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May 28, 2014

What Is Acceptable Bending For Assembled PCBs?

How much bend is acceptable when fitting a PCBA into a box build. We have a PCBA that attaches to a front panel, this then goes into a unit and attaches to a back panel. The problem is that the front panel has a tolerance on the flaps that attach to the back of PCBA and thus this could cause the PCBA to bend when being attached. I've searched for similar issues, but all I can find relates to unpopulated PCBs.

C.F.

Expert Panel Responses

The limits of acceptable bending depend strongly onthe type and size of components mounted. BGAs in particular are sensitive tobending stresses, and the larger the part, the smaller the allowable bendingstress. Given the strong design-dependence of the situation, I'm very reluctantto throw out any numbers, and I would encourage you to talk to someone who canreview the design, model the bending, and give you an opinion.

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Fritz Byle
Process Engineer
Astronautics
Fritz's career in electronics manufacturing has included diverse engineering roles including PWB fabrication, thick film print & fire, SMT and wave/selective solder process engineering, and electronics materials development and marketing. Fritz's educational background is in mechanical engineering with an emphasis on materials science. Design of Experiments (DoE) techniques have been an area of independent study. Fritz has published over a dozen papers at various industry conferences.

It's better practice to avoid any bending on theassembled PCB for two reasons. There has to be some room for coefficient ofthermal expansion in use and secondly to avoid creep loading of solder joints.creep loading i.e relatively small stress on tin-lead solder joints leading tofractured joints over a period of time. this has been a silent killer in manyinstances.

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KN Murli
Head-Quality
Astra Microwave Products, Hyderabad, AP India
Holds Degree in Engineering, started off as Scientist/Engineer in ISRO (Indian Space Research Organization) in Quality Assurance of Space hardware Electronics Production. Worked in the area of Parts, Material and Process; DPA, FA and Process Qualification for space and ground hardware. Later moved into Private sector and worked in the area of Quality Management Systems & ISO 9001 certification. Currently hold a position as Head-Quality in RF/Microwave Product manufacturing for Defense and Aerospace segment.