Hi Jack, It sounds to me if you want repeatable results with stacking heights you would have to fixture the parts when they are soldered wouldn't you? Besides... if they are concerned about the solder thickness, the tolerances must be awfully tight... that would make anybody a little nervous about it. A good tolerance study would show the variances in the extremes... then you would need to compensate for that in the design or use a through hole connector... Obviously, I am guessing because I have not seen the design. The surface mount part will float in the x,y and z to some extent... I have not seen any studies done, but it could probably be modeled if you knew the viscosity of the molten solder it's worst case quantity on the worst case pad and the worst case weight of the connector on the worst case board thickness and plating, etc... The float obviously can't be worse than the thickness of the paste... so you could use the max thickness of the solder paste as a worst case height off the surface of the board to the bottom surface of the connector pins even though the part will most likely settle some. I use through hole parts when stacking boards for this very reason... they are more reliably located by the hole pattern and structurally much stronger. In the rare occasion where I was forced to use a surface mount connector, I used one with mounting holes and hardware and some fixturing to make sure the part ended up where it needed to be. Surface mount connectors are good where tolerances are more relaxed and the mating is not critical like say with a cable or something... Wish I could be of more help... I expect all this stuff I said doesn't help at all. But let me know what you decide to do... it's an interesting problem. Best regards, Bill Brooks - KG6VVP PCB Design Engineer , C.I.D.+, C.I.I. Tel: (760)597-1500 Ext 3772 Fax: (760)597-1510 e-mail:[log in to unmask] http://www.dtwc.com http://pcbwizards.com -----Original Message----- From: Jack C. Olson [mailto:[log in to unmask]] Sent: Friday, March 18, 2005 9:09 AM To: [log in to unmask] Subject: [DC] Floating SMT components? I have been asked a question that I've never read anything about before, so I'm hoping someone will know this... We have a mechanical engineer who is trying to predict the distance between boards after soldering SMT board-to-board connectors. We know the connector mating height, but the question is: "If the connector is sitting in solder-paste, how high will it stay above the surface of the board?" Common sense tells me the connector will not be flush to the surface, there will be some solder under there, and I think we are using 6mil paste screens. Has anyone had to document something like this before? Jack ---------------------------------------------------------------------------- ----- DesignerCouncil Mail List provided as a free service by IPC using LISTSERV 1.8d To unsubscribe, send a message to [log in to unmask] with following text in the BODY (NOT the subject field): SIGNOFF DesignerCouncil. To temporarily stop/(restart) delivery of DesignerCouncil send: SET DesignerCouncil NOMAIL/(MAIL) Search previous postings at: www.ipc.org > On-Line Resources & Databases > E-mail 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 ---------------------------------------------------------------------------- ----- --------------------------------------------------------------------------------- DesignerCouncil Mail List provided as a free service by IPC using LISTSERV 1.8d To unsubscribe, send a message to [log in to unmask] with following text in the BODY (NOT the subject field): SIGNOFF DesignerCouncil. To temporarily stop/(restart) delivery of DesignerCouncil send: SET DesignerCouncil NOMAIL/(MAIL) Search previous postings at: www.ipc.org > On-Line Resources & Databases > E-mail 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 ---------------------------------------------------------------------------------