Solder Reliability Tests

With the advent of lead-free it is essential to evaluate several possible alloy combinations and select the most appropriate substitute for lead-based solder. 

In surface mount technology (SMT) lead is widely used in components, solder paste, and board surface finishes, hence eliminating lead would involves changes to the entire assembly process. 

A complete lead-free transition would require careful modification of several process parameters. Any lead-free alloy replacing lead-based solder should qualify with requirements such as low melting point, adequate wetting characteristics, comparable cost, consistent manufacturability (at the component level and the board level), wide availability, acceptable reliability, ease of reworkability and reparability etc [1].
Several possible combinations for lead-free are available in the market. Most commonly used lead-free solder paste is SAC305. Many alloys in this family typically have melting points ranging from 217°C to 222°C. 

Lead-free solder bumps comprises of combinations such as SAC105, SAC305, SAC405, SnAg, etc. Choosing a combination of lead-free solder bump and lead-free solder paste would
dictate the reliability of the assembly.
Today’s hand held devices are subjected to many stresses and hence requiring them to have to reinforce mechanical strength. For superior protection of solder joints against mechanical strains such as shock, drop and vibration, underfill technology should be adopted. 

Underfill technology aims to ensure that area array packages assembled on PCBs can withstand mechanical and thermal shock [2]. In order to prevent the solder joint strain at corner balls it was decided to underfill only the four corners of the package.

thermal_shock_and_drop_test_smta.pdf

Bono Test

Lead free soldering with no clean solder pastes represent nowadays the most common process in electronic assembly. 

A solder paste is usually considered as no-clean if it passes all IPC J-STD-004 corrosion tests: copper mirror, copper panel corrosion test, Surface Insulation Resistance (SIR) and Elecrochemical Migration (ECM). 

Other SIR and ECM tests are described in Bellcore GR-78-CORE and JIS Z3197 standards.
Although SIR and ECM tests are recognized by all standards authorities to evaluate the solder paste residue corrosivity after reflow, a more selective method, the Bono test, has been developed and implemented in some French companies as a qualification criterion. It has been proven that compared to common corrosion tests, the Bono test better differentiates the nature of solder paste residues.

Bono Test Description

This method is based on an existing test which assesses the liquid soldering flux residue corrosivity after wave soldering. 

The test board has been modified to measure the solder paste residue corrosivity from the one used in the SIR and ECM tests. It is composed of 10 electrolytic cells and is made of an FR4 epoxy substrate with a single copper layer, having a very thin anode between two cathodes.
The solder paste is printed on cathodes through a 120µm thickness stencil and reflowed according to the desired profile.

References

1. J. Guinet, X. Lambert and D. Bono, Soldering and Surface Mount Technology, No 16, February 1994

2. Inventec procedure, MO.SB.10029, January 2007

3. IPC-9201, Surface Insulation Resistance Handbook, 1996

4. L. Lach, R. Mellitz, F. Sledd, L. Turbini, J. Schodorf, Developing a Standard Test Method to Indentify Corrosive Soldering Fluxes Residues, International Conference on Solder Fluxes and Pastes 1992

5. IPC-TR-476A, Sixth Working Draft, April 1995

6. IPC J-STD-004B, 2008

7. Bellcore GR-78-Core, 1997

8. JIS Z 3197, 1986 and Z 3284, 1994

9. H. Daniel, M. Leturmy, S. Lazure, T. Vukelic, “Influence of N2 atmosphere on the contamination effects of lead-free  solder paste during reflow soldering process”, APEX 2005