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Black stain... what causes it?

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Black stain... what causes it? | 25 January, 2013

Hi there, I have a new product today, after reflow oven I've seen a black stain coming from various holes, they can be cleaned with a tissue but some of the contaminant remains inside, we are using Sn63Pb37, the stains are far from soldering areas, they arise on the edges, in the middle, etc. PCB thickness is 4.5mm. Could it be some chemical trapped inside the holes? or could it be related to the reflow process? we have seen 3 PCBs with this condition from the 5 units we have built. Any input will be appreciated, regards JQ


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Black stain... what causes it? | 25 January, 2013

My first guess is that the plating in the barrel (via) has failed and allowed laminate/epoxy egress into the barrel and out to the surface of the board. Just a guess based on the picture.... 'hege

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Black stain... what causes it? | 26 January, 2013

Looks like you have a carbon black process that isn't quite plating to completion, pin prick size holes have remained in the plating. When dipped in an etchant (ie any acid or H2O) the holes grow and they leak carbon. This may be more of a problem than just cosmetics. The reasoning being that the holes are not 100% reliable. The problems can be caused by improper desmear. Which doesn't leave a nice flat surface for the carbon. The smeared epoxy then expands during plating making a hole. The smeared epoxy can have thread like bits that don't get covered by the carbon.Another possibility is the holes did not plate 100% through on a few holes. Carbon black plating is a very tricky process to control and get right.

Talk to your supplier

regards sarason

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Black stain... what causes it? | 14 February, 2013

PCB Supplier made an EDX anaysis, the found a chemical trapped in the holes, and some burnt burrs, according to what they say this stains are over the solder mask & silk, so they do not affect functionality.

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Black stain... what causes it? | 22 February, 2013

Jorge: I took this total posting to several fabs at IPC APEX. None was satisfied with the answers you receive from your supplier or the Forum members.

They suggested that you get your board analyzed. They were surprised that your supplier didn't insist on doing that level of assessment.

BR, davef

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