significance of small dots of solder on PCBs?

[Michael J. Noone]

Hi - I've often noticed small round solder dots on traces on PCBs. They
aren't a part of a component's footprint, and they don't appear to be
vias as they don't have holes going through them. For example, in this
picture of an XBox's motherboard:
http://www.teamxecuter.com/x3/tutorials/pinheader/v10v15/x3_bottom.jpg
they're very visible (for example the circled "D0" points). Can
somebody tell me what these are for? My guess has always been that
they're a sort of test pad - but why would test pads be needed on a
mass produced PCB? I mean my understanding is that these days when a
board like this goes bad it is replaced, not repaired. My only guess is
that they're for testing and/or programming in the factory. Can anybody
shed some light on this for me? Thanks,

-M Noone

 

[Pooh Bear]

Hi - I've often noticed small round solder dots on traces on PCBs. They
aren't a part of a component's footprint, and they don't appear to be
vias as they don't have holes going through them. For example, in this
picture of an XBox's motherboard:
http://www.teamxecuter.com/x3/tutorials/pinheader/v10v15/x3_bottom.jpg
they're very visible (for example the circled "D0" points). Can
somebody tell me what these are for? My guess has always been that
they're a sort of test pad - but why would test pads be needed on a
mass produced PCB?

Why not ?

It's common to test assembled boards that way !

I mean my understanding is that these days when a
board like this goes bad it is replaced, not repaired. My only guess is
that they're for testing and/or programming in the factory. Can anybody
shed some light on this for me? Thanks,

Yes, factory testing on a 'bed of nails'.

Graham

 

[Kryten]

Hi - I've often noticed small round solder dots on traces on PCBs. They
aren't a part of a component's footprint, and they don't appear to be
vias as they don't have holes going through them.

yes, I expect they are test pads too.

I mean my understanding is that these days when a
board like this goes bad it is replaced, not repaired.

Yes, but you still need the test pads there to detect the fault.

They may even help find if the fault is something easily fixed or not.

Even if not, it can help highlight where faults are happening, to detect
problem areas.

 

[John Popelish]

Hi - I've often noticed small round solder dots on traces on PCBs. They
aren't a part of a component's footprint, and they don't appear to be
vias as they don't have holes going through them. For example, in this
picture of an XBox's motherboard:
http://www.teamxecuter.com/x3/tutorials/pinheader/v10v15/x3_bottom.jpg
they're very visible (for example the circled "D0" points). Can
somebody tell me what these are for? My guess has always been that
they're a sort of test pad - but why would test pads be needed on a
mass produced PCB? I mean my understanding is that these days when a
board like this goes bad it is replaced, not repaired. My only guess is
that they're for testing and/or programming in the factory. Can anybody
shed some light on this for me? Thanks,

-M Noone

Are they in sets of 3 widely placed across the board? If so, they are
fiducials. These are alignment references that the auto placement
machines use to get the solder mask and component placement precisely
aligned with both the position and rotation of the board. The ones
labeled D0 appear to be vias that have filled with solder from wave
soldering. They are not fiducials, because traces run through them.

http://www.tkb-4u.com/articles/other/fiducial/fiducial.php

 

[Mac]

Hi - I've often noticed small round solder dots on traces on PCBs. They
aren't a part of a component's footprint, and they don't appear to be
vias as they don't have holes going through them. For example, in this
picture of an XBox's motherboard:
http://www.teamxecuter.com/x3/tutorials/pinheader/v10v15/x3_bottom.jpg
they're very visible (for example the circled "D0" points). Can
somebody tell me what these are for? My guess has always been that
they're a sort of test pad - but why would test pads be needed on a
mass produced PCB? I mean my understanding is that these days when a
board like this goes bad it is replaced, not repaired. My only guess is
that they're for testing and/or programming in the factory. Can anybody
shed some light on this for me? Thanks,

-M Noone

I'm pretty sure those are testpads. Testpads are not only for
troubleshooting, but also can be used to flag boards as bad prior to
function or final acceptance tests. They can also be used to look for
shorts between nets on bare boards.

A few years ago I worked for a small design group within a large company
we are all familiar with. Our group designed Intel architecture SBC's for
CompactPCI chassis. The test guys always wanted us to have test points on
every single net (even both sides of a series 10-Ohm resistor). The layout
guy we had hated testpoints and thought they were stupid (this is because
bring-up and debug and testing were not part of his job). Anything that
made routing a board take longer was stupid, in his view. We had to watch
him carefully. ;-)

Usually the test guys would let us put the test pads on only one side of
nets with small series resistors. And then there would always be some
nets in densely-routed areas where there just wasn't any way to place all
the test pads without creating long stubs. For high-speed nets, I would
usually veto long stubs.

--Mac

 

[Robert Latest]

somebody tell me what these are for? My guess has always been that
they're a sort of test pad - but why would test pads be needed on a
mass produced PCB?

Because at one point in time that very board was a prototype --
with the appropriate test pads. No reason to redesign the board
just to get rid of them.

robert

 

[Rich Grise]

Hi - I've often noticed small round solder dots on traces on PCBs. They
aren't a part of a component's footprint, and they don't appear to be
vias as they don't have holes going through them. For example, in this
picture of an XBox's motherboard:
http://www.teamxecuter.com/x3/tutorials/pinheader/v10v15/x3_bottom.jpg
they're very visible (for example the circled "D0" points). Can
somebody tell me what these are for?

They're vias. You can't see the holes, because they're filled with solder.

Cheers!
Rich

 

[Spehro Pefhany]

Because at one point in time that very board was a prototype --
with the appropriate test pads. No reason to redesign the board
just to get rid of them.

robert

Access to points on the board which are not otherwise brought out can
improve testability. For example, on an analog circuit, you might want
to probe an internally generated voltage to make sure it is within
specs rather than trying to infer that it is from the outside. You
might want access to an internal node on a digital circuit because it
otherwise would take too long to test. The board can be put in a test
jig with probes (eg. pogo pins) and tested without the cost and board
area of connectors or the routing required to get different nodes to
the connector.

This kind of thing is probably more the rule than the exception in
complex mass-produced boards.

Some things don't lend themselves to easy testing- DIPswitches, for
example. If the board is 100% electrically tested and you use a good
quality DIP switch, and inspect you might be willing to assume that
things are okay if the board tests okay with the switches in one
position. However, there are many, many faults that this will not
catch, and will only show up by switching a particular switch from one
position to another. There may be faults that could show up only if
combinations of switches are put in a particular set of positions.
It's clearly impractical to test all combinations of, say, 2
8-position DIPswitches, so brute force is out. Last design I did with
those d*mn things (customer spec'd BTW), I tested each switch in both
positions by forcing the operator to go through a programmed test
sequence flipping one at a time. More than good enough for the
application and it caught an almost invisible QFP-80 pin solder short
in my prototype.


Best regards,
Spehro Pefhany

 

[Rich Grise]

They're vias. You can't see the holes, because they're filled with solder.

You could verify or refute this by looking at the other side of the
board.

Good Luck!
Rich