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Conductive Adhesive Tape Problems

Some 40 years ago, I was working for a well known electronics company in Colorado.  I was working on a new oscilloscope model, which had just been released from engineering for mass production.  The scope had a number of circuit boards.  Most used edge connectors similar to the photo below.  These gold plated contacts on the edge of the circuit board slid into a matching connector.
   
       

During initial oscilloscope production, we discovered some erratic operation in some of the vertical Amplifier channels.  The symptoms were not consistent but they always seemed to effect the Amplitude calibration of the vertical Amps. Sometimes the DC offset was way too high.  Other times the Amplifier gain was way too low, indicating that the signal was being attenuated.  We could solve the problem by swapping out one vertical Amp card for another, so we were pretty sure that the defect was on the card.  But, all too often, the card, which would not work in one machine, would work fine in another.  We were really scratching our heads on this one.  I decided to go back through the production line process and see if I could spot something.  Nothing seemed to be amiss. This new oscilloscope was being made in much the same way as other models, using the same procedures and materials.  I did notice that a protective tape was being used to cover the gold edge contacts during the board assembly and cleaning process and was only removed when the board was ready to be installed in the scope.  Looking at the oscilloscope schematic I did notice that the vertical Amplifier channels were routed through the connector and they did have a very high impedance.  After close inspection of the contacts under a microscope, I could just make out some residue left by the adhesive used on the protective tape, covering the contacts.  The company had been using this stuff for years without any problem but perhaps, just perhaps, this was the first time that such sensitive circuits were being routed through the edge connectors.  I took a board which exhibited problems and carefully cleaned the edge contacts with an aggressive solvent, followed by a deionized water rinse and a hot air dryer. 

Bingo! The board worked perfectly.  We sent samples of the adhesive to the manufacturing engineering group for more testing.  Their tests did indeed indicate that the adhesive was partly conductive and did sometimes did leave a residue on the board when the tape was removed.  They initially kept using the same protective tape but added a step in the assembly process, which required cleaning off the contacts.  Later, they moved to a better contact protection tape, which was not conductive.

The lesson here was that when dealing with very high impedances, even something like adhesive tape needs to be chosen carefully.

 

Protective Connector Tape


November 2010     Issue 13

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