PCB Test Point questions. Solder or no solder

[TheRadioGeek]

Hello all,

I have a question about PCB test points. What is standard practice? Do test points get solder pasted. In other words should test points have solder on them? Or should the test points have no solder on them. Is it better for the test probe to hit a test point with solder or without solder. What are the pros and cons of doing it each way?


Thanks
TheRadioGeek

[free_electron]

Absolutely NO solder. Usually automated test equipment use a kind of pins called pogo pins (which are spring loaded pins) to probe test points.
With solder (and a bulged solder "pillow"), the tip of pogo pins may slip, breaking contact at best, short circuit and kill your board at worst.

nonsense.
the pogo pins are very sharp and will poke in the solder without problem.

[wraper]

If that is board with ENIG, there is no point applying paste. With HASL, no much difference IMO, if OSP, then certainly yes. Pogo pins may be rounded or sharp. Rounded won't pierce flux residue layer which may cover the solder.

[TheRadioGeek]

How bad will the flux build up contaminate the pogos? 

[HHaase]

There's no reason to apply paste on a test point if you're not soldering a component to it, unless the pad material isn't compatible with your test procedure.

If you apply paste you'll now have the convex surface of a solder pillow with flux residue on it if you use no-clean. Both conditions can potentially make it harder to test. 
Even with a sharp probe you can potentially have issues with residue from no-clean fluxes. 

I honestly couldn't tell you the last time I've seen a board with solder intentionally placed on a test point.

-Hans

[HHaase]

How bad will the flux build up contaminate the pogos?

Bad enough.    Causes a lot of false failures in our flying probe when the probes start having contaminants build up, or if they're dull enough to have trouble piercing through the residue on the pcb.

[DavidDLC]

In all the products I have worked on, there is no solder on the test points.

Sometimes there is no way to have a test point, so you need to probe on a soldered pad, and for those cases you use a special test probe.

But no, no solder on them.

David DLC

[T3sl4co1l]

I've seen many boards with paste (or wave) solder on test points.  I've also seen many of those pads having a distinct, very fine pinprick in the center of the blob...

Do ensure the pads are nice and clean.  Crusty flux deposits are common on non-pop pads.

They do indeed tin testpoints on OSP boards.  I've inspected motherboards designed in this way.  (They also make bus traces testable by exposing soldermask along a trace, making a portion of the trace into a very narrow pad.  These need quite precise probes to contact!)

Tim

[wraper]

Raspberry Pi zero (ENIG) and 3 (OSP) PCBs compared.

[noidea]

Absolutely NO solder. Usually automated test equipment use a kind of pins called pogo pins (which are spring loaded pins) to probe test points.
With solder (and a bulged solder "pillow"), the tip of pogo pins may slip, breaking contact at best, short circuit and kill your board at worst.

nonsense.
the pogo pins are very sharp and will poke in the solder without problem.

+2 I work on air conditioners and the boards I look at every day actually have solder pillows on the testpoints and you can make out the impressions where the bed of nails probes made contact, or at least I can when I am wearing my glasses 

[CatalinaWOW]

I have never seen a study on this so I am not aware of an objective reason for going one way or another.  As seen above there are lots of opinions with anecdotal information supporting them.  For boards which will be manually probed my personal preference is to have solder on the test points.  The softer solder surface makes it easier to get a no slip contact.  For those using automated methods (flying probe or bed of nails) that answer is very possibly wrong.

The cadillac way to go for manually probed boards is to provide actual test pins.  The extra BOM cost may well be recovered by reduced test difficulty.

[technotronix]

What is a reason to have test point?

[T3sl4co1l]

What is a reason to have test point?

A need to test that point ofc

Lots can go wrong in production, and the easiest for the test engineers is 100% nodal coverage.

Tim

[elecman14]

How bad will the flux build up contaminate the pogos?

So how about just don't apply paste to them? I've never seen boards having solder on test pads, even for OSP boards. They will sometimes selectively gold plate test points on OSP boards, or they will simply use OSP rated pogo pins which can pierce OSP layer.
Unless you are designing ultra high reliability or ultra low cost things, I see no reason to use OSP. The main goals of OSP are to use less gold to achieve better solder joint reliability and to reduce cost of gold plating.

People are using OSP since it is flatter than HASL and cheaper than ENIG. If you make enough of something that cost difference makes it worthwhile to overcome the shortcomings of OSP like harder to test and shelf life.

I have not evaluated OSP pogo pins in the last few years so they may have improved. My observations from a few years ago of them were that they became clogged with the OSP material and required replacement more often than ones that just probed HASL or ENIG. When you have a large number of them that can get expensive quickly or cause test failure.

[richardlawson1489]

I don't think you should apply paste on a test point.

[julianhigginson]

My understanding is that at least originally, OSP would be destroyed in reflow. leaving you with bare copper. in that case you would paste the test points to stop them oxidising in the field... maybe there are different types of OSP that don't do this now?

I never paste ENIG test points, but you do need to be careful that your test points are not puncturing the pads and leaving room for oxidation to start. pasting them would probably help with that.

if anything, this discussion about one aspect of manufacturing becomes one more story about how EVERY part of a manufacturing process is related, and can be critically important, so you need to think of everything you are doing in context.

[KL27x]

I had some boards done the other year, and turned out they got solder on the ICSP pads. It wasn't my mistake (THIS time, anyway ). I assume they were done with no clean paste, same as most of the boards coming from the large manufacturers, anymore. The spear-heads I used didn't work. My PCB guy gave me some special pins that were supposed to center on the solder beads (I think); they have little cups on the end. They didn't work. This particular board was pretty tiny and panelized; I ended up flashing them on a CNC, using serrated head pins. I programmed the machine to give a little "wiggle" after setting down on the solder beads, to dig in. That worked 100%.

[TheRadioGeek]

Thank you to all that replied to my question. It looks like putting solder on test points is not a good idea. I have seen boards done both ways and just wanted to known what the preferred way was for most manufactures.

 

[julianhigginson]

Thank you to all that replied to my question. It looks like putting solder on test points is not a good idea. I have seen boards done both ways and just wanted to known what the preferred way was for most manufactures.

I know what isn't a good idea:

Going to manufacturing with an assumed generic "one size fits all" approach, without understanding the implications of your board finish, assembly process, and test hardware setup, and how it all relates to the specific requirements for your design.