jan,
We've used solid solder preform made out of Sn63Pb37 (again, no flux)
beneath the FET to do a similar type of attachment. We had to request a
change in the thickness of gold on the part as the FET manufacturers like
to load up the surface with very thick gold (for mechanical attachment
intent). The substrate was solid copper with a nickel and gold plating,
but the gold was immersion at 8 microinches max. Nitrogen was required due
to the lack of flux, but the gold wet just fine
At 10:01 PM 3/30/00 -0500, Creswick wrote:
>Jan,
>
>Actally, there are many solders within range of termperatures which you
>specify. However, practically, I am not so sure I would recommend any to
>you without knowing the requirements of your system.
>
>You probably already knew that MOSFET manufacturers normally attach their
>devices to copper slugs using high lead alloys such as Sn10Pb88Ag2,
>Sn10Pb90, Sn5/Pb95 or other variations incorporating Indium as well. Check
>data books or web sites for International Rectifier, XYIS, and others. Each
>of these alloys has the benefit of being very ductile, absorbing the thermal
>coefficient of expansion mis-match between the high expansion copper slug
>and the low expansion silicon chip - allowing the device to survive many
>power cycles. On the other hand, most of these alloys melt at 300+°C and
>need to be reflowed in a hydrogen or reducing atmosphere. Thermal
>conductivity is so so, but the joint isn't more than 25-50um thick, so it
>becomes relatively negligible, if it is void free.
>
>By going to any number of SnPb, SnAg, or SnPbAg alloys you will gain the
>decreased liquidus temperature which you desire, but give up much of the
>ductility of the traditional high lead alloys. However, since you are
>currently using AuSn, you don't have a very ductile system in the first
>place - the redeeming grace is that the composite thermal expansion of the
>DBC substrate (~7-10ppm/°C - depending on foil to ceramic ratio) is much
>closer to the silicon (2-3ppm/°C) than copper (17-18ppm/°C). Net result
>after device attachment, the device is under compression - better than the
>opposite!
>
>If your substrate were aluminum nitride (4-5ppm/°C) the structure could be
>even better. But, not knowing your design etc. this may not be suitable to
>your application. I believe that Brush Wellman (and maybe Stellar) did
>offer the effective equivalent of DBC on AlN.
>
>In either case, if you choose to go to a Sn, Pb, or In bearing alloy go
>lightly on the gold plating, unless it is required elsewhere for wire
>bonding.
>PbIn or other alloys. Daughter-boards alleviate some of these problems, but
>may create others. There should always be a nickel plating layer between
>the copper and the gold.
>
>The traditional Pb/In alloys work well for this application, but it sure
>helps to have hydrogen or reducing atmosphere reflow capability. Fluxes do
>a great job of creating voids! Scientific Sealing Technology vacuum reflow
>equipment also is a big benefit, but again, thou shalt not use flux!
>
>Depending upon the circuit, in some fault conditions, the junctions can get
>rather hot - hot enough to melt Sn63! Consider what safeguard you require
>in the event of a fault?
>
>Review your solder preform manufacturer's selection of alloys for one within
>your temperature range. Consider your system requirements, and any
>re-qualification liabilities. If you still wish to proceed, buy some ribbon
>stock of the alloy/s you wish to evaluate. Cut them to size manually, and
>attach some devices. Acoustically, or x-ray scan the interface for voiding.
>Do some die shear and power cycling of the devices.
>
>To conclude - even though the AuSn causes you repairablity problems, please
>attack the problem with your eyes wide open. Sometimes, low temp solders in
>situations like this can be too good to be true. Be careful!
>
>Steve
>
>
>
>
>
>
>
>
>
>At 11:11 AM 3/30/00 +0200, you wrote:
>>Hi guys and girls!!
>>
>>I have a question that I think many of you can answer.
>>Today we solder FET chips with Au/Sn solder onto a DBC substrate with Au
>surface.
>>The soldering area of the FET is Chromium/Nickel/Silver.
>>
>>I want to replace the Au/Sn solder with "something else".. I don't want to
>glue the chip to place, only solder.
>>The FET working temperature reaches up to 120degC.
>>
>>Any ideas on what kind of solder to use?
>>__________________________________________________
>>Jan Merstrand
>>Microwave MCM, Design/Production Support
>>Ericsson Microwave Systems AB. Defence Manufacturing
>>Office address: Bergfotsgatan 2, SE-431 84 Mölndal
>>Phone: +46 31 747 0725, Fax: +46 31 747 3515
>>
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Phillip A. Bavaro
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