Ask the Experts
April 17, 2024 - Updated
March 6, 2018 - Originally Posted

Inspection for Hidden Solder Joints

We are integrators working on automating the assembly for a high-volume medical device that requires multiple boards to be soldered together in a complex mechanical arrangement. There are only 50 through-hole solder joints to be inspected, but if we wait to the end of the soldering process, many joints will be hidden from visual inspection. We would like to do the inspection in-process.

Removing the assembly from the fixturing to send it to a standard vision inspection cell for inspection multiple times in the process is going to very difficult. Ideally, we would like to integrate the solder joint inspection into the soldering automation cell. What do you suggest?


Expert Panel Responses

More details are required to respond to this question. What is the soldering process? Hand soldering, Selective, wave or robotic soldering? My assumption is each board is processed as a single item than the assembly is put together.

Michael Kaminsky
Sr Field Applications Support Engineer
Kester Inc.
Mr. Kaminsky has 30+ years of circuit board soldering assembly experience along with a patent for wave solder VOC flux process.

I think the best option in this case is X-RAY. First, determine your soldering process capability (Cp & CpK) for the assembly using X-RAY as your inspection tool.

This will give you an idea of how good your process is achieving Type 1 or Type 2 barrel fill specifications especially the hidden joints. Once this is determined, you optimize the process and it all comes down to do a sample check depending on the volume you run.

Edithel Marietti
Senior Manufacturing Engineer
Northrop Grumman
Edithel is a chemical engineer with 20 year experience in manufacturing & process development for electronic contract manufacturers in US as well as some major OEM's. Involved in SMT, Reflow, Wave and other assembly operations entailing conformal coating and robotics.

I would suggest a custom camera inspection with lighting on top side and bottom.

Terry Munson
President/Senior Technical Consultant
Mr. Munson, President and Founder of Foresite, has extensive electronics industry experience applying Ion Chromatography analytical techniques to a wide spectrum of manufacturing applications.

Great question. You indicate a complex mechanical arrangement with 50 through-hole solder joints to be inspected. Your concern is if you wait to the end of the soldering/assembly process, many joints will be hidden from visual inspection, hence requesting upstream in-process inspection strategy. My advice is an in-line Automated X-ray Inspection(AXI). This could be performed before removing the assembly from the fixture to verify effectiveness.

You might test 2D vs. 3D computed tomography (CT) depending on the system capability to image defects in joints after assembly. You may indirectly calibrate the x-ray to subsequent standard vision inspection cell for inspection defects for accuracy and efficiency. Your quest for an ideal combined solder automation cell with either AOI and/or AXI is to be commended unfortunately I am not sure this is feasible or cost justified?

Mark Northrup
Mark has over 35 years of experience in electronics fabrication, quality, and reliability while working for Raytheon RMD, IEC Electronics, GE, Motorola, ORS, etc. He has most recently taken the role as a Fellow at Raytheon in Tucson, AZ; prior to that, he established IEC Electronics Analysis and Testing Laboratories (IATL), LLC in Albuquerque, NM, for electronics and material analysis testing in the military, medical, and industrial industries. His expertise includes PCB, PCBA, components, and analytical and electrical analysis techniques.

X-ray is one of the better ways to inspect thru-hole solder joints. Using an off-axis view, one can identify hidden voids and quantify percent barrel fill. Complex assemblies may require multiple views at different angles to avoid features in the assembly that would otherwise obstruct the desire view.

Incorporating x-ray into a soldering cell will be straight forward if you are considering a cell made up of a group of systems. Incorporating x-ray into a soldering system will be more of a challenge. Doing so will require shielding and safety interlocks to prevent potential radiation exposure to those working near the cell. However, this is possible. X-ray has often been a feature included with PCB drilling equipment.

Don Naugler
General Manager
VJ Technologies, Inc.
Don is the General Manager of VJ Technologies, Inc., a leading manufacturer of X-ray Inspection and Rework equipment for the electronics manufacturing industry. He has more than 20 years experience in development, manufacturing, and support of a wide range of capital equipment.
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