I take it you mean for solderability. This can be done either with a wetting balance and using a low temperature alloy or by reflowing them to a known good coupon using only a sticky flux and cross sectioning to evaluate IMC thickness and formation.
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.
I would do a solderability test and cross section of a standard assembly made with your standard process. If the spheres adequately wet the pads and the intermetallic is uniform and sufficiently thick, I would say that the spheres are still usable.
Technical Support Engineer
Kay Parker is a Technical Support Engineer based at Indium Corporation's headquarters in Clinton, N.Y. In this role she provides guidance and recommendations to customers related to process steps, equipment, techniques, and materials. She is also responsible for servicing the company's existing accounts and retaining new business.
The pertinent question to consider is, what physical processes do we expect might degrade performance of the spheres, and have we mitigated them effectively?
So to answer the above, let's list the things that might occur in storage:
So based on the above, it seems like(1) and (3) are mitigated, and (2) can be dismissed with a quick visual inspection. That leaves (4). I'd suggest a quick, empirical solderability test. Use a clean, solderable surface much larger than the ball diameter, and a mild tacky flux, less active than you'd use for ball attach. Solder a sample of spheres using a profile at the minimum acceptable peak temperature. Check solder spread against a new lot, or failing availability of a new lot, just look for acceptable wetting angle at the periphery.
- Oxidation; you've effectively mitigated this by using nitrogen storage, which limits both the molecular oxygen from the atmosphere, and water, since nitrogen sources are normally very dry.
- Recrystallization; Sn and Sn alloys are above their recrystallization temperature at room temperature, so microstructure may evolve over time. This should not be an issue, since you are going to re-melt the spheres during use. It might only become an issue if the changes caused surface morphology changes that defeated optical inspection. Such changes would be apparent on visual inspection. Risk is very low indeed.
- IMC formation; not a problem, since the spheres are not in contact with any other metals.
- Organic contamination; potentially the spheres could accumulate organic material if packaging materials outgas and the outgassed products are re-deposited on the spheres. Whether said products would be detrimental is another question. Risk is low, but not zero, and nitrogen storage does not mitigate.
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.
Recommend to perform Shear / Bend Test on entire PCBA to determine the Solder Strength on BGAs. Upon Shear test a Dye-and-Pry test need to be carried out on BGAs to know the Shear / Bend test results. It should identify any cracks or separations within the solder joints. This test is traditionally performed to evaluate the solder joints of BGA components, as their joints are not readily visible.
Additional to that, you can perform,
- Micro sectioning on BGAs: The joints should be examined for voiding, cracking, wetting, and a variety of other features and anomalies, via Optical Microscopy. Once sectioned, the joints should also be examined at high magnification via Scanning Electron Microscopy / Energy Dispersive X-ray Spectroscopy.
- PUSH TEST Analysis: Push Test on BGAs
- Pull TEST Analysis: Pull Test on BGAs
Have 18 years of experience in electronic Industrialization. Specialties in PCB Design & manufacturing process, PCBA Process Development and Continuous Improvement.
Some manufacturers assign a shelf life on 1-2 years to their spheres. One way to re-qualify is by performing a solderabilty test preparing a BGA component and soldering onto a test coupon. If more reliability data is required you may perform a cross-sectional cut to analyze ball composition & integrity.
Senior Manufacturing Engineer
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.