Soldering an MSOP10 with thermal pad....no heat gun available - Page 1
Forum section: electronics / general

author: Faringdon
author_rank: Super Contributor
timestamp: December 16, 2021, 07:36:07 pm
content: Hi,Ive got to solder a MSOP10 (TPS5401 with thermal pad beneath chip) but there is no heat gun, and one won't be purchased.TPS5401https://www.ti.com/lit/ds/symlink/tps5401.pdf?ts=1639653876953&ref_url=https%253A%252F%252Fwww.google.com%252FThe PCB is double sided and 5cm x 1.5cm. I can put it on a heat plate but dont have one, and one will not be purchased. Therefore, i have made a "heat plate" out of a 6R8 metal chassis mount resistor with 4.8A flowing through it, and can rest the PCB on that. (a pair of pliers will also rest on the PCB to kind of stabilise it on the resistor surface, and stop it moving whilst i solder it.)The way i am soldering the underside thermal pad is as follows... There is a thick GND copper pour emerging out from each end of the chip, so i have scraped the older resist off these areas, and will use a soldering iron at each end of this GND pour (so 2 solderign irons simultaneously) , to seep heat into the underside pad (from each end of the chip) so it solders to the thermal pad (first wet the copper foil with solder).Then , once the thermal pad is soldered, i will blob solder over all 10 pins. Then i will mop the solder up with braid (with the PCB still on the hot plate), and hope that i can mop up the excess solder from each row of pins without desoldering them.What do you think?

author: brabus
author_rank: Frequent Contributor
timestamp: December 16, 2021, 08:25:37 pm
content: Quote from: Faringdon on December 16, 2021, 07:36:07 pmIf it's a prototype, duh: whatever floats your boat. I generally prefer the dead bug soldering in these cases.Regarding the methodology: you will not get nearly enough heat under the chip. Best guess: you will burn the substrate or lift the copper from it, and the chip will still not be soldered. Try some lower melting temperature solder, it sometimes helps a lot during prototyping.

author: jpanhalt
author_rank: Super Contributor
timestamp: December 16, 2021, 08:47:55 pm
content: Schmarboad (https://schmartboard.com/schmartboard-ez-qfn-40-pins-0-5mm-pitch-2-x-2-grid-202-0044-01/) makes a QFN BOB. It adds a hole under the chip for soldering. I have used Schmartboards, but not the QFN ones.A couple of years ago, I considered doing that, but bought a cheap hot air station instead.

author: Faringdon
author_rank: Super Contributor
timestamp: December 16, 2021, 08:52:54 pm
content: Thanks, why will it lift the copper off? Also, the PCB has no under-hole, so i cant do that malheuresement.

author: SMTech
author_rank: Frequent Contributor
timestamp: December 16, 2021, 08:55:33 pm
content: Quote from: Faringdon on December 16, 2021, 07:36:07 pmIf the thermal pad has a thermal via to the other side of the board, that would be your way to reflow it with an iron..However a hot air station is a cheap and invaluable tool, one of those Aoyue ones can be picked up for peanuts and is really quite effective

author: Faringdon
author_rank: Super Contributor
timestamp: December 17, 2021, 05:27:03 am
content: Thanks, it has four 0.3mm thermal vias, but with the board being 1.6mm thick, i still think this seems difficult, to be sure of reflowing through those vias?

author: SMTech
author_rank: Frequent Contributor
timestamp: December 17, 2021, 08:30:33 am
content: stick an iron on it and find out

author: Faringdon
author_rank: Super Contributor
timestamp: December 17, 2021, 09:00:45 am
content: There is, i am told, a surface mount oven available at another location.To use this, do you need solder paste?..And apply it with say a pin head to the pads?I dont have a MSOP stencil for the board.Supposing i apply solder paste? do you think inter-pin shorts will tend to un-short themselves when its in the oven?

author: SMdude
author_rank: Frequent Contributor
timestamp: December 17, 2021, 11:26:24 am
content: If you apply a thin line of paste across the pads, if you are lucky, the inter pin shorts will fix themselves as the solder flows onto the pads.But seriously, just a cheapie hot air gun is all you need.I have an old kada 858D that someone gave me 8 years ago and it had had a decent amount of usage when I got it, it's still going strong. You can get these or the "modern day" equivalent for not much at all.It just needs to blow air and have adjustable fan and temp.Heck, if you have a hot air gun, you can use that!For something like the chip you are doing, I'd put some solder on the pads and a little bit on the centre pad(you don't want too much), hot air to get the solder melted, flux on the ic, hold it near the hot air stream to preheat, then when the solder on the pads is melted, drop it on and poke it about with the tweezers to be sure the solder has flowed properly.Go up to the bean counters office and take all their modern equipment and then tell them to hurry up and get their jobs doneSeriously, not having a hot air station in you line of work is just plan stupid.You have put more time into thinking of alternate ways to get that chip on the board than what a hot air station costs and you could have had it on the board before you had the time to make your first post about it.Sometimes I'm sure management just likes to be difficult because they are wankers!

author: Faringdon
author_rank: Super Contributor
timestamp: December 17, 2021, 03:17:02 pm
content: QuoteThanks, do you think it would be better to use solder paste? (assuming that we will get a hot air gun and hot plate).....since wetting the MSOP10 pads with melted solder wire and a solder tip , may result in them getting taken off the board. Or is solder paste just generally less good than solder from solder wire?Supposing we had an Infra-red reflow oven like this..https://montaz-elektroniki.pl/wp-content/uploads/2017/04/TUser-Manual-of-T-962C.pdf..Do you think it would be best to use this with solder paste? It only goes up to 280degC, and i wondered if that will melt the solder on the thermal pad? I mean i know its above the melting point of solder, but what if the PCB doesnt really heat up enough?The thing about Infra-red ovens, is that there are no "properly commercial" ones.......so it makes you wonder if they are any good?This video says IR solder ovens are useless...https://youtu.be/nMv4O8SxiyY...but this video says the same make of IR solder oven is really good...https://youtu.be/cP2SYn7XFl4Who to believe?Also, at 10:37 of this video......it says that the solder resist will help undo shorts which appear between pads after stencil application of solder paste......on our MSP10 footprint, there is no solder resist between the pads....therefore do you believe that stencil and paste reflow of our MSP10 will be a waste of time?...and that our only option is to use hot air gun , and then mop up the excess solder with fine braid?

author: thm_w
author_rank: Super Contributor
timestamp: December 17, 2021, 10:23:58 pm
content: Wetting the pads with melted solder is fine if you use a reasonable temperature. Add paste flux after.If you want to know that you've melted the solder on the thermal pad properly, solder just the thermal pad, then wait until it cools. Look at the side, is it flat to the board? Pull up lightly on the chip, does it pop off?

author: SMdude
author_rank: Frequent Contributor
timestamp: December 17, 2021, 10:45:57 pm
content: You'd have to be pretty incompetent to take the pads off the board while wetting with solder.I do it all the time, just wet the pads so there is a thin bead of solder on them, heat it, drop the chip on.Any bridging, flood with solder and wick it away. A heat gun will assist in this process too. I couldn't work for the people you do, I would lose my sh!t! The older I get, the more intolerant I become.If you do get the oven, then solder paste, as you can't guarantee that the ic won't move when the board is moved into the oven.The oven will be fine, I use a modified t962a and once the profile is tuned properly it will solder anything.Just FYI, it sounds as though my backyard "lab" is much better equipped than that of the company you work for. Make a point of it! This is the problem when the management have absolutely no idea about the processes involved in the company they are "managing".