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PCB Pad become brownish after IR reflow

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#59861

PCB Pad become brownish after IR reflow | 16 September, 2009

We encoutered few PCB after IR reflow the PCB pad change color from silver to yellowish / brownish. Our IR reflow peak temp is ~250 degree. The brownish cannot be remove with IPA as well.

What is the main reason causing this issue?

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#59868

PCB Pad become brownish after IR reflow | 16 September, 2009

Questions are: * What is the solderability protection on the pad that changes color? * What is the distribution of this issue?

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#59871

PCB Pad become brownish after IR reflow | 16 September, 2009

PCB finishing is immersion white tin. The brownish is at random location at PCB pad. The issue only affect visual reject but no solderbility functional failure.

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#59872

PCB Pad become brownish after IR reflow | 17 September, 2009

The colour you see on the pads is tin/copper intermetallic layer. In your reflow the immersion tin coating and the copper pad underneath react to form in intermetalic layer. The effect is worse the hotter the pcb or area of the pcb gets. In IR reflow there will be quite a variation in the temps across your pcb so that is why the effect is in different parts of the pcb. You'll also find it's not as solderable as the original pads were. Keeping the pcb peak temp. between 230~240 will be better then 250 but it will still happen. This is an inherent problem with immersion tin coating.

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#59891

PCB Pad become brownish after IR reflow | 18 September, 2009

I agree with Graham cooper's input. Also note that the brownish color is incremental w.r.t to number of days you hold after the thermal cycle(example: after the reflow)so try to complete all thermal cycles ASAP to lessen the discoloring.

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#59893

PCB Pad become brownish after IR reflow | 18 September, 2009

Also, it wouldn't hurt to perform SERA testing to determine the amount of free tin to begin with. From what I remember, I think you need 30 microinches of free tin to ensure success. XRF won't give you an accurate thickness reading as it counts the intermetallic as part of the tin thickness measurement.

The only reason I bring this up is that even if there is uneven heat, the finish should still be able to survive at least the one pass since the final finish providers state that it is good for 2 reflows.

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