|Ask the Experts|
January 30, 2012
Soldering Wires to SMT Pads
We are lap soldering connector wiresfrom a multipin connector to the surface of a board. I am looking for therecommended pad size for any given wire diameter. Is there a rule of thumb for pad size such as 2x the bare wire diameter? Is there an applicableIPC guide or specification?
|Expert Panel Responses|
The pad, trace, and the wire size are determined by thecurrent-carrying capacity required of the circuit. This is determined by thedesign engineer if the pads and wires are going to be designed-in, and thesecurrent-carrying capacities are spelled out in IPC 2152 of the designstandards. To determine the correct WIRE size for a modification or repair(such as a jumper), IPC 7711/7721 Repair Method #4.2.5 provides a table (Table1) to determine appropriate jumper wire size to use based on the existingtrace or pad, in order to (again) provide ample current-carrying capacity. Inyour case I believe you need to determine the optimum pad size for a given wirespecification such as the connector lead, for this you can use the table in IPC7711/7721 in reverse. The goal here is to provide the same cross-sectional areabetween the original pad/trace and the wire, so a rule of thumb ofapproximately 2X to 3x the wire diameter is probably going to work. However, you need toconsult with the design engineer before attempting such a modification becausethe pad width is critical to other electrical functions such as impedancecontrol, EMF, crosstalk, Hall effect (skin effect in very high frequency circuits),and minimum conductor spacing, for example. Before making such a modificationon many assemblies, I would qualify the pad/jumper on one or two testassemblies and validate it thoroughly through qualification. This should bedone with customer approval.
IfI understood you are about to solder wire of specified AWG from a connector pinto other location on Board on a pad. If soldering directly on a Pad isnot recomendable as solder joint reliability. Pad should be a through-hole PTH.Now what should be PTH dimensions can be derived by IPC7351 land-patterncalculator considering under option hole size calculation give data of wireawg and choose IPC nominal-B option will give Pad dimensions. Possibly IPC7351now is not a free software after acquired by mentor graphics but you can referStd copy also but time consuming.
Supplier Quality Leader
We have a library part for #22 AWG that is .040 X .060. Our requirements are .050 minimum solder joint length with avisible fillet on both sides.
Lockheed Martin Space Systems
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