Soldering Underneath Chips

Intro: Soldering Underneath Chips

I recently had to design a device that used a chip with a heatsink underneath the body of the chip. This heatsink had to be both electrically and thermally connected to the PCB.

Typically these devices (see picture) are soldered to PCBs using reflow techniques, where solder paste is stencilled to the board, robots place the chips and a special oven heats the device till the solder paste melts. Other devices with the same problem include driver chips and high power LEDs.

I originally tried using silver heatsink compound however although it was pretty good thermally it didn't make a reliable electrical connection, the cct malfunctioned with vibration and the magic smoke escaped...leading to much swearing and frustration.

After some experimentation I came up with this method to solder underneath these types of devices for hand prototyping without needing a reflow oven.

STEP 1: Prepare Thermal Vias

Your PCB should have a copper area underneath the chip heatsink for electrical and thermal connection.
First drill small holes (as many as can fit) under where the chip heatsink goes.

Next poke through copper wire through the holes (second picture). Try to use wire as thick as the holes will allow. You need a tight fit. I just used the leads from a diode....they were just right....and made of copper (plated with tin).

Second time around I'd poke the wires from the bottom just enough to poke out, but not too far (third pic).

STEP 2: Solder the Top and Bottom of the Thermal Via

Now solder the top and bottom of the poked through wires.....try to use as little as possible on the top, where the chip will be installed to make the next step easier.

Trim the top wires as close as possible to the PCB without destroying any trackwork. Leave about 2-3mm of wire poking out from the bottom though.....you need to be able to connect the heat from the soldering iron to something when it comes time to attach the chip.

STEP 3: File Back the Topside

Now comes the delicate part.

Carefully file as much as possible without scratching surrounding trackwork. Take your time here....it is very difficult and cannot be rushed.

When it gets too close to file, use a scalple blade to scrape away even more. The copper and the solder should be reasonably soft.

In the first picture you should be able to see the cores of the copper wires that were poked through starting to appear.

STEP 4: Finally Sandpaper the PCB

Using wet/dry sandpaper under a tap, carefully sandpaper the remaining solder off the PCB underchip heatsink area till it is bare copper and as flat as can be.

Don't be too agressive with coarse sandpaper or else you might (as I did) grind away surrounding trackwork. Again take your time, and finish with 2000 grit paper to get a good finish.

Look at the picture, although blurry you should be able to see bare copper with two copper slugs where the wires are.

Also note a couple of scratches on some connecting tracks....oops.....hopefully the tinning will take care of these little scratches.

After this, use some used solder braid to tin the pin tracks where the chip will connect.....but leave the heatsink area bare copper....you may need to remove excess tinning with clean solder braid. It is important to have everything flat.

STEP 5: Hurray, the Solder Paste Enters the Stage.

Now get the solder paste and dab a little in the center of the chips heatsink. Don't use too much and leave a gap around the edges. If you get a little on the outside, remove and try again.

When the chip is placed on the PCB, the paste will sploosh out, which might end up shorting out the chips pins.....so only use as much as is needed.

Next place the chip on the PCB, and tack solder the corner pins to the tinned tracks. Use a multimeter to make sure that there are no shorts.

Be careful with solder paste, it is toxic so wash your hands if you get any on yourself and clean any splatters. Also it should be stored in the fridge when not in use.

When tacking on the corner pins, rely on the tinned trackwork....don't add any more solder. You just need to hold the chip in position. You should have a little play when moving the chip slightly. If you put too much in, remove everything, clean and try again.

STEP 6: Heat From Underneath

Now turn the board over and heat the poking out bits of copper wire from underneath.

Watch the topside of the board and notice that there should be a little bit of fumes as the solder paste melts and fluxes.

When cooled, nudge the chip. It should be rock solid if the paste has melted and solidified. If there is any play....then try heating again, or else remove everything/clean and try again.

Finally solder the remaining pins and the previously tacked pins and clean up with clean braid then flux remover and test for shorts.

Congratulations you've successfully attached a chip with a heatsink underneath both thermally and electrically.

Sorry about the blurry pics, my camera only just does macro.

This technique should be useful for not only chips as shown in the pictures, but also high power LEDs and any other components with a similiar need for a good electrical and thermal connection to PCB layouts.

Comments

[throbscottle]

Absolutely ingenious. I'd been wondering how to solder these - thanks!

[stormwlf]

Cool concept thanks for the info!

[wierd idiot]

I just have to ask this. Why was the name of the chip wiped?

[kllrwolf]

I have been told by many professors at school (Electrical Engineering major), that once you start making things others will use, removing chip identifiers is the best way to protect your design from people who will try to reverse engineer it.

[wierd idiot]

Sad isn't it? That people do such things like being a copy cat.:(

[Derin]

And it makes the harvester's job harder.

[rgbphil]

nothing mysterious, I just wanted to have an uncluttered picture to emphasize the process not the circuit. Its a LTC3477 switch mode controller in case you're interested.

[danlab]

Yay, someone else who calls it the magic smoke! I hate putting the magic smoke back in after it escapes.

[drewish]

"Sorry about the blurry pics, my camera only just does macro." I think you'll find that adding more light and holding the camera still will do the trick.

[andrew101]

yea ither crop the image or hold it farther away and zoom

[jridley]

And if it really is true that the camera won't focus that close, then don't get that close, just crop the image down.

[sumguysr]

very good idea and an excelent instructable. but wouldnt it be easier to just put the solder paste on the heatsink and put the whole thing on a hot plate. I'll try to remember to post a link tommorow I'm too tired right now.

[rgbphil]

yep.....I looked at the PCB on hotplate method, but was a little concerned about the control of heat. Worried I'd cook the chip basically. With this method, the soldering iron is on the component no longer than the normal amount of time for soldering a through hold component, so presumably less time to hurt the electronics. If you had a plated through board, with vias placed under the chip, you could avoid the process of putting wires/sanding etc....however this instructable is primarily for hand prototyping, where you etch and drill the boards yourself.

[starphire]

If you can see the solder paste melting into shiny pools of molten solder, you can use that to prevent overheating - just remove the heat when it makes the transition. People have been doing this with toaster ovens for years - I've done literally hundreds of boards this way. Just don't use the oven for food anymore. Actually, a regular toaster oven has hotspots, and you can get uneven results if you try to do larger boards in them. Nowadays, I use a cheap tabletop convection oven, and it works fantastically well. The maximum temp. on broil is just above the reflow temperature.

[sumguysr]

chips that have no exposed leads are meant to be soldered in reflow ovens and can take the heat. pretty much all smd components can take the heat.

[Myself]

I'm not sure the copper in the vias is important over such a short distance. Why not just drill the holes, apply paste and affix the chip to the topside, heat from the bottom, and let the paste fill the vias?

[rgbphil]

I tried that as well.....the paste doesn't wet the holes, and you end up with solder forming on the bottom side only and a hole filled with flux. I guess the surface tension of the flux has something to do with that. If you made the holes big enough, say 3mm or used a very thin tip and managed to get the soldering iron tip to touch the topside through the hole, then that would work. For larger components that would be fine, but the little chips are too small. Essentially the copper in the vias extends the soldering iron tip.

[Myself]

I think your problem is the flux, because the technique I suggested is precisely the one used by Schmartboard to hand-solder BGAs.

Maybe it has something to do with their vias already being plated through? Hmm.

[rgbphil]

Definately....in fact using plated through vias is good practice under heat generating components (or near them) to better whisk away the heat. Look at some high end LED integrated driver boards. They have the LEDs surrounded by thermal vias, and these are recommended in the datasheets. Nice link.....would be pretty scary doing a BGA even using a reflow oven. This instructable is not for that sort of thing, just for flat plate heatsinks on components. As an aside, in my design I blew the chip a couple of times whilst getting the design right. With the plugged vias, it makes it easy to unsolder the chip to put a new one in.

[theRIAA]

i didn't know that sandpaper was a verb...