What are these black squares for (pcb design question)

[max_torque]

On a dual layer pcb from a large UPS, the pcb has a lot of small black squares visible:



They look to be in the centre of the prepreg/fr4 and can be seen as a pattern of 'bulges' under the large copper pours on the top and bottom of the pcb. I have back lit the pcb for that photo so you can see the copper pours / traces on the back, and the squares between the layers

They are not across the whole pcb, so can't just be an artifact of the material used for the stackup?

They are most prevalent under the high voltage / switching portions of the board, so perhaps they are either some form of emc shield / ground or something to increase (or decrease) the pcbs impedance?

Has anyone seen such a thing before?

[Ribster]

If it's a multi layer PCB, and those pads are copper, it is for warping.
If not, i guess it's just the marking of the prepreg.

[dmills]

Bet that's a multi layer board and they have kept a buried plane out of the high voltage area, which makes those squares copper to help equalise the copper distribution for both warping and etch quality.

It is a thing that PCB houses will sometimes ask to be permitted to do if you have a buried plane on ONE half of the board, especially if the layer is heavy copper.

Regards, Dan.

[max_torque]

It's a scrap board, so i'll attack it and see if those squares ARE copper!  I can't see any internal traces, just lots of squares!

[Doctorandus_P]

Can you post some macro photos?
Preferably from before, and maybe also from after you've "attacked" it, but then you probably know and it is nof of much use anymore to post...

[BurningTantalum]

Many years ago, in the days of Ferric Chloride, Rotring pens, and photo negatives of layouts, these features were called 'robbers' or 'thieves', I no longer recall which was the correct term and which our joke name. They served the purpose of stopping warping and, in leaving more copper on the board, made the ferric in the spray booth last longer.
BT

[max_torque]

OK, i've hacked into the pcb and those squares are indeed copper on two inner layers!  However i can't find any trace of any, er traces, on those inner layers.  The only reason i can see to use a 4 layer board, and not use the inner layers (unless they are for some form of EMC complicance which seems unlikely) is because the board is going through a process with other boards that ARE 4 layer boards and so it would be 'cheaper' and 'simpler' to just make the inner layers into that squared pattern to avoid warping??

The device is an expensive multiphase UPS for scientific equipment so i don't think cost was a concern.......

[dmills]

It might be part of a bigger panel that does have real 4 layer stuff on it I suppose?

Regards, Dan.

[texaspyro]

If it's a multi layer PCB, and those pads are copper, it is for warping.

That is done on ground/power planes and large fill areas on PCBs to prevent warping and delamination during reflow due to heat buildup / trapping.