On Fri, 20 Apr 2018 07:37:13 -0000 (UTC), gregz
wrote:
I think the screw down components can warp under torqueing, but flat seems
good to try and achieve. I noticed some power modules were not perfectly
flat, and I started to sand them on a flat precision table. They were
pretty far off from flat.
Nope. They might be warped on arrival, but are impossible to
straighten without sanding or milling.
Back in about 1976, we were having flatness problems with the
transistors used in a linear power supply. There was a large extruded
aluminum heat sink, and either 4 or 8 2N3055 transistors in TO3
packages. The heat sink was milled and quite flat, but the TO3
packages were warped from what I would guess was a worn stamping die.
Some clever person in production decided that the cases could be
straightened most easily by simply tightening the 6-32 screws holding
the devices to the heat sink. After crushing the nylon shoulder
washer, he torqued the hell out of the screw until the head broke off,
and then gave up. The uneven squashing of the silpad insulator showed
that the case was still warped.
At that point, someone in production decided that I needed some unpaid
overtime. It was now my project, errr... headache. I put together a
dial indicator and verified that no amount of pressure from 6-32
mounting screws is going to bend the TO3 base when the spot welded lid
was acting as a stiffener. Using a strain gauge, a bar of steel, and
brute force, I determined that the best I can do with trying to
straighten the TO3 packages was to bend ears near the mounting holes,
making the flatness problem even worse. I fixed a few packages by
milling the bottom of the package, but the cost was too high. Cheaper
just to buy new transistors from a different vendor that weren't
warped.
The only good thing to come out of this waste of time was a fixture
that I threw together to measure the flatness of the TO3 and later RF
power transistor packages. It saved having the same problem repeat
itself in later years.
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558