BGA - Rework and Repair - 1st Step

STEP 10 Rework and Repair: The Complete BGA Rework Process Tuesday, November 17, 2009 | SMT Magazine Archive Robert Avila and Wade Gay, Finetech, describe the steps to BGA rework, aided by video clips of rework in action. There are at least five steps in successfully completing the cycle for BGA rework. These steps, which include component removal, site cleaning, reballing, and soldering, do not change, independent of whether or not the BGA is on a PCB that is large, small, thin, or thick, etc. Removing the component from the PCB is step one. Cleaning the BGA site (and PCB) follow as step two. Attaching new solder balls (reballing) is step three, and re-soldering the BGA back to the PCB is the final step. There may be others, such as single-ball reballing, that have not yet become mainstream but could add significant value. This is especially true when attempting to re-ball a BGA that has a high ball count or small ball pitch. The BGA package has long been a mainstay in surface mount PCB designs. BGAs are able to meet equally growing pressure to decrease product size and increase functionality. Many types of BGAs are integrated into products worldwide. Step 1: Component Removal As components become more complicated, certain parameters need to be fully understood before attempting to remove a BGA. Prior to removal, PCB preheating is necessary. By saturating the copper within the PCB, then applying top heat to the component to be removed, the heat becomes localized at the component and not distributed throughout the thermally conductive material of the PCB. Figure 1. Inside the BGA. Monitoring the PCB temperature with external thermocouples (TC) is standard practice. When heating the component from the top side, the generated heat should remain localized at that site until the solder becomes molten. Then the component is lifted from the PCB using vacuum. However, if the PCB has not been preheated (saturated), then heat generated from the top will be distributed into the PCB until the copper (thermal conductor) has been saturated. By the time this happens, the component could become unsalvageable or at least difficult to recover. These effects include the die within the BGA package exceeds threshold temperature (Figure 1); thermal mismatch between PCB and component exceeds the limits and results in delamination; heat generated from the top to liquefy the solder begins to liquefy neighboring components; and activation time for flux passes without properly transferring heat between solder and component. Preheating the PCB efficiently is the first of several steps to successfully remove the package without compromising the integrity of the PCB or the BGA. Typical IR heat sources traditionally require little tooling for specific BGA sizes. This approach of one tool fits all has advantages and disadvantages. Hot air rework systems, for the most part, require tooling based on component size.  However, having the capability to control air flow precisely top and bottom while heating the component is critical in rework. BGA components for the most part are black on the top, to cover and protect the die within the package. When comparing the absorption/reflection ratio of IR, the middle area of the package absorbs a good portion of the energy while the peripheral areas remain cooler. Process control is difficult, and reproducibility from component to component and board to board can be an issue. Figure 2. IR rework system concept. On the other hand, hot air convection proves to be advantageous as the absorption of heat by the component and circuit board is essentially independent of a material’s color or texture. Additonally, inducing nitrogen to the site during component reflow has advantages. In air-atmosphere rework, an oxide layer forms around the solder sphere as it reflows. Inducing nitrogen during this step displaces the oxygen and eliminates the oxide layer forming around the molten solder. Whether or not nitrogen is used, the desired temperature and dwell times stay fixed. Figure 3. Effects of using nitrogen during BGA rework. As shown in Figure 3, the pads of a device appear to remain shiny when nitrogen is used, compared to the pads reworked in air. http://www.smtonline.com/pages/zone.cgi?a=60375