there are six common mistakes. These mistakes can be costly leading to the following:

1. Excessive solder joint voiding
This is often due to incorrect solder paste selection or process parameters and can compromise the integrity of the attachment and require additional rework, or result in rejection if the voiding is over 25%.

2. BGA Pad damage during the BGA removal process
This is sometimes an unavoidable hazard, and it's made worse when conformal coatings and underfill are used. Repairing damaged BGA pads is a time-consuming major headache worth avoiding.

3. Incorrect BGA orientation or joint bridging.
This means additional rework thermal cycles, and the increased risk of damage with each successive application of heat.

These problems are preventable. Let's look at the 6 Most Common Mistakes in BGA Rework and how you can avoid them!

1. Inadequate Operator Training
We can't emphasize this enough! BGA rework technicians must be fully trained, their skills practiced and developed. They must understand the materials they're working with, the tools, the process steps, and the interrelationship of all factors.

They must have the skills to evaluate and 'size up' a BGA rework situation knowledgeably and skillfully before commencing rework. And they must be able to recognize the subtle, tell-tale signs indicating the process is off-track.

2. Inappropriate Equipment Selection
It's an old saying, but true, you need the right tools to do the job properly. For BGA rework, the equipment used must have the sophistication, the flexibility, and the capability to sustain a controlled, predictable and repeatable process.

This includes closed-loop thermal sensing and control, the robustness to be able to deliver heat as the process requires, and product handling capabilities for removal and replacement. Use the most capable equipment available; this is not an area to cut corners.

3. Poor Profile Development
The BGA rework profile is as important as the assembly reflow profile, and in most cases duplicates it. Without it, you won't achieve a successful and repeatable BGA rework process.

A poorly-developed thermal profile can result in damage to the assembly or BGA component requiring additional rework cycles to the same site, and damage or reflow of adjacent components. Good profiles must be carefully developed using correct thermocouple placement and analysis of the data that they provide.

4. Improper Preparation
A professional painter knows that a good, lasting paint job is 90% preparation. Similarly, before the first heat cycle is applied to a BGA rework site, there's a lot of preparation needed if the process is going to be done right.

This includes baking out moisture from the BGA device and the board assembly to prevent 'pop-corning' and other problems and removal or protection of nearby heat sensitive components to avoid damage or inadvertent reflow.

The right decisions need to be made in advance, such as whether or not to use solder paste, choosing the right solder paste stencil, and choosing the right chemistries and alloys.

There's plenty of preparation to do, and do right, before the actual rework cycle begins. This includes an accurate assessment of such things as solder ball size; device and ball co-planarity; solder mask damage and missing or contaminated pads at the PCB site.

5. Collateral Heat Damage
Reflow of adjacent component solder connections results in oxidation, de-wetting, pad and lead damage, wicking, starved joints, component damage, and other issues that can create a host of new rework problems.

The technician must be constantly aware of the effect of heat not only on the target BGA device, but also how it is affecting other components adjacent to it, on both sides of the assembly. The goal is to minimize heat migration beyond the BGA component being reworked, and this is a function of a well-developed profile and tight process control.

6. Insufficient Post-Placement Inspection
The world beneath a BGA component is a hidden mysterious place, but not from today's X-ray inspection machines. Problems such as excessive voiding and poor placement or alignment are immediately detectable with X-ray inspection.

But just like a Radar operator, an X-ray system user needs proper training to correctly interpret and understand the image that the machine is providing. The complexity of the BGA component and the different variations in the X-ray image demand it if maximum benefit is to be obtained from this significant - and indispensable - equipment investment.

Avoiding the 6 Most Common Mistakes in BGA Rework is the best way to ensure a successful, robust, and repeatable process with fewer headaches, higher yields, and reduced costs for a better bottom line.

Several members of the Circuit Technology Center team contributed to this feature story.

http://www.circuitrework.com/features/721.shtml

X-ray Analysis

This is one of those things that drive people in this business mad. A very expensive and difficult board was manufactured successfully. All the electronic and environmental testing was completed and the board went into service. Months later the board began to perform erratically in the field.

Of course the failure was "high visibility" and everyone was under the gun to find out how and why this is happening. And fix it! The customer urgently directed all possible resources to bear and after a week or so of testing found the source of this very sneaky problem.

Figure 1: The lead and plated hole center-bottom has insufficient solder fill.

They discovered a dual challenge: a very thick board and leads that were (on occasion) contaminated - most likely oxidized. There was only one component type involved and this component was placed at about a dozen locations per board and not every component demonstrated the problem.

What happened was, at the affected holes on the through-hole component, barrels incompletely filled with solder, which caused the solder connection with the component lead to occasionally fail after the board had been in the field working for some time.

When the boards arrived at Circuit Technology Center our challenge was to first identify the holes that were inadequately filled and fill them. At the same time the customer wanted to ensure that all pins on the problem part were reflowed to ensure they would forevermore demonstrate proper wetting.

Once this was complete, there had to be proof that the holes were filled to everyone's satisfaction. To complicate matters these boards were covered with a thick conformal coat.

The rather large board was placed, solder side up, at about a 30 degree angle to the x-ray emitter head in order to permit full view of the barrel length. Once the board was under view, each location was photographed and a marker placed on the photo noting the holes that were clearly under-filled.

Figure 1 is an example of an x-ray snapshot of a plated hole and lead with insufficient solder.

Now came the hard part. It's easy enough to talk about reflowing ground planed pins on a .150-inch thick conformal-coated board, but to actually do it? In order accomplish this task without damaging non-affected areas, the board had to be prepared by placing thermal resistant tapes and heat deflecting material on the surrounding surface areas and components to protect the unaffected components and the solder side conformal coating.

A vacuum desoldering tool was used to remove the conformal coating from the solder-side leads to permit flow and fill at those locations. The interesting thing is that this rework was going to be performed on a BGA rework machine in order to maintain the board at a steady state high temperature using the BGA rework machine's bottom heater plates to warm the whole substrate and the topside air nozzle to drive heat into the specific rework location.

In order to maintain control of this volume of heat, thermocouples were placed under the rework locations. Thermocouples were also placed to monitor the heating of sensitive devices near the rework area.

The board was slowly heated until the proper base temperature was reached then a vacuum de-soldering iron was used to suction out the existing solder ridding the barrel of any suspect material.

The final step of the soldering phase was to add solder at the target barrels using the soldering iron and wire solder. This process was repeated until all affected plated through holes were properly filled.

The board was then cleaned in a de-ionized water washer. Once again the board was x-rayed at every location.

The rework environment was hot and demanded great patience and skill from the technicians involved. It isn’t always easy, but somebody's got to do it.

Several members of the Circuit Technology Center team contributed to this feature story.

http://www.circuitrework.com/features/658.shtml

0201s and 01005s Soldering / Rework

We are struggling to solder and rework small parts - 0201s and 01005s. I have been using a small nozzle hot air solder rework station with adjustable air flow. The minimum air flow on the unit will blow small parts across the work bench in the blink of an eye. My world of reworking small passives etc. with lead free solder is painful. I am open to any suggestions you may have.

T.G.

Experts Comments

There are several companies that offer mini and micro soldering/de-soldering tweezers just for this type of work. I recommend checking with your soldering tip supplier to see if they can fit your current equipment.

Edithel Marietti Senior Manufacturing Engineer iDirect Edithel is a chemical engineer with 20 year experience in manufacturing & process development for electronic contract manufacturers in US as well as some major OEM's. Involved in SMT, Reflow, Wave and other assembly operations entailing conformal coating and robotics.

I would suggest you rework with an IR rework station and dispense paste with a syringe. Check out the DAGE 3000.  That what I have used for 0201s with some minor modifications you can place and reflow all with one system.

Terry Munson President/Senior Technical Consultant Foresite Mr. Munson, President and Founder of Foresite, has extensive electronics industry experience applying Ion Chromatography analytical techniques to a wide spectrum of manufacturing applications.

For these type of devices we have switched back to using infrared rework station. It is slow but works amazingly. Also see if your team can use a solder iron tweezer. I have not been able to successfully solder 01005s with this, due to the design of our lands. (Changed to almost bottom solder device to eliminate tomb-stoning)

Kishan Sarjoo Process Engineering Manager - Electronics Altech UEC, South Africa Currently with Altech UEC and responsible for technology road map in PCBA electronic manufacturing and technical support for PCBA electronic manufacturing for Altech UEC and its JDM's. Over 7 years in SMT, Radial Insertion, Wave solder & Test Applications.

Thank you for the question. I understand your frustration in using hot air to reflow those small components 0201 and 01005, as the processes are not quite in place to reflow components so small. Additionally the amount of paste to be applied to those pads is very difficult to maintain a minimum amount. The existing equipment has to be special to do this correctly and unless you can help from an equipment supplier it will be difficult. We are developing a process to do this and I would recommend visiting our web site, www.eptac.com for more information.

Leo Lambert Vice President, Technical Director EPTAC Corporation At EPTAC Corporation, Mr. Lambert oversees content of course offerings, IPC Certification programs and provides customers with expert consultation in electronics manufacturing, including RoHS/WEEE and lead free issues. Leo is also the IPC General Chairman for the Assembly/Joining Process Committee.

In order to rework small/special components, you need special tools.  From your question, I do not understand if you are reworking boards and these parts are close to the rework area or your intention is to rework these particular parts. If you want to rework parts like 0201 and 01005 I highly recommend the JBS nano station tweezers. They are great, I have used them in the past and they are the perfect solution for something like this. For stencil design, paste deposition, pick and place and reflow there are more things to consider. Should these components be in an area that gets reworked, you can minimize the damage by protecting the surrounding areas of the component being reworked from the hot air flow using masking or kapton tape.

Georgian Simion Engineering and Operations Management Independent Consultant Georgian Simion is an independent consultant with 20+ years in electronics manufacturing engineering and operations. Contact me at georgiansimion@yahoo.com.

http://www.circuitnet.com/experts/87577.shtml

Adding Weights to Small BGAs During Reflow

We have issues with small, light BGA's. They often have opens after reflow. We have no problems with larger BGA's or fine pitch BGA's with higher ball counts. Do you think it makes sense to add a weight to the top of the small BGA's during reflow to improve soldering and reduce opens? Any advice would be appreciated.

C. B.

Experts Comments

The reasons for opens with small light BGAs may be due to: a. Insufficient flux activity: For the smaller apertures and therefore a smaller paste deposit, the flux fraction burn-off is higher. Therefore the flux may not have a robust oxidation barrier to prevent the solder deposits from oxidizing and therefore cause opens - this is the Head-in-Pillow (HIP) phenomenon. Fluxes with a good oxidation barrier can prevent oxide formation and therefore prevent HIP / opens / poor solder coalescence for tiny deposits - even for long, harsh profiles b. Insufficient / Inconsistent paste volume printed: as the stencil aperture becomes smaller, it becomes more difficult to print consistent paste deposits. Low paste volume means less flux and therefore potentially not enough "juice" to withstand even a regular reflow profile. The opens you are seeing may be a Head-in-Pillow phenomenon. An optimized paste flux chemistry in conjunction to the right particle size will maximize paste transfer from the stencil to the board and eliminate opens c. Insufficient tackiness of paste: If the flux in the paste does not have enough tackiness to hold the component, then there could be loss of contact between the paste & BGA ball during reflow and opens could be caused In addition to being a cumbersome addition,a weight should really not be needed for reflowing small, light BGAs. The paste is expected to do the job of holding all components during reflow and achieve good solder joints.

Karthik Vijay Technical Manager - Europe Indium Corp. Currently with Indium Corporation and responsible for technology programs and technical support for customers in Europe. Over 15 yrs experience in SMT, Power, Thermal & Semiconductor Applications. Masters Degree in Industrial Engg, State University of New York-Binghamton.

At first this sounds a good idea but it is fraught with potential problems. What will the extra mass do to the profile, can the weights be placed without moving the part, will the weight stop the part from self centering, or compress the joints causing bridges. This sounds as if we are treating the symptom rather than the route cause. The problem from the limited description is likely a result of component warpage. What is the warpage specification on these parts? If you can't source parts with less warpage I would suggest extending the time above liquidus. If you have a good solder paste the activity will be sufficient to allow time for adjacent balls to collapse and cause contact between solder and ball.

Neil Poole Senior Applications Chemist Henkel Electronics Dr. Poole is a Senior Applications Chemist in Henkel Technologies, electronics assembly materials application engineering group. He is responsible for all of Henkel's assembly products including soldering products, underfills, PCB protection materials, and thermally conductive adhesives.

Be sure you understand the thermal profile of the environment under this component before you try a mechanical remedy as you have suggested. The forces that take place under the part while the solder goes to liquidous are much stronger than the weight of the part and tend to work in your favor, pulling the part into place. If there is some thermal reason for the "opens" like: too high temperature or too long at temperature, the solution may be to protect the part from the heat rather than trying to force the part closer to the board during liquidous. Of course, adding weight may have the side effect of protecting it thermally, solving the problem for the wrong reasons, so to speak. So please understand the thermal profile and its impact on the solder and its chemistry before making any changes to the assembly fixture to make sure you are solving the right problem.

Paul Austen Senior Project Engineer Electronic Controls Design Inc Paul been with Electronic Controls Design Inc. (ECD) in Milwaukie, Oregon for over 34 years as a Senior Project Engineer. He has seen and worked with the electronic manufacturing industry from many points of view, including: technician, designer, manufacture, and customer. His focus has been the design and application of thermal process measurement tools used to improve manufacturing processes like: mass reflow and wave soldering, bread baking, paint and powder curing, metal heat treatment and more.

You can't put the weight on the BGA at pre reflow stage. Plus the air flow in th reflow Oven and the solder paste application and PCB should be moisture Free before mounting the BGA. The control on Solder Paste printing and Reflow parameter will help you to solve the BGA open Problem.

Sandip Thakor Quality Engineer Matrix Telecom Solution P Ltd Sandip Thakor has 9 years of experience in electronics industry specializing in soldering technology. Thakor has experience in lead free installation, process optimization and developing quality standards.