Alvin; There is no formula since the stencil opening depends on pad size at PCB, as rule of thumb they recommend reduce 10% from pad width to give a good gasketing. Stencil thickness generally is 6 mils, laser cut, trapezoidal shape with eletropolish finish.You can use eletroformed stencil as well, but NEVER use chemical etching type for fine pitch. The big problem to fine pitch apertures is the fact that when you reduces stencil opening the area formed by the four sides of the stencil wall become greater than the area of the pad and there is a limit that the solder paste stay at stencil instead of release to the pad. Some people also recomend the aperture be enough to accomodate 5 solder sphere in line, this way you can define the solder type to be used. I have been using type 3 solder paste for 20 mils without problem. OSP finish is better to print due to planarity of the pad, some bad HASL finish can cause bridging due to solder paste accumulation under stencil during print cycle, if you have automatic stencil wiper just increase the cleaning rate. Related with the process I always worked with No-clean process and has no argument to compare with Clean process. Thanx Jorge Dourado de Santana Maintenance / Process Engr Microtec - Brazil > -----Original Message----- > From: Alvin Leong [SMTP:[log in to unmask]] > Sent: 10 de Agosto de 1999 07:43 > To: [log in to unmask] > Subject: [TN] Stencil aperture opening > > Hi Technetters, > I've a question of what is the formulae that used to calculate for > the > stencil aperture opening on the 20 mils pitch IC? This question is more on > the amount of solder volume that sufficiently to meet the strength as well > as the reliability. Is there any different of the clean vs no-clean > process? HASL finishing vs Entek (OSP)? > Hope to some words for reference. > > Thanks in advance, > Alvin > > ############################################################## > TechNet Mail List provided as a free service by IPC using LISTSERV 1.8c > ############################################################## > To subscribe/unsubscribe, send a message to [log in to unmask] with > following text in > the body: > To subscribe: SUBSCRIBE TECHNET <your full name> > To unsubscribe: SIGNOFF TECHNET > ############################################################## > Please visit IPC web site (http://www.ipc.org/html/forum.htm) for > additional > information. > If you need assistance - contact Gayatri Sardeshpande at [log in to unmask] or > 847-509-9700 ext.5365 > ############################################################## ############################################################## TechNet Mail List provided as a free service by IPC using LISTSERV 1.8c ############################################################## To subscribe/unsubscribe, send a message to [log in to unmask] with following text in the body: To subscribe: SUBSCRIBE TECHNET <your full name> To unsubscribe: SIGNOFF TECHNET ############################################################## Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional information. If you need assistance - contact Gayatri Sardeshpande at [log in to unmask] or 847-509-9700 ext.5365 ##############################################################