Looking for rectangular shim that is thick and can push a MOSFET into a heatsink - Page 1
Forum section: electronics / projects

author: SharpEars
author_rank: Unknown
timestamp: October 24, 2017, 07:23:42 pm
content: I am trying to find a rectangular shim or block that I can place on top of a mosfet to put pressure on it. This shim would sit on top of the square mosfet and push it down against a large aluminum heatsink when pressure is applied to it from above by an aluminum crossbar that is attached on both sides by screws. The aluminum cross bar is not very wide and not very thick, so it may warp and wind up pressing on only the edges of the shim.The keys is that I need for the shim not to warp or distort, but to apply pressure evenly down on the mosfet plastic top. The shim should optimally be 20x20 mm and anywhere up to a 10 mm thick. It should probably be made of steel or titanium, or some other metal that will not warp from uneven pressure along its top.The closest I've been able to find are thick cylindrical valve shims, but they are cylindrical and not square.Here is an ASCII picture of how it would look:=X===========X=X ##### XX ##### XX MMMMMMM XAAAAAAAAAAAAAAAAALegend:=== is the aluminum crossbar which is about 10 mm wide and about 5-6 mm thick### is the shim I am looking forMMM is the mosfet (DE475 square package with a 20x20 mm square ceramic (?) heat transfer pad on its bottom and leads coming out of its sides. The mosfet package is 23x23 mm)AAA is the large block aluminum heatsink on the bottomXs are M5 screws in a bunch of washers that ensure an equal amount of downward pressure on the aluminum bar on both sidesThe M5 screw holes in the heatsink are pre-drilled/threaded, so I cut and drilled holes in the aluminum crossbar to match up with the holes in the heatsink.

author: ahbushnell
author_rank: Frequent Contributor
timestamp: October 24, 2017, 11:03:03 pm
content: What is the part number of the MOSFET. Is this a high power device?Andy

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 25, 2017, 01:52:46 am
content: How about screw-down clips? Or snap-in, or others. Examples:https://www.digikey.com/short/qtpr1vIf you really just want a lump of metal, you can buy those from any machining shop -- it won't be cheap, but you'll get what's on the drawing. If you only need one-offs, hardware store metal will do.I've seen them made of pressed steel, which is great on cost... if you're making a million at a time.Protip: if machined, dimple the face towards the transistor, so it presses in the center of the transistor, even if the screws aren't quite equally tightened. (Or if not machined, place a washer or pad under the beam as you clamp it down.) Shaped properly, the beam can also be used as a spring, so the screws can be fully tightened while ensuring even pressure.Tim

author: SharpEars
author_rank: Unknown
timestamp: October 25, 2017, 06:56:28 pm
content: The device is an IXYS TrenchT2TM GigaMOS HiperFET Power MOSFET with an IXYS custom DE-475 (DE475) form factor.Here is a link to the datasheet for the device in question:http://ixapps.ixys.com/DataSheet/DS100250A(IXFZ520N075T2).pdfI am considering layering several 20mmx20mmx1.5mm copper shims (represented as -----) one on top of the other and using that on top of the MOSFET chip plastic (i.e., between the force transfer element (#####) and the MOSFET (MMMMM)):___ ___|XXX| |XXX|==X===========X== <-- The aluminum crossbar which is pushing down (about 10-12 mm in width / 5-6 mm thick)X ##### X Note: There is a little bit of convex warping due to forces applied at sides,X ##### X but it is a very small amount (balanced/limited by the spacers around the screws).X ##### XX ----- X <-- One of the M5 screws (encased in spacers to balance force applied to each end of crossbar)X ----- XX MMMMMMM XX ~~~~~~~ XAAAAAAAAAAAAAAAAAAA GIANT AAA <-- Huge thick heatsink not drawn even remotely to scale (300 mm^2 x 20 mm thick Al plateAA HEATSINK AAAAAAAAAAAAAAAAAAAA||||||||||||||||||||||||||||||||||||||||||||||||||| <-- ..., with many grooved zig-zag Al fins protruding from the bottom)||||||||||||||||||||||||||||||||||Legend:----- now represents the copper shims##### represents the element that will transfer pressure from the crossbar (====) to the MOSFET top (MMMMMMM) via the shims (-----).~~~~~~~ is the bottom of the MOSFET package that contains the ceramic heat transfer square.I am trying to find the element (#####) that will do the job, preferably made out of something strong like titanium or steel. Again, the shims are 20mm x 20mm square, so that's the "landing area" for the element.