Printed Circuit Board Assembly & PCB Design SMT Electronics Assembly Manufacturing Forum

Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.


Silver finish components

Views: 959

DAM

#38568

Silver finish components | 17 December, 2005

You know, many SMD components have copper alloy leads and then a copper (or Ni) barrier on them. On the copper (or Ni) barrier there is the finish. Usually the finish is made of Sn\Pb (tin-lead), and if I solder with a Sn\Pb solder paste it's all right (with a reflow oven), becouse the finish melts. Now, unfortunately I have some components that have silver finish. I have some questions: 1) If the thickness of the silver is too much (how is too much??), do I risk to solder on the silver instead on the copper (intermetallic area I mean) ? 2) Is the Sn\ Pb solder paste profile already good, or I must increase the temperature?

Thanks

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#38585

Silver finish components | 19 December, 2005

Q1a. If the thickness of the silver is too much (how is too much??), A1a. We have heard stories of using Ag2 to limit silver [or gold] dissolution, but have never seen formal studies to demonstrate that idea.

First, exclude the use of silver plated components, where ever possible.

Second, keep the silver concentration to less than 2 percent in the alloy, if you must use such components.

Q1b. do I risk to solder on the silver instead on the copper (intermetallic area I mean) ? A1b. The IMC that you form will depend on: * Surface to which you are soldering. * Solderability protection used over the solderable surface. * Solder used.

Back in 1982, people saw silver plated PLCC literally falling off the PCB when slight bending, such during fixturing for functional testing, put even very small loads onto the solder joints. As a consequence of this scary news, (nobody knew about the effect of silver yet and compliant leads were just invented and without history), seventeen competitor companies joined forces to form the IEEE Compliant Lead Task Force with Jack Balde as chairman.

Q2. Is the Sn\ Pb solder paste profile already good, or I must increase the temperature? A2. It may work fine. The addition of 2 percent of silver to Sn/Pb solder decreases the liquidous temperature a couple of degrees.

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