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September 10, 2006

Reflow of "high-lead" flip chip device

We can not reflow solder 63/37 to "high-lead" (95%) bumped flip-chip. We have tried Type 6 solder paste and aggressive flux. We have even tried reforming the "high-lead" bumps in N2 to see if contamination may be the cause. The result is that the paste just does not "wet" the "high-lead" bumps and component fall off. At the moment we have started using parts with only the UBM and directly soldering 63/37 to that. Any suggestions?



Doug MacGugan

Experts Comments

Soldering to high lead bumps can be a big problem. First how are the bumps formed? They could be heavily oxidized this would cause the issue. Also you say you are using an aggressive flux. This may not be compatible with the high lead.

Just because a flux is deemed aggressive it does not necessarily mean it is the proper match for a high lead bump. Dwell time and time to reflow are critical. The faster you get to reflow the better fluxing action you will achieve. Also on high lead bumps run hotter I would recommend a 235C reflow spike temperature.

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Karl Seelig

Deck Street Consultants
In his 32 years of industry experience, Mr. Seelig has authored over 30 published articles on topics including lead-free assembly, no-clean technology, and process optimization. Karl holds numerous patents, including four for lead-free solder alloys, and was a key developer of no-clean technology.

With the data that you have presented it is not clear what the route cause of your problem is. It could be either the activity of the flux (wrong type of activity or insufficient) or it could be profile related - it is not clear.

If it is a profile issue with there being insufficient time for the molten solder to form any appreciable intermetallic bond to the solder bump. If this is the case then additional heat and time should improve the situation. Also check the O2 content in the oven if this is to high this could result in flux exhaustion.

This can lead to "graping" where the surface of the solder looks like a bunch of grape with the outline of individual solder spheres still visible in the surface of the solder fillet and poor wetting. It is desirable to use the largest solder powder size that you can. This gives the least amount of solder surface to oxidize and reduces the load on the flux. It may be worth considering a larger particle size if possible.

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.

Years ago we supplied a stencil where they put an eutectic cap on the high-lead balls. The stencil had small domes (3 mils deep) etched into a thick coupon.

63/37 Solder Paste was doctor bladed into the 3 mil pockets. The FC was placed on the coupon with the bumps registered to the pockets. The FC and coupon were then sent through reflow forming an eutectic cap. The stencil was rendered non-solder wetting by coating with Chrome.

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Bill Coleman
Vice President Technology
Photo Stencil
For over 18 years, Dr. Coleman has been the vice president of technology for Photo Stencil, working closely with customers to understand their printing requirements. His efforts have resulted in several new stencil products.
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