You got it Dewey, shorts that give you wedgies when you walk. -----Original Message----- From: TechNet [mailto:[log in to unmask]] On Behalf Of Whittaker, Dewey (EHCOE) Sent: Friday, February 16, 2007 10:03 AM To: [log in to unmask] Subject: Re: [TN] LPI soldermask resolution What are cross shorts? Ones that give you wedge voids or base layer exposure? Dewey -----Original Message----- From: TechNet [mailto:[log in to unmask]] On Behalf Of Lee parker Sent: Friday, February 16, 2007 8:57 AM To: [log in to unmask] Subject: Re: [TN] LPI soldermask resolution Dave Back in the good old days when 40 mil pitch was all the rage, Bell Laboratories carried out several studies to ascertain the benefit of soldermask dams. It was found that the dams cut cross shorts between pads by several factors. Best regards Lee J. Lee Parker, Ph.D. JLP Consultants LLC 804 779 3389 ----- Original Message ----- From: "Steve Gregory" <[log in to unmask]> To: <[log in to unmask]> Sent: Friday, February 16, 2007 10:36 AM Subject: Re: [TN] LPI soldermask resolution Hi David, It's been my experience that having mask between the pads does help to prevent solder bridges. I think it helps the solder to coalesce back to each pad when it is liquidous during reflow, and helps with gasketing of the stencil during solder paste printing. That doesn't mean that it's absolutely necessary, but it does help in my opinion. That's why the term "solder mask dam" is sometimes used to describe the strip of mask between the pads. If an LPI mask is used, I would prefer to see mask between the pads... -Steve Gregory- -----Original Message----- From: TechNet [mailto:[log in to unmask]] On Behalf Of David Greig Sent: Friday, February 16, 2007 9:17 AM To: [log in to unmask] Subject: Re: [TN] LPI soldermask resolution Hi list, I want to ask what may well be a silly question - Why is having a strip of soldermask between the pads of fine pitch (~20mil) IC's so important? Best Regards David Greig ______________________________ GigaDyne Ltd 5 Albany Business Centre Gardeners Street Dunfermline KY12 0RN United Kingdom t: +44 (0)1383 624 975 www.gigadyne.co.uk ______________________________ -----Original Message----- From: TechNet [mailto:[log in to unmask]] On Behalf Of Phillip Bavaro Sent: 16 February 2007 00:45 To: [log in to unmask] Subject: Re: [TN] LPI soldermask resolution Hi Steve, I am accustomed to thinking of LPI soldermask as a +/- .003" material....and for that reason we try to discourage designers from trying to have dams less than .006" wide. Having said that, I will admit that it is my preference is to not use soldermask on certain 20 mil pitch components such as BCC48, yet we do use soldermask on BGAs at the same pitch. I will assume you are talking about stripes (dams) between the pads for a gull wing QFP leads, and yes, the resolution of LPI allows any good pwb fabricator to put a 6 mil wide or larger dam between the pads. Now if your pads are equal or greater than 14 mils, then the yield at the fabricators will be impacted. Hope this helps, Phil 12/2007 -0600, Steve Gregory wrote: >Mornin' All! > >What kind of resolution should a LPI be able to give you? Specifically, >LPI's should be able to give you a solder mask dam between 20-mil pitch >leads if you have 8-mils of space between the pads shouldn't it? > >Thanks! > >-Steve Gregory- > >--------------------------------------------------- >Technet Mail List provided as a service by IPC using LISTSERV 1.8e To >unsubscribe, send a message to [log in to unmask] with following text in >the BODY (NOT the subject field): SIGNOFF Technet To temporarily halt >or (re-start) delivery of Technet send e-mail to >[log in to unmask]: SET Technet NOMAIL or (MAIL) To receive ONE mailing >per day of all the posts: send e-mail to >[log in to unmask]: SET Technet Digest >Search the archives of previous posts at: >http://listserv.ipc.org/archives Please visit IPC web site >http://www.ipc.org/contentpage.asp?Pageid=4.3.16 >for additional information, or contact Keach Sasamori at [log in to unmask] >or 847-615-7100 ext.2815 >----------------------------------------------------- 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