# Reflow Soldering Tutorials

#### fsonnichsen

Joined Jun 6, 2013
49
I am considering buying a reflow oven for our lab. Apparently some of the better ones are less than $5K and with 40 day turnaround on boards these days it makes sense for us to have this capability. I still hand solder down to 402 and TSSOP but it is slow and I use more and more QFN these days. In addition many datasheets these days explicitly prohibit hand soldering. Like any craft there is a lot to be learned regarding details such as paste types, heating curves, stencils, paste dispensers etc. I was casting about the web and amazon-books for some rather detailed information on this topic but didn't find much. Perhaps the oven vendor manuals provide this information. Can anyone suggest any books or online PDF tutorials on this topic? Thanks Fritz #### jpanhalt Joined Jan 18, 2008 11,087 On the one hand, you have the oven. I don't know anything about them. I bought a cheap ($100) T-962, modified per "standard" practice, and ran some profiles, including tests with a couple of unused boards and Kester EP256 solder paste. It worked, and the joints looked good. I have no advice for a commercial unit.

On the other hand, there are all the variables associated with the board. One-sided or 2-sided? Cream (solder paste) aperture compared to pad size and stencil thickness. The Internet is full of opinions -- a lot of them from anonymous sources. I called OshStencil (oshstencil.com) and made my order according to its recommendation: 3-mil SS stencil, solder paste aperture = pad size. Pad sizes were as recommended by the chip manufacturers. After seeing how things lined up*, I decided to make a manual stencil printer out of aluminum rather than depending just on tape and a flat surface. Got everything together, and then Spring came. Yard work, Winter clean-up, and so forth have kept me too busy to touch it.

Maybe these two posts will help "prime the pump." I suspect there are members here with real experience.

John

*Smallest IC's were TQFP and smallest pitch device is a 0.5 mm FPC connector. I expect more problems with the latter.

#### Irving

Joined Jan 30, 2016
3,301
I also use a 'hacked' T-962 or similar look-a-like. I use KiCAD to lay them out and use the default sizes for everything. I now get my PCBs from jlcpcb (other PCB manufacturers are available) because they are super cheap and super-fast. I learn't very quickly to always order a stencil, its crazy not to. Initially I tried making my own on a laser cutter but its just not as good. Admittedly most of my boards are < 100 x 100mm and < 200 parts or so but done a few larger ones with ~500 component. For some larger/more complex jobs I had jlcpcb assemble them too and it turned out pretty good (the boards came back in 7 days, but you must use parts in their catalog - would have taken me three or four times as long). Worst IC on own assembled boards was narrow 48pin 0.5mm TSSOP. Biggest reflow problem a SM JST connector that insisted on melting instead of reflowing, think it might have been fake as next batch from different vendor were fine.

There are 100s of videos on YouTube regarding reflow and stencil techniques. Most important things I've learnt are keep paste in fridge until a couple of hours before use, make sure the stencil is in direct contact with pcb (don't tape board down then put stencil on top) - I now use a jig to clamp board & stencil, dont put too much paste on, squeegee paste in one pass and don't go over it a second time!

#### jpanhalt

Joined Jan 18, 2008
11,087
@Irving
Melting the Molex 0.5 mm connector is my other main concern. Some users report better luck with JST. I intentionally chose one I have hand soldered in case that happens.

As for the stencil, I wanted SS, and when I compared the prices at JLCPCB, shipping of a SS stencil was a killer. Also, plated slots using Eagle 7.x at JLCPCB is still an unanswered question.

#### Irving

Joined Jan 30, 2016
3,301
True, it adds £14 (stencil + additional postage via DHL Express) so if you're using the special offer of ~£17 for 5 off 100 x 100mm PCB its a relatively big jump (but at £31 it still overall quicker & cheaper than say Multi-CB from Germany at £54), but go to 10 off 200 x 100mm boards and its a much smaller %age.

#### fsonnichsen

Joined Jun 6, 2013
49
Thanks for the tips.
Looks like Yahoo has pretty good hits compared to google. I'll keep that in mind.

I had been using MacroFab to do all my boards offsite in about 15 days--single boards, around $150 each. Their service was great but lead time crept up to about 20 days. I attributed that the the Coronavirus. Then one morning I got up for a quote from Macrofab that is usually around$120 and it was $1200! They explained that they had "changed their model". Now -- I still prefer to go offsite to have my boards done, but when duilding a 5 board system and maybe 3 recuts per board I am up to$15K for a project that usually costs me around $1000. That is is just to much- I work in a small physics lab and we do mostly 1 offs. And with my limited engineering/drafting skill on KiCAD I usually need at least on re-cut and maybe two. Then the other scientists always want "just a little change" here and there so one-offs are a way of life in our lab. I started looking for a new assembled board vendor-JCLPCB looked promising but when I tallied up I found they would only use in-house parts. That explained their quick turn around. Most of my parts are not in their inventory. It is apparent that for reasonable turn around, going overseas doesn't work. Apparently the overseas suppliers still go to Digikey here in the US for parts and it adds about 10 or more days. If you have a long time--up to 40 days, it appears that PCBway and ALLpcb have reasonable prices. I never could figure out the T-962. Who makes it? It is a great start and I would be willing to pay about twice that if they would just fix it up and give a vendor name and email. I really don't have time to play with it at least now. The tips on solder stencil are appreciated--I have used them before and they are finicky. I think some vendors making solder-paste microscope/syringe combos out there that I would like look into. I always hand solder my own connectors even when I send the boards out. Those blasted Molexes melt even when I use my iron. I just tell the other guys that this is my "Salvidor Dali" art connector. So-for this month I have recast my boards in SOIC and will get the boards cut in a few days using ExpressPCB--always relied on them. I will look over the Yahoo data and see if I can come up with a plan for the future. Thanks All Fritz Last edited: #### jpanhalt Joined Jan 18, 2008 11,087 If you go the T-962 route, you probably want the T-962A, which has a bigger drawer and a 3rd heater (I think). Mine (T-962) says it was made by Puhui. I have seen that name quite a bit and sometimes with another name in front. As usual, pictures and products don't always match. I did see an ad awhile back about an"improved" version. No info on what, if anything, was fixed. I suspect company support is very limited. Adding the cold-junction reference temperature and new software is quite easy, once you get your programmer to recognize the chip. If you go toaster oven, you might check out the RocketScream controller (https://www.rocketscream.com/blog/product/tiny-reflow-controller-v2/ ). It's from a small shop in Malaysia. Shipped quick, and a test program seems reasonable. Not very sophisticated. I got it for the T-962 in case I had insolvable problems with updating its software. John #### Irving Joined Jan 30, 2016 3,301 Hi Fritz The T-962, like many items from China, is re-badged & resold by so many outlets I doubt anyone knows now who originated it! In the UK at least, we have a company ES|Technical, who import and upgrade the T-962 with their own controller. They ship worldwide and provide worldwide support as well. I'm surprised there isn't anyone in the US doing the same... OK, its not as cheap as buying one from China and upgrading yourself but its still very competitive compared to other products. I bought one (A+ model) for our lab, the DIY route wasn't an option if I wanted to get it funded, but the available commercial alternatives were way outside our budget. #### MrSoftware Joined Oct 29, 2013 2,069 The place I used to work had one of those small reflow ovens and there were hot spots and cold spots in the drawer, so it took a little trial and error to figure out how to use it. Thread Starter #### fsonnichsen Joined Jun 6, 2013 49 Thanks for the further replies. The T-962 seems to be ever present in the reflow field so I will definitely look into modifying one or buying the refab from the UK. When you consider that vendors are now asking over$1000/board here in the US the price tag of a reconstructed oven is not bad. Maybe I will even find time to rework it myself.(yeah--right!)
Per MrSoftware, the uneven heating seems to be a commonly mentioned problem with these devices. I have a bunch of thermocouples around here somewhere and it would probably make sense to profile the oven. (I once heard of people doing this on their microwaves using a piece of web paper towel!). It may make sense to install a gentle blower in the system to even up the heat-
thanks all
Fritz

#### jpanhalt

Joined Jan 18, 2008
11,087
The T-962 has 2 thermocouples, one situated about midway on each side (left and right). The software allows independent adjustment of gain (slope) and offset (intercept) for each. I don't know how linear they are, but compared to my older Fluke TC meter, which I calibrated against a mercury thermometer, I could get relatively good agreement (ca. 5°C) side to side. Front to back and bottom might well be different.

One of the modifications is to place the board being reflowed on standoffs from the tray (ca. 1/2") to allow circulation of hot air on the bottom.

The RocketScream controller and perhaps the UK board use the MAX31856 digital and linearized TC amplifier.