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On several PC motherboards, I've noticed some solder blobs on traces and vias. What's the deal with that? Are they some kind of test points? If so, how are they used?

Solder blobs 1

Solder blobs 2

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    \$\begingroup\$ I swear I've never seen anything like that on professional boards, that's quite interesting. \$\endgroup\$ Commented Jul 15, 2014 at 20:48
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    \$\begingroup\$ Same although it is probably more evident on large boards that are made by a cheaper board-house that still use wave soldering. Undoubtedly hired by some entity less than professional. \$\endgroup\$
    – Funkyguy
    Commented Jul 15, 2014 at 20:50

3 Answers 3

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They are testpoints for bed of nails testing. See wikipedia article:

http://en.wikipedia.org/wiki/Bead_probe_technology

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  • \$\begingroup\$ Yes! Thank you. I was not sure how they would be used as test points, but it makes perfect sense when you remember that there are serrated pogo pins. \$\endgroup\$
    – Armandas
    Commented Jul 18, 2014 at 13:59
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This is a kind of defect called solder attachments (or solder balling), and is mostly a problem when using wave soldering. They are basically a failure of the solder mask, and are directly attached to the underlying traces. Because of that, you should be very careful to avoid touching them, as you might break the track.

You can see a few more examples of this happening here

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  • \$\begingroup\$ I didn't realize that QFN (on the 2nd photo) can be wave soldered. \$\endgroup\$ Commented Jul 16, 2014 at 0:18
  • \$\begingroup\$ +1 For providing the term and a reference. The blobs seem somewhat consistent though, so I'm still not 100% confident. From the pictures it looks like there is only one blob per trace. Also, in the upper picture, one trace is offset around the blob, as if it was done intentionally. Coincidence? Any more comments? \$\endgroup\$
    – Armandas
    Commented Jul 16, 2014 at 8:01
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Motherboards have many, many through-hole solder joints (mostly connectors for I/O and memory). Those joints are made by wave soldering, whereas the majority of the components are surface-mounted and reflow soldered by conventional means.

What you are seeing are vias that were incompletely tented in the solder mask application and the wave solder wetted the inside of the via and reached the top from the pressure and capillary action.

The plot thickens somewhat because you can find small groupings of SMT parts on the bottom of motherboards, bereft of adhesive and spaced too closely to allow them to immersed in a solder wave without risk of solder shorts.

I believe the sequence is probably to populate and reflow solder the SMT parts on the top of the board, populate and wave solder the through-hole parts on the top of the board, with the bottom SMT areas masked, then to peel the masks, populate and reflow solder the bottom SMT parts.

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  • \$\begingroup\$ Good reasoning, but doesn't explain blobs on the traces. \$\endgroup\$
    – Armandas
    Commented Jul 16, 2014 at 8:05

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