Please look at this image(see the soldered/un-soldered dotted-shapes which are identified by arrows).


This circuit belongs to a laptop adaptor.

  1. What are these soldered/un-soldered dotted-shapes?
  2. As I can see, each trace just has one of them. why?
  3. Why aren't they connected to ̶t̶h̶e̶ ̶ ̶"̶c̶a̶r̶g̶o̶ ̶c̶u̶l̶t̶s̶"̶? those soldered traces?
  4. Does this question have something related to this question.

Why are these traces shaped in such way?

  • 1
    \$\begingroup\$ My guess is test points for a bed-of-nails. \$\endgroup\$ – Oldfart May 25 '19 at 14:24
  • 1
    \$\begingroup\$ What do you mean by "cargo cults" here? \$\endgroup\$ – Hearth May 25 '19 at 14:26
  • \$\begingroup\$ @Hearth electronics.stackexchange.com/questions/440343/… \$\endgroup\$ – Roh May 25 '19 at 14:27
  • 3
    \$\begingroup\$ You're misunderstanding the idiom "cargo cult" here (and the person in that comment is using it in a slightly nonstandard way); it just means "something people do because they see other people doing it, without understanding why", and was used there because there is little actual benefit to having all that extra solder there. \$\endgroup\$ – Hearth May 25 '19 at 14:29
  • \$\begingroup\$ @Hearth Holy s*t! going to edit it. Really thought that you call it "cargo cults"! damn! thank you \$\endgroup\$ – Roh May 25 '19 at 14:32

What are these soldered/un-soldered dotted-shapes?

These act as test points. As pointed out by @oldfart, these could help facilitate bed-of-nails test. It looks similar to how it sounds:

bed of nails

This is important in case of mass manufactured items. In this case, before you label it as working, you might want to run some tests. For ex - is the output voltage correct, is the peak voltage appearing on mosfet is under limits, is the feedback signal correct etc. These can be either measured manually using a multimeter or by bed of nails test. Manual process will be time consuming. However you can make a jig out of spring loaded pins which will make contact to all traces of interest and then you can perform all measurements using a program within a second or so. You just hold the pcb in place and press a button. If all tests passed, you get a green signal otherwise you send it back to assembly line for repairs (and some low wage worker gets scolded upon by his manager).

As I can see, each trace just has one of them. why?

Answered above.

Why aren't they connected to ̶t̶h̶e̶ ̶ ̶"̶c̶a̶r̶g̶o̶ ̶c̶u̶l̶t̶s̶"̶? those soldered traces?

Most of these are electrically connected. However in some cases, you might see some unconnected dots. They are called fiducial marks (pointed out by @Hearth in comments). These help the pick and place machine align the component correctly while placing on the pads. It looks like this:

fiducial marks

There are two fiducials on the PCB image that you shared:


Does this question have something related to this question.

Yes, old question explains the cargo cult joke. :)

| improve this answer | |
  • \$\begingroup\$ One or two of them may also be fiducial marks, used not for bed-of-nails testing but for alignment during manufacture. The one in the bottom left, not connected to any trace, especially looks somewhat fiducially. \$\endgroup\$ – Hearth May 25 '19 at 15:07
  • \$\begingroup\$ right. i'll include it in the answer. totally overlooked it. thanks @Hearth \$\endgroup\$ – Whiskeyjack May 25 '19 at 15:13
  • \$\begingroup\$ There's another fiducialish mark in the top right, too, that I just noticed, if you want to mention that one too. \$\endgroup\$ – Hearth May 25 '19 at 15:15
  • \$\begingroup\$ I guess it's necessary to put at least two fiducials, preferably at the diagonally opposite corners if you want to compensate for thermal expansion or contraction of the PCB during assembly. If fiducials were placed at 10 cm in CAD but the machine is measuring 10.1cm, then it needs to calibrate accordingly for placing components. Local fiducial gives additional precision while placing complex ICs. \$\endgroup\$ – Whiskeyjack May 25 '19 at 15:20
  • \$\begingroup\$ beautifully answered! \$\endgroup\$ – Pranit Pawar May 25 '19 at 17:19

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