On 23 January, Ty wrote:

> Need information on highest probable cause of missing smt passive components  
> that are on the solder side of cca's.

> ty


Mornin' Ty!

    Well, that's a pretty w-i-d-e o-p-e-n question ya' got there. There's a 
quite a bunch of different reasons why ya' might be shy a few parts on the 
bottomside.

    It depends on the bottomside process your using, and where you're actually 
losing the parts. I'll tell ya' what I've found as some of the causes when I've 
had problems along those lines running bottomside in the processes I've used...

    
Bottomside Epoxy, then Wave Soldering:

* Amount of epoxy dispensed prior to component placement is insufficient or not 
  accurate. Dot shape/height not enough to contact the bottom of the body of the
  component.

* Design guidelines not followed as far as not putting via's between the pads.  
  The epoxy flows down the via and causes the component to be barely secured.

* Epoxy curing profile not hot or long enough to fully cure the epoxy.

* Rough handling during the PTH insertion step of the process knocking off the 
  parts prior to wave. This has been found to be caused both by auto insertion  
  machines and from the way operators handled the boards during manual stuffing.

* Epoxy past shelf-life.

* Contaminated components and/or PCB.

Double-sided Reflow:

* Reflow profile not optimum. Too hot causing the board to "bow" or "warp" and 
  upset the balance of surface tension and/or wetting forces that are holding 
  the component in place while the solder is liquidous on the bottom. A "jerky" 
  or vibrating conveyer belt in the reflow oven can cause the same thing.

* Contaminated components and/or PCB, solder not wetting properly.



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