Stressing of samples prior to solderability testing

Stressing deposits prior to solderability testing sometimes called incorrectly aging of parts.

 

Stressing plated samples, whether components or PWB’s, when done correctly, can be a great predicator of plating quality, age of the part, the potential shelf life remaining and most importantly the potential for producing reliable solder joints through a complicated assembly process.

Aging of parts using exposure to steam was the de facto stressing technique prior to the introduction of Pb-free devices and assembly. It is well documented that exposure to 8 hours of steam vapor, at 93⁰C at sea level, equates to 1 year of storage under “normal” industry conditions. This works ONLY for devices or PWB’s plated with SnPb. The requirement to steam age is still in use today, primarily for military applications or other high reliability applications that have not changed over to Pb-free.

It is important that if exposing parts to steam that the guidelines in the IPC JSTD002/003 solderability specifications are followed and that the equipment is cleaned regularly and that the parts being stressed are not lying in a pool of condensate.

 

Automatic steam age chamber at S T and S – four individual sample drawers

 

“dead bug” orientation for SnPb plated DIP devices

Steam stressing requirements from the IPC JSTD002D

 

For non SnPb plated parts, there are alternate stressing techniques used and while they evaluate the quality and robustness of the plating deposit, it is NEVER a predicator of shelf life potential although it can be inferred that if the parts solders following stressing then it “should” have a useable shelf life of perhaps a 1 year or more if stored correctly.

For PWB’s with non SnPb based surface finishes, example OSP, ENIG, etc., the prescribed stressing technique is exposure to 72⁰C/85% R.H. for 8 hours followed by drying at 105⁰C for 1 hour. Again it is imperative to prevent condensate from pooling onto the surface of the PWB and the samples should be placed vertically in the chamber. It is imperative not to overload the chamber as this will result in localized cold area where condensate can form and impact the stressing of the samples.

The Thermotron™ Temperature Humidity chamber used at S T and S for stressing of PWB surface finishes

 

Example of a Wetting Balance test coupon array orientated “correctly” in the chamber for temp/humidity stressing.

An alternate stressing technique for PWB surface finishes is a 2X (reflowed twice) exposure through a reflow oven. There are many different reflow profiles used in Industry and so when the IPC JSTD003C WAM1 introduced the use of a reflow profile as an acceptable “alternate” stressing method, it was decided to use the two standardized reflow profiles detailed in the IPC TM 650 method 2.6.27. There is one profile for SnPb assembly and one profile for Pb-free assembly. Both these profiles have elevated peak temperatures compared to most assembly profiles. All the PWB surface finishes currently specified in the IPC 45XX have been tested following this stressing technique. If the deposit is robust then the surface finish will show evidence of solder wetting, advancement of flow and continue to produce positive wetting forces if tested by wetting balance.

DIMA four zone reflow oven used for reflow stressing of PWB’s at S T and S Group.

 

 

The standardized SnPb profile used to stress PWB’s per the IPC JSTD003C WAM1

 

The standardized SnPb profile used to stress PWB’s per the IPC JSTD003C WAM1

 

 

 As per the IPC JSTD002D, the use of dry baking for stressing is now the default method for stressing components. This is done at 155⁰C with a four-hour duration specified. Additionally higher temperatures may be used to evaluate plating quality and adhesion with typical annealing temperatures in the 170 to 175⁰C being used with varying duration times prior to analysis by cross section or in the SEM following fracturing of the plating layer.

 

 

 One of the three dry bake ovens at S T and S Group used for stressing, annealing or long term high temperature storage conditioning