Where holes are drilled for thru-board components but of diameter far too big , thru-hole plated , but no eyelet/inserts used to fill the gap. So
"N_Cook" wrote in news:iouqr0$pl0$ snipped-for-privacy@dont-email.me:
perhaps PCBs designed for machine parts insertion have those sort of "oversize" thru-holes? Perhaps the insertion machine prefers big holes and besides,they crimp over the leads anyways. Maybe it's too much trouble to drill PCBs for different size thru-holes,and/or not worth the effort.
certainly stocking and inserting eyelets would be an additional,unnecessary expense.
We had a board house add 'thermal rings' to the mounting holes in a
500 MHz synthesizer. It played hell with the modules and sent the phase noise through the roof but they said that they would no longer make them the way we needed them. it also dropped the center frequency by about 100 MHz. I had to take some of the copper foil we used to seal the shields on the modules and cover them, then solder them to the surface and the plated through holes, till we could get the boards from another supplier. They routinely made a couple dozen different boards for us, prior to that. When they decided to change our layouts without permission, we dropped them.
Microdyne stopped bending the leads or using the special cutters that cripmed the leads back in the '80s to reduced damage to the PTH.
It's more likely that there was no design review, or the cad operator was too lazy to verify the hole sizes.
and
additional,unnecessary
As the drill bits tend to be very brittle carbide rather than HSS I imagine doubling the drill bit diameter drops the breakage rate by 1/10 or so, reduced bit replacement costs, plus reduced down-time manual intervention to rectify stoppages, due to such breakages.
A properly run PCB drilling system doesn't break the bits, and they are replaced before they are dull enough to cause a problem. If it is a cheap, in house product, all bets are off. You can get properly made boards, if buying the cheapest you can find is at the top of the list. Boards with over sized holes use more chemicals to plate the PTH, and waste solder in the hand or wave solder process. We stuffed and placed our boards at Microdyne, but used outside PCB houses to produce the blanks. Some of our boards were 16 layer and cost over $8,000 to populate.
"N_Cook" wrote in news:ip109c$f1n$ snipped-for-privacy@dont-email.me:
Tektronix still used the crimped over leads on their TH PCBs all the way up to conversion to SMD boards,and that didn't happen until the late 1990's. We didn't have much trouble with damaged thru-plated holes.
It was always my understanding that the "oversized" PTH was a deliberate choice. If significant current needs to pass to both planes, a (proper, of course, not RoHS) solder plug enhances the through-plating's capability.
It makes more sense to just specify a thicker plating on the PTH. Solder his a higher resistance, and is much weaker.
It does (doh!) but obviously the hole plating and the solder fill also contribute to the overall conduction.
The blob of solder has much less resistance than the very thin copper surrounding the hole. Mass counts here, and the copper is at a big disadvantage there.
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