With quality SnPb Solder Leveling I would not expect the maximum thickness of the solder at the crest to exceed 22 microns, with a mean of 14 microns. So unless your apertures are significantly smaller than the pad I don't see how the stencil would contact the solder. But I take you point with small isolated features without a corresponding aperture and thin masks; not least because small features can electroplate thicker. We have measured fidutial thicknesses BEFORE leveling 20 Microns thicker than the earth plain areas; so presumably you can have gasketing problems with non fused coating and thin masks too? Is it practical to etch apertures part way into the stencil to provide clearance around fidutials and like features? But it's a different story with LEAD FREE Solder Leveling where minimum deposits are typically double that of SnPb and the solder crest is significantly lower. We are routinely measuring 0.010 X 0.060ins. QFP foot prints with a maximum crest height of 10-11Microns with a mean of 5-6. Interestingly BGA pad's come out thicker than you would expect; 0.18ins Dia. typically measuring 16-17Microns at the crest with a mean of 9-10. -----Original Message----- From: Stadem, Richard [mailto:[log in to unmask]] Sent: 14 October 2005 06:31 PM To: [log in to unmask] Subject: RE: [TN] Hasl When I say gasket, I mean it as a verb, not a noun. What I mean to say is that, yes, the HASL buildup on the vias and other plated-through holes does rise higher than the solder mask in certain designs, thus preventing the stencil from coming down flat to the board everywhere on the circuit board's surface. So if you have via holes and through holes next to SMT pads, there is a small gap between the stencil and the board that causes the solder paste to squeeze out. Also, the stencil apertures are typically a few thousands smaller than the pad features. If the pads have solder buildup due to HASL, there again that HASL solder prevents the stencil from sealing flat to the board. Many people are not aware of this, and they struggle with the paste, the print pressure, the squeegee speed, etc. trying to obtain a crisp clean brick, but they do not realize the problem. This is no longer as much of a problem as it used to be, because board designs are changing to keep the solder features below the surface of the mask height. But it is still a problem when trying to place very fine pitch parts precisely onto pads less than .020" pitch. -----Original Message----- From: [log in to unmask] [mailto:[log in to unmask]] Sent: Friday, October 14, 2005 11:14 AM To: [log in to unmask]; Stadem, Richard Subject: RE: [TN] Hasl Thank you for the explanation Richard, whenever I have been shown this kind of problem I have found the solder coating to have been out of control, but as we have limited contact with assemblers perhaps you can clear up some confusion for me. You say that solder on via's and other surface features that do not have an associated aperture in the stencil will prevent gasketing. Given that it is unusual to have the solder coating thicker than the solder mask, is this because the copper is generally electroplated thicker on small areas and towards the outer edges of multiple image panels? And could this problem be eliminated by etching part way into the gasket? You then say the aperture is smaller than the pad; are you suggesting that the stencil can be held away from the surface by the thickness of solder this close to the edge of a pad? -----Original Message----- From: Stadem, Richard [mailto:[log in to unmask]] Sent: 11 October 2005 07:03 PM To: [log in to unmask] Subject: Re: [TN] Hasl The idea of the flat surface finish is for both stencil gasketing to the board and for placement accuracy. To form a "perfect brick" of solder paste, there has to be a certain amount of pressure (a lot less than most people think). To prevent squeezout of paste between stencil and board, there has to be good, really good, gasketing or sealing between the stencil and the board surface. The problem with HASL, as Dr. Parker stated below, is that the round bump (meniscus) of HASL sticking up above the surface of the pad on vias and on surfaces where there is no accomodating hole in the stencil prevents this tight seal, and allows the paste to squeeze out. (The aperture design is typically slightly smaller than the pad). It is also a problem when you go back and perform pick-n-place, as now you want to place the SMT components onto your perfectly formed bricks of paste, but with about 50-100 grams of pressure. If the paste is not stable to a flat pad, the parts slide all over the place. While some of this is forgiven by the surface tension of the solder in liquidus, bridges do form, components misalign, etc. This is especially critical in a no-clean process. If large (relatively) amounts of the solder paste are not "tight to the brick" and fully registered on the pad, the paste does not fully agglomerate into a single solder joint. Separated solder paste does not usually conglomerate back into the central solder joint during reflow. Extraneous solder balls form. This is very bad news if you did not plan on washing them away. -----Original Message----- From: TechNet [mailto:[log in to unmask]] On Behalf Of Lee parker Sent: Tuesday, October 11, 2005 12:03 PM To: [log in to unmask] Subject: Re: [TN] Hasl Ramon After reflowing a soldered surface, you will form a meniscus which will interfere with SMT component placement. To avoid the problem I suggest a flat metal finish such as immersion silver or flash gold (not ENIG) Best regards Lee J. Lee Parker, Ph.D. JLP Consultants LLC 804 779 3389 ----- Original Message ----- From: Dehoyos, Ramon<mailto:[log in to unmask]> To: [log in to unmask]<mailto:[log in to unmask]> Sent: Tuesday, October 11, 2005 12:52 PM Subject: [TN] Hasl Hi technetters: Is there a fine pitch limit to the use of HASL board plating finish due to its variation in thickness? Does selective solder strip and reflow plating, SSSR, have a significant advantage over HASL? Any comments relating to platting finish would be greatly appreciated. I thank you for your comments. Ramon ME 410.552.2210 --------------------------------------------------- Technet Mail List provided as a service by IPC using LISTSERV 1.8e To unsubscribe, send a message to [log in to unmask]<mailto:[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]<mailto:[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]<mailto:[log in to unmask]>: SET Technet Digest Search the archives of previous posts at: http://listserv.ipc.org/archives<http://listserv.ipc.org/archives> Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16<http://www.ipc.org/cont entpage.asp?Pageid=4.3.16> for additional information, or contact Keach Sasamori at [log in to unmask]<mailto:[log in to unmask]> or 847-615-7100 ext.2815 ----------------------------------------------------- --------------------------------------------------- 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 ----------------------------------------------------- --------------------------------------------------- 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 ----------------------------------------------------- --------------------------------------------------- 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 -----------------------------------------------------