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Date: | Fri, 18 Feb 2000 16:38:24 EST |
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In a message dated 02/18/2000 1:14:55 PM Central Standard Time,
[log in to unmask] writes:
<< We have a sole source for TSOP components, in which, they are only
available with alloy 42 leads. The vendor refuses to build the parts with
Copper leads because the CTE of alloy 42 better matches the die CTE and as
result they have fewer component failures and defects.
Does any one know of a PWB material that would better match the CTE of alloy
42 leaded TSOPs? >>
Ahh that's the rub ain't it? The component vendors dictate what materials
they use to improve their numbers, it's up to us assemblers figure out how to
deal with it...just like Texas Instruments palladium plating. I understand
that when they switched from tin/lead plating to palladium their yeild
increased dramatically...
Well seeing how alloy-42 CTE is down around 5.5ppm /° C., there's a few
exotic laminates that come a little closer than FR4 to matching that CTE, but
I said just closer. I got this outta my Printed Circuits Handbook:
Laminate Type(Fabric/Resin) CTE in X/Y axis
Aramid/Epoxy 6.5 - 7.5ppm /° C.
Thermount/Polyimide 6 - 10ppm /° C.
Thermount/Epoxy 7 - 9ppm /° C.
S-2 Glass/Cyanate Ester 7 - 10ppm /° C.
Quartz/Epoxy or Polyimide 8 - 11ppm /° C.
Also in the book there is this statement; "While adequate solder joint
reliability can be achieved with TSOP's in many situations, some vendors have
opted to encapsulate the solder joints with a filled epoxy to better
distribute the stresses."
I have never gone to that extreme, and have used TSOP's in quite a few
assemblies.
Worked at a memory company where we built literally thousands of PCMCIA Flash
cards using TSOP's.
There is also the option of turning the TSOP into a MicroBGA if it's Flash
Memory. A lot of memory component vendors are going that route, and
supposedly a MicroBGA is more reliable than a TSOP...smaller too.
-Steve Gregory-
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