Solder Mask

[OlekSz]

As far as I know most PCB manufacturers require following parameters for solder mask:
Solder mask pad clearance per side: minimum: 0.002" , preferred: 0.003".
Solder mask dam between pads: minimum: 0.004", preferred:0.006".
For smal pitch (0.5 mm or smaller) componets, meeting this requirements may be impossible.
One of the solder mask function is to prevent solder bridges between component pads.
According to IPC-SM-782A, gang solder mask can be used for small pitch components (it means continuous solder mask opening over a component pads, so no solder mask between pads).
They claim that most of the bridging occurs between component leads rather than between pcb pads.
I wonder if it is true.
Does anybody have experience with gang solder masks?

Another solution to the problem is to decrease width of the copper pads to gain space for solder mask between pads.
However this may decrease solder joint relability.
Which solution is better?

[robert Tarzwell]

hi olekSz
The fiqures you quote are normal solder mask requirements , however most board shops can do much better, which leads me to the answer you are looking for. todays better quality LPI solder masks can in fact image to much closer tolerances, better then .002 . this means that for solder mask between the pads on a chip tey can be done quite easly with LPI they are strong and will image and devlop right to an edge. we typicaly did .001 tolerance to a pad and solde mask line widths of .005 betwen chips and any finger such as your second question..
hope this answer helps

robert Tarzwell megadawn.com

[OlekSz]

Thanks for reponse Robert,

Most of the board shops charge more for manufacturing of precise solder mask.
I'm still looking for information about relability of gang solder masks.

Aleksander Szychlinski

[robert Tarzwell]

hi Most shops I know do not charge extra and LPI should be the standard by now, in replying to your question gang openings to open up the pads of a chip is not the prefired way, it works but you are allowing more chance of solder wicking between pads and more chance of shorts. We did that method for years so it does work but todays LPI masks are so much better to not use them is a waste. sort of a answer.
Robert tarzwell
megadawn.com

[OlekSz]

Thanks,

Aleksander