Bob,

The IPC standard for soldermask is IPC-SM-840, which has recently been
revised. It covers the properties and application of soldermasks.

The IPC-D-275 covers the design parameters surrounding the use of
soldermask. When dealing with liquid photoimageable masks, they can usually
provide a dam of 0.004 - 0.005 inch wide. Provide a misregistration
allowance of approximately .005inch and small clearance. You will find that
you can bring the lands closer. 

However, a poorly controlled wave solder operation will still result in
solder bridging problems. Be sure that the process is well under control,
when attempting to reduce clearances.

Regards,

Gary Ferrari
Manager Technical Support
Tech Circuits
[log in to unmask]






At 08:16 AM 4/19/96 CST, [log in to unmask] wrote:
>     HI,
>         YOU HAVE NOT STATED WETHER THIS IS A SMT OR THROUGH HOLE 
>     COMPONENT, PRIMARY OR SECONDARY SIDE OR THE SOLDERMASK THICKNESS.  
>     THAT HAVING BEEN SAID A GOOD RULE OF THUMB WE'VE BEEN USING FOR YEARS 
>     IS NO LESS THAN .025'.  THIS ALLOWS FOR A .005" ANNULAR SOLDER MASK 
>     CLEARANCE AROUND THE PAD LEAVING .015" SOLDER MASK  DAM BETWEEN PADS.
>     DEPENDING ON THE TYPE AND THICKNESS OF YOUR SOLDER MASK YOU CAN GO 
>     SMALLER. SOME TIMES GOING SMALLER CAUSES THE SOLDER MASK TO DRAW AWAY 
>     FROM ITS SELF, LEAVING A DASHED PATTERN OF SOLDER MASK BETWEEN THE 
>     PADS.  THERE IS AN IPC SPEC ON SOLDER MASKS OUT THERE, BUT I DON'T 
>     HAVE IT READY TO HAND.  GARY, CAN YOU HELP OUT HERE?
>     
>     REGARDS,
>     
>     ROB BUTTERWORTH
>     SR PWB DESIGNER
>     ADC VIDEO SYSTEMS
>     999 RESEARCH PARKWAY
>     MERIDEN, CT. 06455
>     [log in to unmask]
>
>
>______________________________ Reply Separator
_________________________________
>Subject: FAB:ASSY:pad spacing
>Author:  [log in to unmask] at internet-mail
>Date:    4/18/96 10:26 PM
>
>
>     
>     Typically, what is the minimum spacing between printed circuit solder 
>     pads that would allow for enough LPI soldermask to prevent bridging 
>     for a wave soldering process ?
>     
>     Is there a design guideline that specifically addresses this ? If so, 
>     can you point me in the right direction.
>     
>     Any assistance would be greatly appreciated.
>     
>     Thanks
>     
>
>