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I have a batch of new PCBs with ENIG (electroless nickel / immersion gold) surface finish. My assembly house has had two unsuccessful attempts at populating the boards. We're trying to troubleshoot the situation.

The assembly house is suggesting that it is Black Pad [1] [2], which is caused during PCB manufacturing but isn't exposed until assembly/soldering. This would, of course, shift the responsibility away from the assembly process.

With ENIG, the nickel is the bonding surface whereas the gold is simply there to protect the nickle. During reflow, the gold dissolves into the solder. You don't know there is "black pad" until the underlaying nickel is exposed.

In my case, we have a 15-up array but we are only populating a few of the individual boards. The remaining boards do not have solder applied to them; all that happens to them is that they go through the oven twice.

We are seeing something that looks like black pad on these post-oven boards:

blackpad

My question: Will the gold get burned away somehow when exposed to reflow conditions, even if there is no solder applied to the pads? Or might this be an entirely different problem?


Additional info: The image makes it appear that if might be soldermask on the pads, but in real life you can see that it isn't. Here's the best image my equipment will give me. You can (kind of) see that the discoloration is different that the intentionally-placed soldermask.

blackpad2


Another edit. I was able to get a better image, shown below. I think I know what's happening, but I don't want to influence any upcoming answers.

blackpad3

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  • \$\begingroup\$ Those pads need to be clean before a BGA is put on them, it looks like it's the same material as the soldermask. It would be unusual for an assembly process to deposit material. \$\endgroup\$ – Voltage Spike Sep 26 '19 at 18:48
  • \$\begingroup\$ @VoltageSpike Agreed :-) They were clean before they went through the assembly process! \$\endgroup\$ – bitsmack Sep 26 '19 at 18:53
  • \$\begingroup\$ If they went through an assembly process, what was it? why are there no components or solder on the board? Assembly usually means 'putting the components on the board' \$\endgroup\$ – Voltage Spike Sep 26 '19 at 18:54
  • \$\begingroup\$ @VoltageSpike This is from a partially-assembled array. They soldered/populated some of the individual boards, and left the others alone. These images are from the left-over boards which weren't soldered, but went through the oven because they were on the same array. \$\endgroup\$ – bitsmack Sep 26 '19 at 18:57
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    \$\begingroup\$ Is that the bottom side glue? \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Sep 26 '19 at 20:43
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For completeness

https://blog.epectec.com/urban-legends-of-pcb-processes-enig-black-pad

All reports and analysis point to the conditions of the nickel bath as the main cause of black pad, with excessive phosphorus being the primary issue. Affected joints are easily broken, and the corroded, “black” nickel is exposed, providing basis for the term "black pad". Even the tightest controlled production houses, which closely monitor all chemistry levels, temperatures and incoming raw materials, fall victim to this ongoing problem at some time or another

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Try scrubbing it off with a toothbrush and some IPA (isopropyl alcohol). If it comes off, then your good to go. My bets are it'll come of with IPA.

Regardless the assembly houses process shouldn't be depositing material on the board, especially in areas that are far away from assembly, I'd ask the assembly house what it is.

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