Knowing your PCB assembly process

From the Circuit Mechanix magazine:

Circuit Mechanix Mar 2016 - Cover

Knowing your PCB assembly process

It’s always a time of mixed emotions when a new batch of PCB’s arrives on your desk, fresh from the subcontractor.
Excitement of testing the design that’s had time, blood, sweat and tears put into it. Hoping they work after all the
time and money that’s been spent on them. Knowing the assembly process can give a designer or engineer greater
confidence in the finished boards. Different aspects that can be considered are:

  • Paste Screens – How the paste apertures are defined in a design can be important to the assembler. Find out
    how the paste apertures are used in the solder screens. What often happens is the paste aperture in the gerber files for a PCB will be a 1:1 representation of the pad and this is used by a separate stencil fabricator to make the paste screens. Check what a subcontractor does though, an incorrect assumption
    could lead to the incorrect quantity of paste being applied.
  • Handling – How well setup is your  subcontractor to handle ESD (Electro Static Discharge) and moisture sensitive components? Are there equipment and controls in place to handle them? Controlling static is often regarded as unnecessary, however it is a real risk to  components. The same applies to moisture sensitivity, which components are rated as moisture sensitive in the design? What impact will this have?
  • Reflow – What reflow process does your subcontractor use as standard? Normally this doesn’t matter, however some components can be damaged by vapour phase reflow processes. Not only is a good knowledge of the assembly process required but an intimate search of component datasheets is sometimes necessary. Checking individual assembly
    requirements of the components is often hard as the information that’s important is buried deep in the datasheet on page 12 in small text which isn’t highlighted.
  • Cleaning – What kind of cleaning process does your subcontractor use as standard? How a PCB is cleaned can again affect components, check your datasheets. It’s also not a great idea to clean components like transformers as drying them can be hard and will have a
    negative effect on safety clearances (if it’s a safety / isolating transformer). With the cleaning process there should be a drying process as well, making sure it’s sufficient to completely dry out the board this is especially needed if using Rogers materials.
  • Inspection – AOI (Automatic Optical Inspection) is employed by most medium to large sub assembly companies. Check and make sure this is the case, do they inspect a sample or each PCB? Finding and curing a short or dry joint you can’t see is difficult to say the least
    and many component packages have connections underneath the component body. Your subcontractor should be able to check this with some kind of xray or specialist visual equipment. Depending on the product type and the PCB being made there could be more
    that needs to be known.

The point is – know your subcontractor, know what they’re doing and as much as possible
tell them what is needed.
Circuit Mechanix © 2016

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