I'm making a doublesided PCB,1/32" thick,that the datasheet for the IC calls for 4-6 thermal vias to the ground plane,under the IC. Is there any way of making them on a home-etched PCB? Google didn't turn up anything.
I was considering cutting a slot and soldering a piece of copper the same thickness as the PCB into the slot.
The IC (LM3410xmy)will be soldered to the thermal pad/ground.
It's a boost LED driver driving two Cree high power LEDs at 340ma.
-- Jim Yanik jyanik at kua.net
There are ways of making DIY *electrical* vias, but none of them make good *thermal* vias (i.e. conductive ink). Perhaps the "expanding barrel" type vias might work. Otherwise, I think the only two ways of getting this to work (short of home electroplating) are:
Personally, though, I've never done any type of thermal via at home.
This would probably be the least of a PITA, when you consider what a nightmare it is to do PTH's at home. I haven't done an in-depth search
but the consensus seems to be solder to the tops of pins or use eyelets, so a dearth of PTH advice. )-;
Good Luck! Rich
It is quite easy based on your idea. Either you can cut a slot or drill holes and use copper wire placed through the hole. If the holes are too small then a large slot or hole will be easier to work with. In fact if you use the slot idea and have the the copper base undersized you can solder it to the base before soldering the IC to the pcb. In any case it's not too critical which way you do it but it will probably look ugly. Remember, The whole point is simply to supply a path for thermal flow so how you do that is not too critical as long as it does the job.
Make a topside ground pour with soldered connections to the power pad and the ic ground pins. Make the pour as big as possible. Add a big ground pour or plane on the backside of the board. Drill 4-6 via holes just outside the chip footprint and solder wire vias topside and bottomside to conduct the heat from top to bottom. #20 wire should do it... roughly 5 K/W per via, close enough to zero. The bottomside pour to ambient will likely be the dominant thermal resistance.
What are your input and output voltages? That will determine power dissipation and allow an estimate of junction temperature. Unless you have a huge boost ratio, this chip won't be working very hard at 340 mA.
John
I think I'd just drill a hole around 1.7mm dia and stick a snippet of solid AWG14 wire through, file/sand the IC side flat first and solder both sides. That'll be at least as good as half a dozen tiny PTH vias.
DJ Delorie wrote in news: snipped-for-privacy@delorie.com:
I just received the parts,and am flabbergasted at the size of them. The IC is smaller than a pencil eraser's diameter.How to put 4 vias underneath it is a puzzle. All of the parts will fit on a 1 inch circle.I don't know if I can even make a PCB that small. :-(
I find it hard to believe that the IC can handle the power for which it's rated!
-- Jim Yanik jyanik at kua.net
Thats pretty well all I do. I use 22AWG when they did some phone wireing in my use they left behind almost a full roll of 300m.
Standard design rules at a typical Chinese proto house will allow 4 vias inside a 48mil x 48mil square with 12 mil radius corners. That's way smaller than 1.6mm x 1.6mm. Of course much finer is possible..
You are free to make the pcb 3" square if it pleases you. A copper pour (especially on multiple layers) will help it run cool. In fact the ratings are based on a four layer 2-oz-copper board that is 3" x
3"-- quite an effective heat sink, and not something you're going to be able to match without soldering copper sheet onto the board.See above. Always read the fine print on the data sheet.
Some phone wire is copper plated steel. Check it with a magnet. Its thermal resistance is much higher than copper.
John
Yipes! Jim needs to pay serious attention to this point.
Jon
Your time must be worth very little to you.
Did'nt know that.
Nope tested it with a magnet it's copper.
Please, not this argument again. You don't know *why* he's choosing to do it at home, so you can't argue why he should or shouldn't be doing it himself.
This from a guy who has been screwing around with some cheap crimp tool for how long now?
Yea it makes much more sense to spend $50.00 plus for a couple of boards. They have a bargain $28.00 shipping, Christ what a joke. Do you work there or something?
APC ESTIMATE FOR TWO BOARDS
Taxes and all the other shit they can dream up to pad the bill still not included.
He might as well buy an led driver.
It should take all of about a half-hour to do a board that size, from printout to etching.
That sounds to me a thermal improvement over the vias.
Meanwhile, it appears to me that the thermal requirements are not great, unless your ambient temperature will be high.
Two modern Cree LEDs at 340 mA sounds to me like 2.2-2.3 watts. The datasheet for this IC makes me think that efficiency for driving 2 LEDs will probably be at least 82%, meaning the IC produces about 22% as much heat as the amount of power going to the LEDs. This means about half a watt of heat.
A slot or a hole stuffed with a cut-to-size sheet of copper is more than adequate, and will have extremely low thermal resistance for the .3-.4 watt or whatever of heat flowing through.
Next: This IC is the 8-pin one, right? That leaves a way to get some copper on the top heatsinking this IC, as well as copper on the bottom. Copper plating on PCBs is fairly thin, and it helps to use both sides to get half a watt of heat spread over the PCB from an IC that small.
One more thing I would do, especially if the PCB is going to be small: Cover the heatsinking copper with solder mask or some sort of paint; something non-metallic and somewhat thicker than coloring it with a "sharpie". My experience is that this improves emissivity of thermal IR enough to make a significant difference - especially with smaller boards and ones in cramped containers, where convection currents are slower.
- Don Klipstein ( snipped-for-privacy@misty.com)
e
Sound like the best idea to me so far.
George H.
Outsourcing a dirty chemical job that's been solved for decades for a few dollars make sense. Is it a pride thing? I don't get it.
Hammy wrote in news: snipped-for-privacy@4ax.com:
I have a lot of time and not a lot of money. Plus I like to "roll my own". It's a hobby project,not any commercial venture.
ExpressPCB wasn't cheap,either.
I couldn't find one that used the supply V I want(4.8v) and drive the two LEDs in series.Thought I could do it cheaper,the 7 parts only were about $3.00,and I have Cu-clad FR4 and etchant.I didn't realize the IC was SO tiny,beyond my hand layout ability.(and probably etching ability) It's only 3mm square,lead pitch of .65mm;eMSOP pkg.
It will take me longer than that to download a PCB layout program.(dialup) any recommendations for that program?
-- Jim Yanik jyanik at kua.net
A few dollars? Hell, if they made PCBs for five bucks in one day, I'd never make one again.
Oh sorry, they're upwards of $50? And several times that for one day service? To hell with that. Only cost effective for production runs. For one-offs, I'll take the satisfaction of hand-made, TYVM.
Tim
-- Deep Friar: a very philosophical monk. Website: http://webpages.charter.net/dawill/tmoranwms
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