Design Specs for Selective Soldering

Is there a PCB design spec to help with solderability during the selective soldering process? We have focused on the keep-out area around the leads but I would like to know if there are any other best practices?

J.S.

Expert Panel Responses

In an ideal world to keep the Selective area to be soldered close together so the jet and nozzle isn’t flying all over the place which would save time which in turn doesn’t stress the solder flux too much. Also spray flux the PCB a little bit before the component and after the component so the wave stays wetted with flux and isn’t offered a Dry PCB. Too often we see drop jet flux going up into the hole so the bottom surface isn’t actually wetted but the top side components are covered.

Greg York
Technical Sales Manager
BLT Circuit Services Ltd

Greg York has over thirty two years of service in Electronics industry. York has installed over 600 Lead Free Lines in Europe with Solder and flux systems as well as Technical Support on SMT lines and trouble shooting.

If you have got the clearances around the leads you’ve covered the most common problem. Good panelization will make it easier to process and help with throughput. A common issue that applies to wave or selective soldering would be the connections to large copper planes. If a PTH is connected to a plane using “spokes” instead of just a drill through the plane it will allow better hole fill due to the reduction in thermal mass.

Kevin Mobley
PCBA Engineering Liaison
General Atomics Electromagnetic Systems Group

Kevin has over 30 years of experience in process and manufacturing engineering serving in both EMS and OEM companies. Expertise includes all aspects of SMT as well as wave solder and CCA materials such as PCBs, solder material, and component finishes. Kevin has developed processes for thousands of assemblies from stencil printing to conformal coating and testing.

That depends on the component(s) being soldered. For bigger PTH components with bigger pins, we use bigger nozzle which requires larger keep out area.

Amit Bahl
Director of Sales and Marketing
Sierra Circuits

Amit Bahl started to work at Sierra Circuits in 2006 where he formed strong relationships with his customers working with them on flex PCBs, HDI, controlled impedance, etc. In 2009, he was promoted Director of Sales and Marketing.

Design to ensure flatness / rigidity – this is for boards and panels (arrays of boards) – if the panel is not flat, you will not have soldering consistency. TH parts holes design: lead / hole ratio and lead type vs hole type (shape) to allow solder flow from to the top of the boards (vertical fill per IPC requirements).

Nozzles sizes vs soldering adjacent areas – to avoid collision of the machine with the parts. Shadowing that might prevent flux to be sprayed on the targeted locations.

Georgian Simion
Engineering and Operations Management
Independent Consultant

Georgian Simion is an independent consultant with 20+ years in electronics manufacturing engineering and operations.
Contact me at georgiansimion@yahoo.com.

The experts have given you some good information, but nobody addressed the original request, which design standards have this info? IPC-2221 is the first place you should look, as it is a general design standard for printed wiring boards and includes manufacturing guidelines for PTH layout to address your accessibility concerns for selective soldering processes.

It also has all of the standard SMT layout info, panelization methods, and everything else. IPC-2222, 2223, and 2226 have the same type of information for flex, rigid-flex, and high-density layout information. But the best reference is a free download packed with really good design data, it is the new Design Desk Reference IPC-DR-DES-2022. I use this almost daily for my PWB design and chip layout work and I can tell you no designer should be without it.

You can download it free to a single PC, or you can purchase a hard copy for around $75 from IPC.org/standards/design standards. Between 2221 and the desk reference you are in good shape. There are also free downloads from the CAD companies so you should refer to your CAD supplier, they can provide a wealth of good info also. Good luck!

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.

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