Thermal relief dilemma - Page 1
Forum section: electronics / projects

author: Pedro
author_rank: Newbie
timestamp: July 11, 2011, 06:23:55 am
content: Hello all,I have a doubt about thermal relief in the specific situation of power components.Thermal relief is recommended to allow for better soldering of both thru-hole and SMD parts. Without thermal relief, the pad has a very high thermal conductivity to the plane and it is very difficult to heat, it cools down much quicker than the other pad's, creating all sorts of problems. So as I understand it, by adding thermal relief we are effectively increasing the pad's thermal resistance, solving many of these assembly problems. Up to this point everything is fine.The problem comes when we have a heat-producing part, such as a D2-PAK or SO-8 power transistor. In these type of components, the tab or bottom pad is used to dissipate heat. We might have connected such thermal pad to a big plane to lower thermal resistance. But if we placed a thermal relief to help manufacturing, isn't the added thermal resistance of the relief making dissipation worse? Wouldn't it be better to have a direct connection of the power part to the plane?I guess there must be some accepted compromise in this regard.Thanks guys and gals,Pedro

author: Mechatrommer
author_rank: Super Contributor
timestamp: July 11, 2011, 08:38:59 am
content: i think you've questioned and answered it in one post.

author: cyberfish
author_rank: Regular Contributor
timestamp: July 11, 2011, 03:09:36 pm
content: Usually the pad on D2PAK is connected to plane without relief. It just takes a long time (~30 seconds with a 50W iron) to solder.There won't be quality problems if you make sure the solder flows completely, just like soldering with thermal relief. Also, the temperature cannot be too high or it may damage the part.

author: ivan747
author_rank: Super Contributor
timestamp: July 12, 2011, 04:09:48 am
content: What about these SMD mpdified "power" SSOP or SOIC motor drivers with huge pins at the sides connected to ground to disipate heat. It must be quite tricky to solder in some situations, because there are two huge tabs and some tiny pins next to them. The situation gets even worse when one of these "Powerpads" (TI) are included in the package.

author: Mechatrommer
author_rank: Super Contributor
timestamp: July 12, 2011, 04:40:01 am
content: once the whole thing goes into reflow machine, everything will go above the solder melting point, no heat will be able to be "relieved", no issue.the same with wave soldering for through hole, the heat is just too massive at mass production scale. my imagination tells me from looking at documentaries.

author: ivan747
author_rank: Super Contributor
timestamp: July 12, 2011, 05:01:37 am
content: Quote from: Mechatrommer on July 12, 2011, 04:40:01 amWhat about hand soldering, I still think it must be challenging in some situations to do it.

author: tycz
author_rank: Regular Contributor
timestamp: July 12, 2011, 06:00:41 am
content: Thermal relief is benificial when hand soldering and servicing. It doesn't aid production (most consumer products don't use it), and of corse is a bad idea if the part dissipates heat!Quote from: ivan747 on July 12, 2011, 04:09:48 amYes! And audio amplifiers in DIP20 with half the pins connected to a ground plane... horrible. I think a bigger iron is required.

author: Mechatrommer
author_rank: Super Contributor
timestamp: July 12, 2011, 06:17:49 am
content: Quote from: Pedro on July 11, 2011, 06:23:55 amOP was concerning about manufacturing. but if its hand soldered job (manual labour at factory or simply diy at home), then a bigger hand solder could solve the problem.

author: Pedro
author_rank: Newbie
timestamp: September 29, 2011, 09:48:54 pm
content: Thanks all for your answers!I have been offline for some time because of my doughter's birth, and I forgot about this post. I will definitely remove the relief to improve heatsinking.Bye!