SMD assembly: Hot air, plate or oven?

Manufacturing
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I currently work for a (very) small manufacturer, about a dozen employees in total.

For several years we have successfully used an electric skillet as our “tabletop reflow oven”. (Here’s one, of several, online pages discussing the method.) There has been a fair amount of cut-and-try to the process, but we made it work and you can’t beat the price. (Our skillet was US$5.00 in a second-hand store.)

Our boards range from about a dozen parts, up to around 100 parts. Passives are 0805 or 1206; semiconductor packages have lead pitch down to 25 mils or 0.65mm. (But it is MUCH easier to work with 50 mils and 0.95mm.) Hand-placed parts want to have a larger courtyard area than automated placement.

We typically do boards in batches of 5 to 20 units. Depending on size (obviously!) there is room in the skillet for 2 to 8 boards at a time. We actually stumbled onto the skillet method by accident, after having problems doing hand-soldering of a chip with an exposed back-side thermal pad. Even using two soldering irons at a time, we couldn’t get consistent, thorough, solder-wetting of that thermal pad. Since the skillet heats the entire board plus all the components at once, soldering the thermal pad is not a problem.

It’s important to be consistent and uniform for the process to work. Definitely apply solder paste with a stencil, and be prepared to try different stencil thicknesses and/or stencil-shrink parameters. Our solder is Kester EP-256 from CML supply. The specified shelf-life is 6 months under refrigeration; we get about 2 years when it’s kept in a desk drawer (and rejuvenated once or twice with a squirt of isopropanol).

A glass cover on the skillet is not essential, though watching things happen speeds up the initial process calibration.

Half an inch (1 cm) of fine sand in the skillet improves uniformity of heat distribution.

We pass everything under a 10X binocular microscope to find bridges and dry joints. Decent wide-field microscopes start around US$200 new; used units from top-line manufacturers (Nikon, Bausch & Lomb, et al) are sometimes as low as US$100.

Dale

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Do you ever do pcbs with parts on both sides? If yes, go to the “toaster oven”. I put parts on 1 side and toast the board. After it cools, I pasted, place and toast the other side. You don’t dare touch the oven or pcb until it has cooled down. Otherwise the bottom parts get knocked off.

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What method would you recommend to a newbie to find this “sweet spot” on the temperature dial ?

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Can you recommend vendors for these ?

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I’ve not had good luck finding a hot plate that gets hot enough, so I use the hot air method. I bought a Dremel drill press and modified it to hold my hot air unit. I use a large nozzle and slowly lower the nozzle to the board in some sort of emulation of a soldering profile (in my imagination, but it seems to work). Rarely blow a component off the board. Working with lots of 0402 parts, and with a bit of practice. it goes quickly. I’ve built about 200+ boards that way with good success.

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Does the nozzle reflow solder on more than one component at a time, or do you repeat the process for each SMD ?

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About 3 years ago, my employer bought a binocular microscope from “Amscope”. It has a 1X objective lens and two pairs of wide-field oculars, 10X and 20X. The microscope head is mounted on a boom arm, cantilevered out from a weighted base.

We have used the 10X magnification almost exclusively. I recall only two or three times when I popped in the 20X oculars, to get a closer inspection of a potential problem after locating it under 10X magnification. If I could change any feature of the optical system, I’d ask for a wider field of view, or possibly greater depth of focus. Even those would be “convenience” or “wishlist” requests, not really a shortcoming of the instrument we have. One thing I WOULD like to have is a diffused lighting source, rather than the single-point illuminator.

You definitely want an extended boom-arm stand, or possibly an articulated arm. An articulated arm would let you fold the microscope up out of the way when not being used, but I wouldn’t put out a lot of money for an articulated arm until I was convinced it doesn’t have problems with vibration or jiggles.

I don’t know what the working distance (from the objective lens to the target workpiece) is, but there is adequate clearance for my fumbly fingers, small tools, meter probes, soldering iron, etc.

A photographer may be critical of our microscope’s optical performance but I have no complaints. Focus seems to be flat across the field of view, I haven’t noticed any geometric distortions, nor have I seen any color fringes or halos around sharp edges. Of course, my super-annuated, tri-focaled eyeballs may not notice such things unless they are very severe.

I believe the company’s bottom-line cost was right at US$200, after shipping charges (the thing weighs at least 10 lbs (4.5 kilos)!) and a new-customer discount. Amscope offers several hundred models, with various combinations of magnification, stand styles, illuminators, CCD camera mounts, etc. I have also noticed several vendors offering microscopes with very similar appearance to Amscope’s products. I don’t know if these are identical instruments with different marketing nameplates, or different performance grades of the same design, or significantly different designs in similar enclosures. And, of course, there are the “usual suspects” offering used/surplus/pre-owned equipment.

Dale

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I used the largest nozzle I have, and once the board is sufficiently pre-heated I bring the nozzle closer and watch as the paste begins to coalesce. Once the melting has started, I move the board and nozzle slowly toward more components. The larger nozzle usually covers multiple passives or one side of a chip at a time.

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I have read that amateur solution for thermal pads are just big plated holes under it and soldering it from backside.

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As I recall, that design had copper zones on both sides with a lot of stitching-vias linking them. We tried several approaches, using multiple soldering irons and heat guns, but couldn’t get reliable, consistent results. Using the skillet to solder that one IC - the rest of the board was still thru-hole parts - made all the difference. We gauged the effectiveness of the soldering job by observing how the paste solder flowed through the stitching vias to the back side.

Dale

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From what you write looks that you didn’t tried to replace lot of stitching-vias with one 3mm hole to be fully soldered from backside.
But I only read that it is the solution. I have never tried it myself.
We were soldering our products ourselves till 2005. At the beginning (88-96) manually and later using semi automatic placement and simple oven (but destined to SMD).
Our oven didn’t had any inspection window so we couldn’t see soldering process. We used a thermocouple (made of very thin wires) inserted in soldering paste at element pads and registered the temperature during the process and based on it we setup our oven parameters.
In 2004 we tried to order PCB production at contract manufacturer who not needed the orders to be 10 000 pcs or more but was able to do 50 pcs for us. We found that calculating our time used for element and PCB ordering and then soldering everything ourselves using external service costs about the same or even a little less. During 2004-2006 we gradually moved all our production outside.

We are very small (without “()” around very). 7 people working, from that 2 in direct manufacturing.

Using external services has one more advantage. In 2006 we got an order to do 800 pcs of that:
https://www.google.com/search?source=univ&tbm=isch&q=Dydaktyczny+system+mikroprocesorowy+DSM-51&client=opera&sa=X&ved=2ahUKEwiLp-3mgNvyAhUq8uAKHW43Cc0QjJkEegQIDxAC#imgrc=DWwSz1bEmY7R6M

If we were to do it on our own, we would not be able to meet the 3 months in which it had to be delivered.

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800 units in 3 months! That’s quite a complex board to build. 8051 training computer? I remember using something like that back in the 80’s.

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As a set to each education unit we also had to manufacture probably (don’t remember well) 5 external boards allowing for some exercises (some of them are visible at link I have given previously). We had designed 10 and I just don’t remember how many of them were ordered those time. So in total we had to deliver 4800 PCBs or more.
We got order in April 2006 and the original delivery date (30.06.2006) was ‘kindly’ shifted by officials to 31.09.2006. But in my opinion it is a perfect example of clerical stupidity. After all, everyone knew then that the RoHS directive came into force on July 1. The contract manufacturer with whom we have recently cooperated said he has not yet mastered the lead-free process and does not know if he will reach it by September. We also preferred to have it done in classical Pb-process as we were forced to give a 3-year warranty. So, our decision was - we have to do it before 1.07.2006.
We used other manufacturer for all those external boards. Our main job was to check and run all of these boards.
We were lucky to have just recently mastered outsourcing production to a contract manufacturer. If not, we wouldn’t have had a chance to complete this order (the biggest one we ever made).

Yes.
When we designed it (1993…1995) some schools in Poland used it without PC (they couldn’t afford computers). So we embedded into it simple assemble code editor (display has only 2 lines). You selected mnemonics and their parameters from the menu. Then you were able to run your program with full speed or step by step. There were two reset levels - absolute or ‘to your program’.
If you had PC you could use written by me 8051 assembler and environment allowing to debug your program.

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When I was young, I lusted after the Micro Professor 1

https://duckduckgo.com/?q=micro+professor+z80&t=hx&va=g&iar=images&iax=images&ia=images

These days I do not care a hoot about big / clumsy / expensive development boards that do not even have the circuitry on them that I want.

I do like the trend to small universal breadboard compatible modules such as the Blue & Black pills, Arduino nano, Wemos D1 Mini. Such modules are easy to get started with because the’re small & cheap. The breadboard lets you easily add your custom circuitry, and at the same time provides a good interface to add logic analyzer or scope probes.

And because those boards are so cheap and easy to work with big pitch, they are also Ideal for simple one-off projects on a piece of matrix board.

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Education needs are different than development needs. 10+ years ago I was speaking with my friend who is working at technical university. He said that he is using some of our DSM-51’s since 5 years and never had any problem with any one of them. But for other controllers they buy development boards and they have to replace 50% of them each year. They are simply not ‘student-proof’. And it is technical university - what about technical schools. He postulated that we should do something similar with other processors, but we didn’t have time for that.

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I think I had the wrong hat (or head?) on when I wrote my previous comments.

Part of education is breaking things, but some students seem to break things just for the fun of it, or because they don’t care.

Such small breadboard friendly modules are also cheap enough to have the students buy the parts, or you give them one at the beginning of the course, and they have to buy another if they break it.

I also do not understand why students are not teased to be a part of the educational system itself. For example, the design of a multiplexed 7-segment module or a button matrix are easy design projects to do as add-on to various development boards, and over time it will build op a library of cheap and easy to replace parts. It also gives the students lots of opportunity to mix and match modules for further projects.

I especially like the way Cornell University handles their FinalProjects. For example:
https://people.ece.cornell.edu/land/courses/ece4760/FinalProjects/

The students have to find a project, design it, make it work, make documentation for the website, and over the years it has become a collection of lots of fun projects.

I also do not know much about the educational market.
I may over estimate what beginning students cando with microcontrollers.
I started myself with programming a microcontroller via a few wires hanging of an LPT port.
Getting the first microcontroller to blink a led was quite an achievement.

There are also limits in amount of hours that students can work on such projects, and such complete PCB’s as you have may be a necessity to get students going in the limited available time.

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Sidebar:::::::

Nice collection of projects. I wonder what the licensing of those projects is. On the couple that I looked at I didn’t see any copyright (or copyleft) notices in case someone else wanted to use their work as a jumping off point for their own project/product.

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I just use a standard heatgun. Place the PCB in a panavise (the kind with the long metal board-holders) and heat the underside until you see the solder turn shinny and the components shifting around a little as the solder pulls them into their pads, remove the heat and you’re done. Works like a charm and miraculously does not damage the PCB, as long as you’re using quality boards from Osh Park, etc.

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Those time (1994) teachers even in electronic school knew very little about microcontrollers (at least here in Poland). They asked us to allow for HEX edition a program (without PC). We said: Yes, but not HEX editor but assembler editor and we did it just for schools having no computers.
Teachers asked for help how they should teach about microcontrollers so we have written a book which then in 1996 got the title “book of the year” from the best electronic magazine those time here.
We also didn’t had a big experience. I had got first microcontroller in my hands in 1987 I think.

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