Simple, when they are putting the solder mask screen over the top of the board to screen it, they need a certain amount of play, as it may not always be placed 100% in the exact location.
I.E. if it was 1:1 there is a chance that a slight misalignment can cause solder mask to be put on the pads.
It is usual for solder mask to be 8 thou bigger than the pads, as for standards I have no idea, it's the manufacturers that insist on this, if we do not provide a soldermask that is bigger, then they will simply increase it to suit.
For fine pitch IC's etc it is common to have a block of mask missing rather than individual pads as it is very hard to put it accurately on fine pitched pads, the tolerances won't match.
Although, this has the effect of sometimes allowing more solder shorts so the design has to be done to take this into account, as are the assembly depts required to be informed to watch for it.
Matthew.
-----Original Message-----
From: DesignerCouncil [mailto:[log in to unmask]]On Behalf Of No
Name
Sent: Wednesday, September 06, 2006 7:00 AM
To: [log in to unmask]
Subject: [DC] Soldermask - question?
Hi All,
I am recently shifted into PCB design filed; interesting field off course!!! satisfied being a part of product development.
Now started learning the reason for each task I perform. Hope I can get more suggestions from expertise here :-).
I understand the reason that slodermask is used for better solderability, but I want to know why we provide certain clearance like 0.005" or 0.004" more than pad size, sometimes I see soldermask missing (say on BGA). Any problems you have experienced due to larger or smaller soldermasks?
Is there any specific standards or calculation used for soldermask creation based of padstack,Assembly process say wave soldering or reflow soldering? How IPC calculates this? I have IPC SM782.
Awaiting for your suggestions.
Thanks,
Raja.
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