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September 9, 2009

Low Silver Soldering Problems

We are using a vapor phase oven which we have used successfully for a number of years with both leaded and lead-free solders.

Recently we experienced problems with a no lead BGA that has low silver solder balls. The problem is that the soldered boards do not hold up to shock testing.

We have increased the dwell to 85 seconds at 245C and this helped but did not fully correct the problem. We increased the temperature to 253C and still the problem persists. We have changed solder paste to one that has a more active flux, still no help.

Any suggestions will be appreciated.

E. R.

Experts Comments

Low-Ag in SAC alloys such as SAC105 in BGA balls are very good for drop performance and are primarily used for hand-held devices to prevent the ball from breaking when the device is dropped
On the flip side, low-Ag reduces fatigue resistance and therefore SAC105 is poor at thermal cycling / shock when compared to SAC305
Industry standard compositions are SAC105 & SAC305 where you unfortunately need to sacrifice either drop or thermal shock
- Moving to SAC305 is an option if your assembly does not have stringent drop requirements
- Considering doped alloys that satisfy both drop & shock requirements - see attached
Karthik Vijay
Technical Manager - Europe
Indium Corp.
Currently with Indium Corporation and responsible for technology programs and technical support for customers in Europe. Over 15 yrs experience in SMT, Power, Thermal & Semiconductor Applications. Masters Degree in Industrial Engg, State University of New York-Binghamton.

I represent KIC, maker of thermal profiling equipment and software. I spend most of my time on reflow since there are so few vapor phase systems out there in the Americas. The following advice I got from Mike Young of Aligned Solutions. Hope it helps!

Vapor Phase, in my opinion, is much more consistent than convection. It gives a good delta across the entire PCB regardless of topography that convection heat normal has to overcome. E.R.'s temperature of 253C, seems high.

The melting point of most low silver solders is 227-228C. With the consistency of most VP units you shouldn't have to be that much higher than the melting point. The most common issue that I have seen when it comes to BGAs has more to do with a mix of material.

The solder paste may be one type, while the ball of the BGA is another and the HASL finish of the PCB is yet another. Once the profile is dialed in on a consistent process, the material typically becomes the most common defect.

It would be good for E.R. to not only talk to the component vendor about the solder balls but also run his own metallurgy test. Here are two links that E.R. might find helpful. Taken at face value you would think the low silver content would be helping. http://www.sanmina.com/partners/QAR-0009-C.pdf http://smt.pennnet.com/articles/article_display.cfm?article_id=302519

Brian O'Leary
Global Account Manager
Indium Corporation
Mr. O'Leary is the Global Account Manager for Indium. He has and extensive global network of contacts in the electronics industry with expertise in SMT equipment and processes.
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