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July 28, 2008

Wetting Problem SAC finished component

We are having solder wetting problems on a component with leads that are finished with SAC solder, using SnPb solder paste specified by the customer to form the joints. The leads are gull-wing type and the component is finished with a dip into a SAC loaded solder pot.

Any suggestions to solve this problem?


Experts Comments

We would really need to know the temperatures and profile parameters being used but in general:

Since the lead free paste has a 217-220 deg liquid temp and the SnPb paste will melt at 183 deg, the 34+ deg. peak temperature delta can make a big difference.

If the customer uses a peak temp that is too low (215-225 deg), then the lead free paste won't properly melt and alloying won't occur.

If they use a peak temp that is too high (240-245 degrees), or stay high for too long (90+ secs above 200 deg) then the flux in the SnPb paste will be depleted and again wetting will be affected

Some studies have been done that show a peak of 230-235 degrees with 45-80 secs above 200 degrees can be effective in these mixed technology cases.

Marc Peo
Heller Industries Inc.
Mr. Peo has been with Heller Industries for over 20 years and has been President for the past 8 years. Marc has authored several industry articles on Soldering, Flux collection, nitrogen use and Lead Free conversion.

Although I cannot speak much to the SMT side of you question, I do have some comments regarding the tinning process and alloy interaction that occurs when mixing solders.

AIM has an excellent white paper in their Lead Free section on their web sitethat spells out exactly what happens when leaded and lead-free solders are mixed. Although this may not be directly related to the wetting issue, it is worth a read and may shed some light on your specific application.

How are the parts being dipped? Do you use an automated lead tinning machine to accomplish the re-tin process, or are you having an operator dip the parts? Obviously a manual process will introduce some variance into the re-tinned plating thickness. It may be worth looking into an automated lead tinning machine to eliminate this factor if this product justifies the expense.

What profile are you using for your SMT oven? If you are using the approved profile for the SnPb solder paste, there may not be enough heat in the system to melt the SAC solder, which may also contribute to your issue.

ACE Production Technologies manufactures and sells a lead tinning machine that may fulfill your tinning requirements should you decide to pursue that route.

Jeff Fouche
Design Engineer
ACE Production Technologies
Jeff Fouche has 2 years of experience as a Manufacturing Engineer for CM/OEM and 2 years as a Design/Process Engineer with ACE Production Technologies. Jeff is also an IPC certified trainer. He received his BSME in 2003 from the University of Idaho.
NOTE: Mr. Fouche is no longer working at ACE production Technologies.

SAC alloy is typically compatible (wetting) with SnPb. I would suspect the problem is a poor plating or contamination issue rather than an alloy incompatibility.

Kris Roberson
Manager of Assembly Technology
Kris Roberson has experience as a machine operator, machine and engineering technician and process engineer for companies including Motorola, and US Robotics. Kris is certified as an Master Instructor in IPC-7711 / 7721, IPC A-610 and IPC J-STD 001.
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