2:02 PM
  OFFICE MEMO                                                 Time:
                      Subject:
                      ASSY: Saponification shorts                   3/12/97
                                                              Date:


TecNetters:

Information, and a query, regarding CCA shorting anomalies and aqueous cleaning residues.

Our company uses an aqueous cleaner after wave soldering to remove flux residues. It works very well in removing flux, passing the resistivity of solvent extract routinely. However, we have recently encountered situations where the detergent in the aqueous cleaner has been left behind in certain portions of the CCA, causing high (and sometimes not so high) resistance shorts. The regions in which the detergent gets left behind have been:
1. In cavities formed by the presence of mechanically attached heat sinks.
2. In cavities formed by adhesively bonded heat sinks where the adhesive has not created a 100% continuous bond under the heat sink. 
3. In cavities created under chip capacitors, this situation being produced by the presence of an epoxy dam used to prevent solder shorting between electrodes, the dam's design essentially preventing the proper rinsing of the detergent residue.

The cause of the shorting problem has been verified by SEM/EDS (x-ray spectroscopy), which found the detergent's active ingredients in the cavities in each case. It has been further corraborated by electrical testing in dry and humid environments, the shorting being directly related to humidity. Our current approach to work around this problem is to not allow CCA's with entrapment sites to be subjected to aqueous cleaning, the offending parts or assemblies being attached and hand-cleaned with isopropyl alcohol after aqueous cleaning. This requires a visual review of each assembly by manufacturing engineers.

My question: Have others encountered this problem, and what have they done to alleviate it? If you would like to discuss our findings or your experiences, you may contact me directly at: [log in to unmask] 

Thank you. 


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