QFP solder mask expansion and min. width

[shadewind]

I'm learning to do PCB layout as of lately and I noticed a problem that I'm not sure how to solve.

I have an ATMega8A in a 0.8 mm pitch TQFP package on my board and the spacing between each pad is well within the minimum clearance (6 mil) for iTead Studio which is the service I'm going to use. On the other hand, the solder mask between the pads is less than 0.1 mm which is the minimum width iTead Studio's rules allow. This is with a solder mask expansion of 4 mil which is the default in Altium.

Now, 0.8 mm is a fairly wide pitch QFP I assume so it surely has to be possible to use QFP footprints with iTead, right? So what am I supposed to do? Reduce solder mask expansion? Ignore the warnings?

[jahonen]

There is no requirement in PCB manufacturing that there must be solder mask between pins. It makes sense to open the mask entirely in pad area (four big openings) in this case. With denser pitches like 0.65 mm and below, solder mask is usually removed this way.

In fact, very thin slivers of mask might cause problem, and it is another reason why PCB manufacturer might edit your gerbers, to remove the thin slivers in soldermask.

Regards,
Janne

[shadewind]

So should I remove it or should I just send it as is?

[jahonen]

I think it would be perhaps safest to remove it by yourself, that way you can be sure how it has been done. This can be done by just drawing appropriate rectangles in the solder mask layer, on top of the pads. Another way would be to enlarge the openings in the footprint so that they touch each other.

Check the result with some gerber viewer, to view the final result (wise operation, which I'll always do).

Regards,
Janne

[shadewind]

Allright, I've draw rectangles across the spaces between the pad. Will it be a lot more difficult to solder this?

[jahonen]

I think not, the <0.1 mm wide mask between pads wouldn't been much of a help anyway, even if it would exist.

Regards,
Janne

[shadewind]

Well, then, we'll see! Thanks for the help.

The advantage of using iTead is that it isn't a lot of money if anything in the design is wrong.