April 2, 2011 Thanks. The volume is low so step 2 may be viable option. For next production the OEM will fix the design. -----Original Message----- From: TechNet [mailto:[log in to unmask]] On Behalf Of Graham Collins Sent: Friday, April 01, 2011 2:54 PM To: [log in to unmask] Subject: Re: [TN] Soldering two SMT resistors to lands originally designed for only one resistor; is it technically OK? Hi Gerald What kind of build volume are you talking about? #2 below is easiest if the volume is low. Were it my issue I would look at two possibilities: 1) modify the board, attach bigger pads. or 2) Solder one part on the pads as per standard. Glue the second part on nearby and run jumper wires to connect it. regards, - Graham -----Original Message----- From: TechNet [mailto:[log in to unmask]] On Behalf Of Bogert, Gerald L (Contractor) Sent: Thursday, March 31, 2011 7:29 AM To: [log in to unmask] Subject: [TN] Soldering two SMT resistors to lands originally designed for only one resistor; is it technically OK? March 31, 2011 Folks, we have an OEM who needs to incorporate a design change into an existing design circuit card assembly (CCA) used in a Class 3 military application that will see shock, vibration and high humidity environment. In the existing design, a chip resistor is soldered to land areas with the following dimensions: Width is 0.080 inches length is 0.035 inches. Spacing between lands is 0.040 (makes the overall length 0.110). The OEM needs to remove the existing resistor and solder two new resistors to the existing land areas. The new resistors are M55342K06B4E02R (MIL-PRF-55342/6, 0.15W, 1%, 4.02K). From the military specification, the overall part length is 0.080 inches; the width is 0.050 inches, and the termination width is 0.016 inches. The tolerance on the 0.050 dimension is plus or minus 0.005 inches. From the above information, I calculate that when the two resistors are soldered onto the existing lands, the parts will overhang on both sides of the lands by 0.015 inches (assumes the resistors are spaced 0.010 inches apart to account for the 0.005 inch tolerance on the 0.050 dimension). From IPC-A-610E, paragraph 8.3.2.1,and Table 8-2, the maximum allowable overhang for Class 3 product would be 0.0125 inches, and for Class 2, 0.025 inches. This means that side overhang would not meet Class 3 for Dimension A, but would meet Class 2 requirements. Is this design change technically acceptable for Class 3? Given the above information, is there sufficient solder joint formation (assuming perfect soldering process) to provide the requisite level of reliability? What is the relationship of side joint overhang to reliability? What data exists to support the A Dimension criteria in Table 8-2 of IPC-A-610? The OEM is considering obtaining additional solder joint volume by soldering a jumper wire from the end terminations of each resistor to the land area such that the jumper wire would act as an additional soldered conductor. He is also considering whether the parts should be glued on the sides for support. It appears that it would be acceptable to just solder both resistors as described above, without the need to add the jumper wire or glue, which would mean we would accept the violation of Dimension A for Class 3. Note also that there is no electrical clearance concern using this design fix and there is no known other acceptable design fix; if this fix is not technically acceptable the CCAs would need to be scrapped.. We considered stacking the resistors on top of each other but this was rejected because; 1) not recommended by the military specification; 2) cannot guarantee that the top surface of the terminations are solderable - the mil spec exempts this surface from solderability test verification; and 3) possible heat transfer issues. Any advice folks could provide to the above would be appreciated. ______________________________________________________________________ This email has been scanned by the MessageLabs Email Security System. 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For more information please contact helpdesk at x2960 or [log in to unmask] ______________________________________________________________________ --------------------------------------------------- Technet Mail List provided as a service by IPC using LISTSERV 15.0 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.aspx?Pageid=E-mail-Forums for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815 ----------------------------------------------------- ______________________________________________________________________ This email has been scanned by the MessageLabs Email Security System. For more information please contact helpdesk at x2960 or [log in to unmask] ______________________________________________________________________ --------------------------------------------------- Technet Mail List provided as a service by IPC using LISTSERV 15.0 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.aspx?Pageid=E-mail-Forums for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815 -----------------------------------------------------