Closely positioned components on this one sided PCB have red resin separating solder connections and underneath surface mount components.

  1. What is it?
  2. How is it applied and why do only some components have this resin separation?

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  • \$\begingroup\$ Found an interesting article mtarr.co.uk/courses/ami4805_map1/poly/pae/poly_pae.asp Though I still don't understand the usage of the resin under C23 (for example) \$\endgroup\$
    – Sachin
    Jul 27, 2017 at 7:18
  • \$\begingroup\$ Note the big (unused) blob where C106 might be. Also note that the glue may not be screened on, but directly written with a dispenser. \$\endgroup\$
    – Jon Custer
    Jul 27, 2017 at 17:45

2 Answers 2


That is glue used to stick components on the lower side of the board during pick&place manufacturing, so that they don't fall off the board before the board completes its course through the reflow soldering oven, i.e. until all the components are held in place by the solder joints.

See, for example, this NASA document about the placement of glue dots (not that all manufacturers follow those guidelines, but those images are good to understand the thing).

Most probably you may notice that the other side of the board (the "primary" side) is populated with other components, which don't need the glue, because that side is the upper side during reflow soldering.

Why they put the glue even on places where no component lies, I don't know for sure. Probably they use the same mask for similar boards where those places are populated. This is due (thanks to @Asmyldof for pointing that out in a comment) to cut down production costs: wasting a little bit of glue on each board is much cheaper than setting up a new "glue mask" for each little variant of the board they may want to manufacture. Note that setting up the machinery for a new configuration takes considerable time, and times is big money in a mass process where dozens of boards are churned out of the assembly line each minute.

If you are curious about PCB manufacturing processes, here are some relevant videos from Dave Jones (EEVBlog):

  • 2
    \$\begingroup\$ As you guess at, in large series of production with several variants, $(set-up-time) >>> $(glue-at-bulk). \$\endgroup\$
    – Asmyldof
    Jul 27, 2017 at 7:25
  • \$\begingroup\$ @Asmyldof thanks for confirming my guess. I'll integrate it in my answer. \$\endgroup\$ Jul 27, 2017 at 7:51
  • 5
    \$\begingroup\$ What I find interesting is that there is glue between through-hole pins. Maybe it's also a bit of wave-solder bridging insurance? \$\endgroup\$
    – W5VO
    Jul 27, 2017 at 13:03
  • 1
    \$\begingroup\$ @w5vo I think that's far more likely. Most of those dots are not in position to hold a (possibly unpopulated) component in place, and it seems unlikely you would bother to reuse a glue mask when component positions have changed. "Large production runs" are a red herring too, IMO, setup time/costs matters less (and per board costs matter more) for large runs than they do for small runs. \$\endgroup\$
    – mbrig
    Jul 27, 2017 at 17:40

The 3-dot sets between some of the closer pads is a fairly routine fix for when wave soldering can create a short (it acts in a similar way to the solder resist mask). When I worked in contract manufacturing, we often used this trick to deal with cases where the PCB wasn't really designed for wave solder manufacture. The majority of these dot groups will be in the same orientation, as bridges will generally be seen in the same direction as the board passes over the wave.


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