when hand soldering prototypes, how do you deal with thermal pads? - Page 1
Forum section: electronics / projects

author: KTP
author_rank: Frequent Contributor
timestamp: June 08, 2012, 12:06:48 pm
content: I am finishing up a prototype board using the A6282ELP led driver. This is a 24 pin TSSOP package with an exposed thermal pad on the bottom (but one that doesn't go to the very end edge of the chip).This low side chip regulates the current to each LED so you don't need series resistors. It is recommended to keep the LED driver voltage low enough such that Vds on the chip fets is between 1 and 3V. Easy since I am using an efficient buck regulator and can set the LED driver voltage to whatever I want....except I am using RGB leds where the red led drop is significantly less than the blue and green elements. I predict having to use a LED voltage of around 4.5V, meaning the Vds on the red led driver fets will be around 2.3V. At 50mA per channel average current and using 12 of the 16 channels in my design, I get a package power dissipation of around 1.38 watts plus about .12 watts for the rest of the chip electronics...so around 1.5W. (note I could use series resistors on just the red channel lines to the leds, except that there is no standard in the industry on these PLC6 RGB LED packages so I don't know which pins will actually be red if the supplier changes!)Sorry for the wall of text, but wanted to provide a background to the question.1.5W seems a fair amount of heat for such a tiny package and I plan on using a layout with thermal vias to a large ground plane. The question I have is how do you guys hand solder these type of parts ensuring good contact of the "exposed pad" underneath the TSSOP 24 pin chip to the circuit board landing? I won't be using an oven, so I guess solder paste is out. I could design the top landing pad to be large enough that it sticks out a little on each side of the chip and then try to wick solder under the chip across this pad or I could try to wick solder through the thermal vias from the underside, orThanks for any suggestions!

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: June 08, 2012, 12:14:20 pm
content: Frying Pan, Hot air station. Just put that chip on first.Put the pads for a resistor on each channel and short the ones you aren't using, might be acceptable.

author: nctnico
author_rank: Super Contributor
timestamp: June 08, 2012, 12:50:28 pm
content: Its simple: always put several 0.6mm to 0.8mm vias in the thermal pad. When handsoldering you fill one via and keep applying heat until the solder comes up in the other vias. The holes also help during reflow soldering to suck away excess solder.One of my boards:http://imageshack.us/photo/my-images/502/p1010217w.jpg/http://imageshack.us/photo/my-images/51/p1010221i.jpg/

author: KTP
author_rank: Frequent Contributor
timestamp: June 08, 2012, 12:53:49 pm
content: Quote from: nctnico on June 08, 2012, 12:50:28 pmExcellent! thanks. I had never actually had to use a part with this type of pad and was not sure if the soldering iron could get enough heat through the via to cause the solder to bond the thermal pad to the landing. It sounds as though it will work like a champ though.

author: 8086
author_rank: Super Contributor
timestamp: June 08, 2012, 12:54:34 pm
content: I wouldn't bother doing it by hand - you can do it with a £30 toaster oven, no need to spend loads of moneyOr just use lots of fairly wide vias and solder through, but oven is the winner in my book

author: KTP
author_rank: Frequent Contributor
timestamp: June 08, 2012, 12:58:17 pm
content: Quote from: 8086 on June 08, 2012, 12:54:34 pmThanks. I definately have heard of the toaster oven reflow used by hobbyists but I had always thought it was a bit sketchy...maybe I should revisit the idea. I had concern that the reliability of the chips could be in question if the manufacturer allowable component temperature were exceeded for a given length of time. There is also the problem of the 144 SMT RGB leds I will be using...they are plastic and would probably melt if heated incorrectly in an oven. I could use the oven just for the driver chip though...

author: 8086
author_rank: Super Contributor
timestamp: June 08, 2012, 01:12:52 pm
content: Quote from: KTP on June 08, 2012, 12:58:17 pmGet a thermocouple in there (cheap bundled DMM one will do) - find out the melting point of the solder you're using (usually about 180-185C for PbSn and 215-220C for Pb-free) and keep the temp constant there by simply cutting the power when you reach that temperature (there may be a little overshoot) - watch the solder reflow carefully through the window, to make sure it is actually melted - and after 30-60 seconds just open the door slightly, and let the temp ramp down. You can ignore the temperatures in "absolute maximum ratings" - they are only for when the device is operating, unless it specifically says otherwise.LEDs will be just fine with this process - the material they are made of is designed to be able to be reflowed in an oven, the only problem I could see is if you have an oven with element at the top and bottom, the ones at the top may introduce too much radiant heat and cause some issue - but I have never seen this happen.Also depending on where you are I reckon you should be able to get a stencil made fairly cheaply

author: KTP
author_rank: Frequent Contributor
timestamp: June 08, 2012, 01:26:54 pm
content: Yeah but to be honest, I only plan on hand soldering one or two of these, then after the design is proofed I will move directly to pick and place with a board house.There are only so many boards you can hand solder with 144 six pin RGB leds before you go crazy (not to mention the other 100 or so parts).

author: nctnico
author_rank: Super Contributor
timestamp: June 08, 2012, 01:30:12 pm
content: Quote from: 8086 on June 08, 2012, 12:54:34 pmIn that case you'll need a stencil as well. Or do you dispense paste with a siringe? Anyway, without vias there is a chance the chip will float on the solder on the thermal pad and making bad contact to the other pads on the side.

author: 8086
author_rank: Super Contributor
timestamp: June 08, 2012, 01:31:32 pm
content: Quote from: nctnico on June 08, 2012, 01:30:12 pmStencils can be cheap - I get mine made for about £12 a go. I find syringes to be frustrating.

author: poorchava
author_rank: Super Contributor
timestamp: June 14, 2012, 01:28:02 pm
content: Where do you buy those stencils? Are they plastic-like (kapton and such) or stainless steel? What size comes for 12 pounds?

author: 8086
author_rank: Super Contributor
timestamp: June 14, 2012, 02:40:32 pm
content: Quote from: poorchava on June 14, 2012, 01:28:02 pmFor £12, I can get an A4 sheet of PET laser cut to spec.http://www.smtstencil.co.uk/Not the best, but certainly more than adequate. And a hell of a lot better than syringe.

author: free_electron
author_rank: Super Contributor
timestamp: June 14, 2012, 02:55:54 pm
content: -edit- cleaned up thelysdecticdylsecticdyslectic keyboard .. @#$# ipad ...If you make a lot of boards : get a craftrobo vinyl cutter for 200$ and cut your own.3M sells adhesive backed kapton foil. Works pretty well.As for thermal pads : get a heatgun. Steinel , aoyue or a weller HAP-1 work well.Or, simply make sure to have a reasonable large hole under the heat pad , poke the tip of the soldering iron in the hole so you touch the pad and simply fill it with a slug of solder....

author: T4P
author_rank: Super Contributor
timestamp: June 14, 2012, 04:14:16 pm
content: Quote from: free_electron on June 14, 2012, 02:55:54 pmElectron ... has your keyboard gone nuts or were you in a rush?

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: June 14, 2012, 04:21:58 pm
content: Assuming it's a proppe PCB wth thermal vias, just put some solder paste on the pad, place the chip, and heat from below til it flows with a suitably powerful iron. Helps if you leave a big hole in the resist on the underside.You may want to tack down a few pins to stop it floating around.