Another try, hopefully this time with the text itself.
         
              We will be building a product for a customer that has three 
         devices that need protecting from electrical access for security 
         reasons. The customer wants a coating over the leads that will 
         prevent people from probing them to find electronic access codes 
         used in the system. Obviously this can only be a deterrent, as 
         someone sufficiently determined could get past any coating or 
         barrier. 
         
              The coating needs to cover the leads only, so potting the 
         entire assembly isn't on. It needs to be a fast in line process. I 
         would like to limit any off line processing, like oven curing, to 
         under one hour. The cured material needs to be resistant to 
         soldering iron heat, the sort of solvents and acids that the PCB 
         will withstand and the sort of mechanical attack that won't wreck 
         the leads. 
         
              After discussion with material suppliers I can see two ways 
         of doing this. One is a filled rigid epoxy material of the type 
         used for glob topping bare die, that will encapsulate the leads. 
         This will be very resistant to attempts to remove it. The Cte will 
         be closely matched to the package, lead and board materials, but 
         the elastic modulus will be high. The other group of materials are 
         of lower modulus. These however tend to be easier to remove. 
         Processing will probably be dispensing from a barrel via an X,Y,Z 
         dispensing system. Curing will either be heat, or UV. 
         
              My principal concern is compromising the fatigue life of the 
         solder joints by having a high modulus encapsulation material that 
         constrains the movement of the gull wing leads and puts more 
         stress on the solder joints during temperature changes. The 
         product is a relatively high value consumer item for use in the 
         home. Has anyone done something similar? What materials did you 
         use? What were the results? What was the processing like? I will 
         be doing some reliability tests, but the time constraints mean 
         that the material choice needs to be right first time. 
         
              One device is a 28mm body 0.5mm pitch PQFP. The other two 
         devices are smaller in size. They will be conventionally attached 
         to an FR4 board.
         
         Jeremy Drake.  
         Design to Distribution Ltd. Kidsgrove, Stoke on Trent, England.
         
         +44 (0)1782 771000.
         [log in to unmask] 

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