Question about Soldermask

[lorth]

Dear all,

I've been working with the same PCB manufacturing for a couple of years now, but after a couple of bad experiences I'm switching to a new one.

There is one rule on the new company that is bothering me regarding Soldermask: "openings in the solder-mask layers should be 0.006" larger than the copper feature."

What does that mean? If I have a pad that is 40 x 9 mils, how should the soldermask aperture be?

Thanks!!!

[c4757p]

52x21?

[free_electron]

ehhh.  46 by 15 mils..

6 mils larger means they want a 3 mil wide ring around the copper.
3 mil is standard opening these days. 2 mil will cost extra. i use 4 mil by default.

for a 40 by 9 pad you will have a 46 by 15 opening in the soldermask centered on top of each other.

[ddavidebor]

That their soldermask is not able to attach on pcb surface, but only on copper, and you must leave 6 mils of copper between nude pcb and soldermask.

[gxti]

Don't be ridiculous, of course they can put soldermask on bare PCB. Like free_electron said, it means the "registration" between soldermask and copper could be up to 3 mils off, so the soldermask aperture radius must be 3 mils larger in order to guarantee that soldermask will not get on the pad.

[c4757p]

I really wish they'd just say something like "Solder mask alignment tolerance: +/- 3 mil". Would anyone have any question what that means? It answers two questions very quickly: What can't you do? Why can't you do it? No need to do the job of interpreting what the tolerance means for my design, I can do that myself TYVM...

[lorth]

Thank you very much!!!!

As for the rules, yes, they make it a little complicated. When I was back in Spain, the company I used had this very nice rules with drawings and everything that made life so much easy...

[free_electron]

I really wish they'd just say something like "Solder mask alignment tolerance: +/- 3 mil". Would anyone have any question what that means? It answers two questions very quickly: What can't you do? Why can't you do it? No need to do the job of interpreting what the tolerance means for my design, I can do that myself TYVM...

it is not the tolerance of the mask. That is something else.

there is 3 things

- the opening around the pad
- the tolerance of that opening ( dictated by the process. depends on dry mask , liquid mask or printed mask )
- the registration of the mask

if you spec 4 mils this means

for a square pad of 20x20 that the opening will be 24 by 24. giving essentially a 2 mil border around the pad
the tolerance may be 0.5 mil. this tolerance is process related.  drymask or liquid mask are photoprocesses and pretty accurate. printed mask is an inkjet style mask squirter that is afterwards baked. there you will have larger tolerance but it is cheaper as there is no exposure step or photoplot involved.

the registration : this is how good the centering is on the pad. the machine needs good coordinate registration or else all pads will be off by a certain x-y delta.

so , your 4 mil opening may turn out to be 3.8 mils in reality (tolerance) and the registration error could be -1,-1. giving you a pad with  0,9mil opening left ,0.9 mil opening top and 2.9 mil opening right and bottom ...

[lorth]

Very instructive!!!