Lance,
All of our assembly vendors prefer/require a solder mask dam between leads to improve manufacturability, something about the mask helping prevent solder balls from bridging across gaps between pads. Many pcb mfgrs prefer a solder mask dam of 0.1mm min, and will work with a 0.05mm clearance around pads. Some are able to make dams as narrow as 0.075 mm, and may be able achieve less clearance to the pad. Sometimes it may be necessary to make the copper land a little narrower to make this work. Check with your board fabricator. We have been using TSOP-I packages for years with no significant problems, once the assembler had the pcb house put mask between the pads.
The fellow I work for on the night shift (read, at home) always requires mask between pads at the request of his board assembler.
If your P&P machine has a placement accuracy of +/-0.1mm or better, placement should not be a problem. Besides, at times (typically?)parts will self-align during the reflow process, if they are light enough to "ride" the solder "wave". You may also want to check the TechNet archives; it contains a lot of good solid advice from various industry gurus.
Hey, it's after 5 and I am outta here!
Have a good one all!
Roger M. Stoops, CID+
Trimble Navigation Ltd., Dayton, OH, USA
Ph: +01 937.245.5288
Fax: +01 937.233.7511
-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Lance Mayes
Sent: Friday, May 14, 2004 4:40 PM
To: [log in to unmask]
Subject: [TN] Solder Masking SMT
Importance: High
Something for the weekend.
We have a SMT prototype board in the works and around the leads for the
IC's there is no mask. Is there a standard that states whether mask is
required or not? These are fine pitch leads also. Another thing is how
much of pad clearance should there be on either side of the lead? As I
see now there appears to be less than .001 on either side of the leads.
This doesn't allow for any variance of component placement with our pick
and place machine. Recommendations, suggestions and or directions please.
Anything to help prevent changing a process further down the road, rev
changes are really cost eaters.
Thanks Net Buddies and Buddettes
P. Lance Mayes
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