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Subject: Small pitch components

Date: 07/01/03 at 9:22 AM
Posted by: Olek
E-mail: Aleksander.Szychlinski@smithsdetection.com
Message Posted:

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?

Another question: What are advantages of using finger shaped pads for componets footprints?
Should they be used instead of rectangular pads for small pitch components?

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