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January 30, 2012

Soldering Wires to SMT Pads

We are lap soldering connector wires from a multipin connector to the surface of a board. I am looking for the recommended 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 applicable IPC guide or specification?


Experts Comments

The pad, trace, and the wire size are determined by the current-carrying capacity required of the circuit. This is determined by the design engineer if the pads and wires are going to be designed-in, and these current-carrying capacities are spelled out in IPC 2152 of the design standards.

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 (Table 1) to determine appropriate jumper wire size to use based on the existing trace or pad, in order to (again) provide ample current-carrying capacity. In your case I believe you need to determine the optimum pad size for a given wire specification such as the connector lead, for this you can use the table in IPC 7711/7721 in reverse. The goal here is to provide the same cross-sectional area between the original pad/trace and the wire, so a rule of thumb of approximately 2X to 3x the wire diameter is probably going to work.

However, you need to consult with the design engineer before attempting such a modification because the pad width is critical to other electrical functions such as impedance control, EMF, crosstalk, Hall effect (skin effect in very high frequency circuits), and minimum conductor spacing, for example. Before making such a modification on many assemblies, I would qualify the pad/jumper on one or two test assemblies and validate it thoroughly through qualification. This should be done with customer approval.
Richard D. Stadem
Advanced Engineer/Scientist
General Dynamics
Richard D. Stadem is an advanced engineer/scientist for General Dynamics and is also a consulting engineer for other companies. He has 38 years of engineering experience having worked for Honeywell, ADC, Pemstar (now Benchmark), Analog Technologies, and General Dynamics.
If I understood you are about to solder wire of specified AWG from a connector pin to other location on Board on a pad. If soldering directly on a Pad is not recomendable as solder joint reliability. Pad should be a through-hole PTH. Now what should be PTH dimensions can be derived by IPC7351 land-pattern calculator considering under option hole size calculation give data of wire awg and choose IPC nominal-B option will give Pad dimensions. Possibly IPC7351 now is not a free software after acquired by mentor graphics but you can refer Std copy also but time consuming.
Subrat Prajapati
Supplier Quality Leader
Ge Healthcare
Subrat has 10 year of extensive experience in PCB assembly process optimizing for quality, process includes screen printing, wave, reflow. He has a copyright in stencil design published in Apex Expo2010 at Las Vegas US.
We have a library part for #22 AWG that is .040 X .060.  Our requirements are .050 minimum solder joint length with a visible fillet on both sides.
Mike Green
Design Engineering
Lockheed Martin Space Systems
Mike Green is co-chairman of the IPC Terms and Definitions Committee. He has been working with board design and manufacturing for 33 years.
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