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I see these white/cream colored blobs all over the board (I've surrounded them with red rectangles in the image above), something which I've never encountered before. They're plastic like in texture, deforming with small amounts of pressure applied to them. Does anybody have any idea what these might be, and whether or not they're indicative of some kind of failure?

Circuit board with blobs highlighted

Context

My motorcycle's EFI control unit is complaining about not getting enough supply voltage (it is, I've checked the input pins) after breaking down recently (and this is now preventing the bike from starting up), so I figured this electronics degree must be worth something, right?

Hoping it might be as simple as a blown capacitor or damaged track, I removed the control unit from my bike and pried off the metal cover covering the pcb, which was attached with some kind of waterproof sealant (somehow without damaging the circuit board myself). The image below is what I found myself presented with.

Also, if anybody happens to notice anything obviously wrong in the circuit board above, I'd appreciate very much if you could post a comment and let me know!

p.s. The supply rails on either side of the board measured ~5mV across with the bike's ignition turned on (what the hell? basically nothing). The Infineon CPU seems to be rated for 3.3V of supply voltage... so one thing I tried was putting a small 3V DC supply in parallel across the rails, hoping it might stop complaining about having low voltage (it didn't), was this stupid? Yes it was.

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    \$\begingroup\$ The "supply rails" are probably both connected to ground. All you did was short out your 3V supply. \$\endgroup\$
    – Dave Tweed
    Mar 28, 2015 at 20:35
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    \$\begingroup\$ Damn. I promise I have been googling "PCB white blobs" (and etc.) for the past several hours :) Even a friend working in circuit design had no idea. \$\endgroup\$ Mar 28, 2015 at 21:23
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    \$\begingroup\$ ...But it's a question asking whether or not such a thing is a commonly seen defect when repairing PCBs, while I'm in the process of trying to repair said PCB. I don't see why it would be considered irrelevant to repair. \$\endgroup\$ Mar 28, 2015 at 22:14
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    \$\begingroup\$ The question is about what the blobs are on the PCB. I just felt there was value in explaining the context of the situation, and mentioning that I would appreciate any comments about the state of the PCB in general, since I am not experienced in PCB repair (though it was not intended as a separate question or part of the main question at all). \$\endgroup\$ Mar 28, 2015 at 22:36
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    \$\begingroup\$ @user31131 I too am curious as to what the blobs are for. The title of the question is fine. The pictures are excellent. But the text of the question starts with "My motorcycle's EFI control unit is complaining [...]", and two-thirds of the text talks about your repair woes. So, it doesn't surprise me that some EE.SEers see this as a repair question. \$\endgroup\$ Mar 28, 2015 at 22:45

3 Answers 3

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I've read @Matt's answer. The adhesive for holding the components during reflow is usually red in color, it's less runny, and there's usually less of it. Here's an example of such adhesive.

I've got a different hypothesis. I would guess that this is a thermally conductive adhesive that's holding something on the other side of the PCB. It may be improving the thermal conductivity between the PCB and the heat sink (or case). Notice that this white adhesive occurs mostly around components which are heat-sunk to the PCB.

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  • \$\begingroup\$ Absolutely! I've posted my findings as a comment on the other answer. \$\endgroup\$ Mar 28, 2015 at 23:57
  • \$\begingroup\$ By the way, red doesn't seem to be the only color for SMT assembly glue. Apparently it comes in yellow too: coprise.com/Heraeus_SMT_adhesive.html \$\endgroup\$
    – Fizz
    Oct 19, 2015 at 2:11
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    \$\begingroup\$ I found one PCB manufacturer that offers this: eurocircuits.com/PCB-design-guidelines#heatsinkpaste Basically it's a paste that becomes solid after baking. It's poured thorough vias so on the other side you see those bubbles. \$\endgroup\$
    – Fizz
    Oct 28, 2015 at 22:23
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The first picture has pads for a couple DPAK components that have white stuff around the body pad. It looks like the populated DPAK next to it has the same stuff under the component. There are a couple other places where missing components have white stuff.

I would venture a guess that there are components on both sides of this board, and that is the glue to hold the components in place for reflow. The random dots throughout the board are glue seeping through vias from the other side.

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  • \$\begingroup\$ Excellent! I wouldn't have thought of that. I'm guessing that the OP will confirm. \$\endgroup\$ Mar 28, 2015 at 23:22
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    \$\begingroup\$ We have a winner! Sort of.. It's very difficult to look on the other side of the pcb, as it's covered by another metal plate with a very difficult to remove seal. I peeked under the plate and there were no visible components (just some solder joints). However, it looks like some parts of the PCB have been bonded to the metal plate with some type of sealant (possibly thermal paste to use the plate as a heatsink?) and it certainly does look like this is what has been leaking through the vias. Good eye! \$\endgroup\$ Mar 28, 2015 at 23:56
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Based on my experience they are blobs of rtv used to seal moisture sensitive low level and/or analog signals against potential moisture and/or condensation.

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