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Gold contamination in solder joints?

Bill Boles

#2967

Gold contamination in solder joints? | 21 September, 2000

Does anyone have a quick reminder for a PCB with immersion gold surface finnish going through a reflow profile: Which of the following statements is more correct?

A: In order to minimize gold in the intermetallics of the solder joint, the board temperature should exceed 215�C.

B: In order to minimize gold in the intermetallics of the solder joint, the board temperature should not exceed 215�C.

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#2968

Re: Gold contamination in solder joints? | 21 September, 2000

Well, I'm going to assume you: * Have a nichel barrier between the copper on your board and the gold / solder solution * Use a lead / tin near euthectic solder

If so, I pick "C" - It doesn't matter.

When concerned about copper tin intermetallics, the time at a higher temperature is better. When concerned with gold tin intermetallics: * There's beaucoup tin relative to the gold * At ~200�C gold moves into solution very (blisterin') fast, like 4um/sec. The um was used as units because the speed I'm refering to is based on watching wire disappear into solder.

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#2969

Re: Gold contamination in solder joints? | 22 September, 2000

That "C - it doesn�t matter" choice sounds good because it doesn�t give you a headache when profiling but it gives me a headache when I read the article in the current "newsletter" about profiling where I found this: "If soldering to gold over nickel coated pads, a peak temperature of at least 220�C should be met; this will prevent post-reflow thermal reliability issues, as tin and gold form a secondary eutectic at 217�C."

In other sources I found that you should give it a bit more time over liquidous in order to allow the gold to disperse more evenly in the joint to prevent gold enriched zones which may lead to early failures and of course to give a bit more time for the formation of Sn/Ni intermetallics.

Well I think I should look for more information

Wolfgang

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#2970

Re: Gold contamination in solder joints? | 26 September, 2000

Wolfgang: I think that we were answering different questions. * I thought Bill wanted to talk about minimizing the amount of gold in the solder connection that is available to make intermetallics * You were talking about the formation of intermetallics

Bill: So, what IS the question?

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