I am starting a new project to assemble an amp that will use two LM1875 TO220 packages in a small, lightweight board. The large heatsink I have is more than adequate for thermal dissipation for the amp, but it is not drilled and I don't have the tools for drilling and tapping to be able to screw the LM1875s to the sinks.
I see thermally conductive adhesive glues and even adhesive tape advertised. If I supported the weight of the boards in the case with standoffs, would the thermal adhesives do the job of conducting heat effectively to the sinks?
thanks!
I see thermally conductive adhesive glues and even adhesive tape advertised. If I supported the weight of the boards in the case with standoffs, would the thermal adhesives do the job of conducting heat effectively to the sinks?
thanks!
I wouldn't trust an adhesive alone. You could find a cheap used drill, or borrow one.
If you can drill a plain hole all the way through the heatsink plate between the fins, then
just use a lock washer and nut on the other side.
If you can drill a plain hole all the way through the heatsink plate between the fins, then
just use a lock washer and nut on the other side.
that's a good idea but the sink I have is fairly massive; base is more than 1" thick solid aluminum, which I would not attempt to drill all the way through. It weighs at least as much a full amp camp amp and chassis!
But if the chips will not mount reliably with adhesives I will need to think of a plan B.
But if the chips will not mount reliably with adhesives I will need to think of a plan B.
That kind of drilling requires a machine shop with suitable equipment. No hand drill.
However, the sink sounds like considerable overkill for 30W parts.
However, the sink sounds like considerable overkill for 30W parts.
You could take it to a local machine shop and ask for two blind tapped holes about 6mm deep.
Metric M3 screws will fit through a TO-220 and an insulating bushing if you need one.
The metal tab of the LM1875 is connected to the -VEE pin (3), so if you use a single voltage power supply you don't need to worry about insulating it from the heat sink because pin 3 will be the ground.
However, if you use a dual voltage supply you will need to insulate the tab from the heak sink or the heat sink will be at the -VEE voltage. The heat sink may already be part of the case ground anyway.
Metric M3 screws will fit through a TO-220 and an insulating bushing if you need one.
The metal tab of the LM1875 is connected to the -VEE pin (3), so if you use a single voltage power supply you don't need to worry about insulating it from the heat sink because pin 3 will be the ground.
However, if you use a dual voltage supply you will need to insulate the tab from the heak sink or the heat sink will be at the -VEE voltage. The heat sink may already be part of the case ground anyway.
The board is designed for dual voltage AC supply so the chips will need to be insulated.
And yes, as rayma mentioned, the heatsink is certainly overkill but I hate to see it go into the landfill.
I do have a dremel. A table top drilling guide for it is well under $100 on Amazon. So I will order a guide, metric M3 TO-220 mounting kits, a cheap m3 metric drill and tap set, and give that a try. Machine shop will be plan B....
I will be needing to do this again I am sure, so a drill guide is a useful investment anyway for the dremel.
And yes, as rayma mentioned, the heatsink is certainly overkill but I hate to see it go into the landfill.
I do have a dremel. A table top drilling guide for it is well under $100 on Amazon. So I will order a guide, metric M3 TO-220 mounting kits, a cheap m3 metric drill and tap set, and give that a try. Machine shop will be plan B....
I will be needing to do this again I am sure, so a drill guide is a useful investment anyway for the dremel.
I've also glued ICs (TDA7293) with superglue before, and it worked well, without any problems. Still, screwing and insulating the ICs is safer!
Also you will very much need "Tap Magic Drilling Fluid" to do this successfully.
https://www.drilldoctor.com/products/tap-magic
However, I have not found those drilling guides to work very well. Try it on some scrap first.
Make sure you can return it before ordering.
With the pre-existing tapped holes, you may be able to work out a clamping arrangement
for the two devices instead, with a bar across each one, clamping them onto the sink.
One vertical bar per device, with the devices arranged with leads directed left and right,
would allow them to be in the center of the height dimension.
https://www.drilldoctor.com/products/tap-magic
However, I have not found those drilling guides to work very well. Try it on some scrap first.
Make sure you can return it before ordering.
With the pre-existing tapped holes, you may be able to work out a clamping arrangement
for the two devices instead, with a bar across each one, clamping them onto the sink.
One vertical bar per device, with the devices arranged with leads directed left and right,
would allow them to be in the center of the height dimension.
It will take a special drill size, but I'd use a forming tap on heat sinks. Works way better in soft aluminum and no chips to worry about. Use no Harbor Fright or eBay stuff- get real tools from McMaster, MSC, Travers or similar places. A decent battery hand drill will work if you're careful and practice on scrap aluminum. The whole thing is an investment, but you'll use it for many projects. Examples, but you can probably do better if you shop around-
https://www.lowes.com/pd/DEWALT-20-...JXZ108taFn9Wzc3WNkSK_bW3fDA84-IC54aAlio&gQT=1
https://www.mscdirect.com/product/details/09247909
https://www.mcmaster.com/2896A553/
https://www.lowes.com/pd/DEWALT-20-...JXZ108taFn9Wzc3WNkSK_bW3fDA84-IC54aAlio&gQT=1
https://www.mscdirect.com/product/details/09247909
https://www.mcmaster.com/2896A553/
There are thermal conducting silicon glues meant for LED mounting, about $1 here for a 50 gram tube, enough for you.
I have used cyano acrylate glue to bond zinc blocks on my hot stamping machine, so yes it works.
Best known as Super Glue , many trade names for this instant adhesive.
The Loctite product was inferior to the local Fev-Kwik, go figure.
You can get silicon pads, then no need of glue.
Also, try self tapping screws, a pilot hole of sightly less than minor thread diameter is needed, can be done on a drill press or milling machine, a starting punch is good to prevent drill wandering.
And high temperature silicon glue, used for engine tappet covers, should work, quite easy to obtain.
Make sure there are no air gaps on any method above, use acetone to slow down super glue, it would be my last choice of methods above, as it is too fast, and not easy to re-work.
I have used cyano acrylate glue to bond zinc blocks on my hot stamping machine, so yes it works.
Best known as Super Glue , many trade names for this instant adhesive.
The Loctite product was inferior to the local Fev-Kwik, go figure.
You can get silicon pads, then no need of glue.
Also, try self tapping screws, a pilot hole of sightly less than minor thread diameter is needed, can be done on a drill press or milling machine, a starting punch is good to prevent drill wandering.
And high temperature silicon glue, used for engine tappet covers, should work, quite easy to obtain.
Make sure there are no air gaps on any method above, use acetone to slow down super glue, it would be my last choice of methods above, as it is too fast, and not easy to re-work.
Lots of very helpful info and ideas, thank you to all!
Clamping arrangement using existing tapped holes will probably be my initial experiment since it won't cost much. Maybe self tapping screws - if they are available in M3 metric - will be second. Then adhesives if #1 and 2 don't work out.
I will report back with pics when I have a solution that I feel is reliable.
Clamping arrangement using existing tapped holes will probably be my initial experiment since it won't cost much. Maybe self tapping screws - if they are available in M3 metric - will be second. Then adhesives if #1 and 2 don't work out.
I will report back with pics when I have a solution that I feel is reliable.
Not to be contrarian, but assuming the chip amp is soldered directly to a PCB, then why not just use thermally conductive adhesive to mount the chip to the heat sink itself using standoffs glued via epoxy or cyanoacrylate glue to the heat sink? That way, the weight of the PCB is borne only by the standoffs and not by the chip amp's adhesive.
The local brand is Fevi-Kwik, quite popular here, no ties, as usual.
You can actually mount the 1875 on the heat sink, and use flat ribbon cable to connect to the PCB.
The 1875 has only a single hole for mounting, this method offers an altrernate way if it is awkward to align the PCB and heat sink, a short (say) less than 100 mm length of flat cable, about 0.5 square mm, should not cause any issues.
Possibly induced hum, but unlikely I think.
You can actually mount the 1875 on the heat sink, and use flat ribbon cable to connect to the PCB.
The 1875 has only a single hole for mounting, this method offers an altrernate way if it is awkward to align the PCB and heat sink, a short (say) less than 100 mm length of flat cable, about 0.5 square mm, should not cause any issues.
Possibly induced hum, but unlikely I think.
[[using standoffs glued via epoxy or cyanoacrylate glue to the heat sink?]]
could work, thank you. It will be an option if I cannot get clamping configured.
could work, thank you. It will be an option if I cannot get clamping configured.
Please don't. Buy a quality bottom tap and a sharp drill bit. +1 for McMaster-Carr. They're not the cheapest on the block, but they do have quality tools.
If you buy a cheap tap you're almost guaranteed to break it, especially if you don't use cutting fluid. Then you have a piece of hardened steel stuck in your heat sink that you then either need to get out (there are tap extractors) or grind flush and abandon in place.
Don't make the project any bigger than it needs to be. Pre-drill with a 2.0 mm drill bit. Then drill that up to 2.5 mm, which is appropriate for M3x0.5 mm. Then tap using a tap guide.
I use a TiN coated spiral flute tap. It works great. McMaster-Carr 26355A81. You'll also need a tap handle. 25605A75 for example.
Buy quality tools and take care of them. They'll last forever. Some of the taps I use and the tap handle are 30+ years old. They work just fine.
You'll need some aluminum cutting fluid. I use Relton A-9. A small bottle of it will last forever for the average hobbyist.
If tapping isn't your thing, you could consider self-tapping (aka thread forming) screws. McMaster-Carr can help you there as well. And, no. They're not paying me to say that. I just shop there. 🙂
Tom
Aluminum is sticky when drilling and tapping, and small taps are easy to break.
Go quarter turn in, then 1/8 turn back, go carefully, you have to be patient for this tedious job.
I tend to drill straight through, and use nuts stuck in place with contact cement or silicon. Slightly larger hole than screw, allows wiggle room...for a 3 mm screw, 1/8" or even 3.5 mm holes are my choice.
Of course, you can always use a different heat sink, possibly with already tapped holes,metal window sections will do, even corrugated iron ... or an old bike piston crown!
Go quarter turn in, then 1/8 turn back, go carefully, you have to be patient for this tedious job.
I tend to drill straight through, and use nuts stuck in place with contact cement or silicon. Slightly larger hole than screw, allows wiggle room...for a 3 mm screw, 1/8" or even 3.5 mm holes are my choice.
Of course, you can always use a different heat sink, possibly with already tapped holes,metal window sections will do, even corrugated iron ... or an old bike piston crown!