step-down dc-dc converter question (BUCK) - Page 1
Forum section: electronics / projects

author: kvresto
author_rank: Regular Contributor
timestamp: March 08, 2015, 07:25:10 am
content: Hi Everyone.I have a question about power pad devices and what can happen if you rout and connect the exposed pad to ground as recommended in the data sheet but do NOT actually solder the exposed pad of the ic to the PCB. I don’t have any surface mount manufacturing tools so actually soldering an exposed pad is a problem. I’m referring to TPS62110, but can be any exposed pad device.Cheerskv

author: mij59
author_rank: Frequent Contributor
timestamp: March 08, 2015, 07:49:49 am
content: Don't think its a good idea, read the data sheet page 5.Exposed thermal pad.Must be soldered to achieve appropriate power dissipation and mechanicalThermal pad reliability.

author: ccayser
author_rank: Contributor
timestamp: March 08, 2015, 07:56:54 am
content: It depends, if you drive small currents with it, it might not be an issue, but it is highly recommended to connect it to a pad. You can solder thermal pads at home if you get yourself a solder paste and manage to get a hot air gun with controlled airflow and temperature. Both these are things that hobbyist should have in their lab.

author: ajb
author_rank: Super Contributor
timestamp: March 08, 2015, 08:05:21 am
content: You don't even need paste if you've got a hot air gun. Use your regular iron to put a pillow of solder on the thermal pad, add some flux to the pad and the bottom of the IC, then reheat the solder and plop the chip down on top of it.If you don't have hot air, you can place a large plated through hole in the middle of the thermal pad and solder it from the bottom.

author: Monkeh
author_rank: Super Contributor
timestamp: March 08, 2015, 08:11:44 am
content: You will cook it. There's no ifs, ands, or buts, if you attempt to use a device like that without soldering the thermal pad it will overheat. It would desolder itself if it didn't have a thermal shutdown.

author: kvresto
author_rank: Regular Contributor
timestamp: March 08, 2015, 10:16:49 am
content: QuoteThat sounds interesting, how have you gone with this have you done it? what size? How did you stop the ic from moving around due to heat or other? etckv

author: MrAl
author_rank: Super Contributor
timestamp: March 08, 2015, 11:47:07 am
content: Hi,It would be interesting to see if it would work if you used thermal adhesive, like Arctic Silver epoxy.You'd have to ensure the electrical connection too though.Alternately some sort of clamping mechanism with thermal paste.

author: paulie
author_rank: Frequent Contributor
timestamp: March 08, 2015, 01:35:34 pm
content: Quote from: Monkeh on March 08, 2015, 08:11:44 amLOL Typical internet naysayer. Unlike balanced views from guys like ccsayer who do take into account actual power requirements.

author: Monkeh
author_rank: Super Contributor
timestamp: March 08, 2015, 01:50:36 pm
content: Quote from: paulie on March 08, 2015, 01:35:34 pmI look forward to seeing your ~1MHz integrated switcher with no thermal slug.These are power devices, the thermal pad isn't for show.

author: paulie
author_rank: Frequent Contributor
timestamp: March 08, 2015, 03:20:14 pm
content: Quote from: Monkeh on March 08, 2015, 08:11:44 amMaybe some "butts".LOL Typical internet naysayer. Unlike balanced views from guys like ccsayser and AcHmed99 who do take into account the applications actual power requirements.BTW since there are several ground pins on the device unlikely the pad is needed for electrical reasons. If I were prototyping for a few hundred milliamps I'd probably not bother with the pad. Full 1.5a may be different.

author: Monkeh
author_rank: Super Contributor
timestamp: March 08, 2015, 03:23:01 pm
content: Quote from: paulie on March 08, 2015, 03:20:14 pmBroken record.Just for the record, broken as yours is, I've recently built a switcher using a 3x3 DFN. With the thermal pad soldered, and three layers of copper attached to it, it still gets quite noticably hot. ... and it doesn't have the switches integrated.The pad is required to be soldered to meet thermal and mechanical specifications. The device will not perform as advertised without it. It really is that simple.

author: Nerull
author_rank: Frequent Contributor
timestamp: March 08, 2015, 03:24:16 pm
content: The OP asked what can happen, not what might work if you really really careful and only use the device for a tiny portion of its rated current.

author: paulie
author_rank: Frequent Contributor
timestamp: March 08, 2015, 04:33:02 pm
content: LOL Typical internet naysayerS. Unlike balanced views from guys like ccayser and AcHmed99 who do take into account the applications actual power requirements. YouASSume too much, when without actual input from OP, you make absolute statements like:Quote from: Monkeh on March 08, 2015, 08:11:44 amBecauseIFdissipation is not too high no pad is needed,ANDwhen dissipation is low enough the pad is not required,BUTunless power consumption warrants the pad may not be necessary.So w/o input it's just as ridiculous toASSume maximum consumption asASSuming it's 1ma.BTW have I ever mentioned there are guys like ccayser and AcHmed99 with more balanced views who take into account what actual power requirements might be. Don't get me wrong, without naysayer/irrational posters (and sidekicks coming to their defense) internet would be no fun at all.

author: Monkeh
author_rank: Super Contributor
timestamp: March 08, 2015, 04:37:46 pm
content: Quote from: paulie on March 08, 2015, 04:33:02 pmDo you have anything to contribute, or is your sole purpose here to insult others?

author: Nerull
author_rank: Frequent Contributor
timestamp: March 08, 2015, 04:42:55 pm
content: Quote from: paulie on March 08, 2015, 04:33:02 pmI don't get the feeling you have a particularly strong connection to rationality.

author: paulie
author_rank: Frequent Contributor
timestamp: March 08, 2015, 04:48:27 pm
content: Quote from: Monkeh on March 08, 2015, 04:37:46 pmWell I think my contribution is that there may be some cases where soldering the pad (specially if it's difficult for some builders) is not needed rather than making misleading blanket statements that no matter what that pad must be soldered.Unlike some, I can't make any more concrete suggestions without knowing at least approximately what power requirements are.Quote from: Nerull on March 08, 2015, 04:42:55 pmWhy am I not surprised you might have that opinion? I do love the internet.

author: Monkeh
author_rank: Super Contributor
timestamp: March 08, 2015, 04:54:26 pm
content: Quote from: paulie on March 08, 2015, 04:48:27 pmIn the case of an integrated switcher like this I see no rational situation in which you could get away without the thermal pad being soldered. The same goes for non-integrated switchers, MOSFET drivers, MOSFETs.. Power devices in general. With some examples (low frequency FET switching at severely derated current) yes, you can get away with it..In any case the pad should be soldered properly for mechanical reasons. But if going "I'm right! I'm right! These guys are assuming and misleading! I know better!" on the internet makes you feel good, you go for it. You know what they say about arguments on the internet.

author: ajb
author_rank: Super Contributor
timestamp: March 08, 2015, 04:59:25 pm
content: Quote from: kvresto on March 08, 2015, 10:16:49 amI've done it. Nowadays I'd stencil it, but it works okay. You'd start by soldering all of the other legs of the IC, just as if it didn't have a thermal pad. That's what keeps it from moving around. The hole in the thermal pad should be ~2mm or maybe 3mm if you can fit. The trick is to be sure that the solder wets to the pad on the IC. Drop a little bit of flux in the hole, then put your iron on the edge of the hole, so you can see that pad on the IC. Feed some of your skinniest solder in to the hole, and try to get it down to where the edge of the hole meets the pad. When you see the solder wet to the pad, you can then feed in more solder and fill in the hole.

author: kvresto
author_rank: Regular Contributor
timestamp: March 09, 2015, 11:35:32 am
content: shiiiiit!! nothing like a robust discussion. Max current I'm thinking is around 0.4A, max excursions to 0.8 or so every so often. My issue is these damn powerpad chips are a pain. ajb has a good idea, but again some of those ic's in sot/msop/qfn packages are just too small. I might have to get the correct tools, and try a heat gun.

author: Monkeh
author_rank: Super Contributor
timestamp: March 09, 2015, 11:38:53 am
content: Quote from: kvresto on March 09, 2015, 11:35:32 amAt what input and output voltages?

author: kvresto
author_rank: Regular Contributor
timestamp: March 09, 2015, 11:54:06 am
content: VIN=max ~14.2V, VOUT=5V.

author: Monkeh
author_rank: Super Contributor
timestamp: March 09, 2015, 12:03:45 pm
content: Quote from: kvresto on March 09, 2015, 11:54:06 amSo something like half a watt dissipation at 400mA, just in switching losses. The package is 48.2C/W junction to ambientwhen mounted on a sea of copper several times the package size.

author: ConKbot
author_rank: Super Contributor
timestamp: March 09, 2015, 02:14:27 pm
content: Quote from: Monkeh on March 09, 2015, 12:03:45 pmHey no worries! Hes only driving it tohalfnormal output curent for an unspecified period of time on occasion, I'm sure the QFN 16 package will have plenty of thermal inertial so it wont overheat in a split second. And dont even worry about doing something simple like

author: kvresto
author_rank: Regular Contributor
timestamp: March 09, 2015, 09:50:31 pm
content: ok, the video shows something I can do on my bench if I get a board made up with plated vias. I've been making my own 2 layer boards in my lab, and I don't have plated vias, I think the board would smoke before enough heat transfers over the other side.kv.