Creating reflow oven to build printed circuit boards

Printed circuit boards have evolved from using components whose pins or wire leads fit through a hole in the board, to use of surface mount devices that do not require any hole. These permit boards to be composed of many layers practically, because the pins/wires of the devices are not interfering with any of the layers other than the top one that contains the component. This has been accompanied by the continual shrinking of the size of these components, all of which makes hand-soldering more and more difficult. Since the mainstream uses these surface mount devices (SMD), manufacturers of chips and other parts have shifted to SMD. It is impossible to get certain chips in a traditional, non-SMD packaging. Such is the case for a few chips that were ideal for use in my project.

Factory production of PCBs make use of reflow, where all the components are soldered to the board in a single pass. In one method, the devices placed on their intended copper connection pads with a sticky solder paste, then heated in an oven with a careful ramp up and cooldown process that minimizes thermal shock to the parts while producing excellent connections. The boards are placed in a reflow oven to heat and cool them. Commercial ovens are many thousands of dollars, but hobbyists have found ways to produce workable ovens for under $100 - I took on such a project to deal with the boards I am creating for the 1130 project.

I leveraged an old case I had in my garage, placing the circuitry for my oven into it. I bought an inexpensive 1500W toaster oven to use - the boards and solders contain lead and other elements that are not good to ingest - even being absorbed from the fingers at low rates. This will never be used to prepare food, because it is now contaminated by lead, bismuth and other metals.

My reflow oven and controller
I inserted a temperature probe (thermocouple) into the oven and removed the power plug from the end of the cable. In the old case to the left, I inserted a PID controller, SSR and the power cable from the toaster oven. I wired them up and put on a power cable with plug. The PID controller is a unit that will get the oven to a selected temperature and hold it there very accurately.

The name PID stands for proportional integral derivative - but the essence is that this is a smart controller that looks at feedback from the probe and figures out how to provide power to the oven to reach and hold the temperature.

PID controller to hold the oven at a target temperature

The SSR is a solid state relay, a device that will switch the power to the toaster oven on and off as needed. In operation, the PID is usually pulsing or cycling the power to the oven, which you see by the light on the oven flashing on and off.

As the oven approaches the right temperature, the light flickers on for shorter and shorter periods. If it simply stayed on till hitting the temperature, the glowing heating elements in the over would keep adding heat as they cooled, pushing the temp inside the oven way over the target before the elements cooled all the way down. The controller autotunes itself, learning how to 'sneak up on' the right temp by lowering the heat of the elements at the right rate.

I put a thin layer of solder paste on the copper pads where devices should be soldered. The device plops into the paste and is held on the board by the stickiness of the gooey paste. The board goes in the oven and begins heating. When the paste gets hot enough, the goo part, which is called flux, melts and evaporates. This cleans off metal corrosion and dirt from the metal surfaces, ensuring that the solder can bond. The heat goes up gradually enough to make sure that the flux is fully evaporated before turning the metal part of the paste into flowing solder - otherwise air bubbles might cause poor solder joints.

The solder paste then reaches its liquification temperature and flows. Surface tension of the liquid solder causes it to form a bulging surface on the copper pads and device pins, while sucking away from the non metal part of the board. This puts the solder on each pin where it belongs and not bridging between the pins. As an added bonus, the power of the surface tension will pull the component into alignment, so that it does not need to be placed with extreme precision before soldering.

The designer of a PCB will usually have a 'stencil' made from the design of the top layer - generating a plastic sheet with openings where paste should be applied atop copper pads. This is placed atop the naked PCB, paste is then applied with a squeegee like tool (like a putty knife) to produce the right depth of paste on each pad. This adds $10 or more to each board designed, thus some will apply the paste without the guidance of a stencil.

I bought some junk circuit boards and surplus SMD chips at a local electronics shop to practice with, and have worked out the right amount of paste and timing for my oven. This was tricky to get right. Too much paste and a big ball of solder formed, which would bridge more than one pin if the SMD device had very small, closely packed pins. Too little and it didn't reliably form solder joints nor pull the device into alignment.  Fortunately, the minimum amount is not that sensitive, even pretty thin swipes yielded satisfactory joints as long as the device was fairly well aligned.

Not using a stencil produces some waste goo placed on undesired spots of the PCB, which yields teeny dots of solder to flake off after using the oven. Since a 5cc syringe of solder paste is about $15, waste costs money as well as inconvenience. If solder bridges do form because of too much paste, they can be fixed by a process called 'reword' where you place a copper braid onto the blog while heating it, the copper braid will suck off the excess solder (wicking it up the same way that the wick of a candle will draw molten wax up to the tip of the wick where it burns).  One tries to minimize the need for rework, as each application of heat shocks the parts and potentially could lead to some damage to teeny pins if I move them too much during fixes.

Another interesting problem concerns boards that are designed to mount parts on both the 'top' and 'bottom' faces of the PCB. If you solder one side, then flip the board over and solder the second face, the solder on the bottom is becoming molten again during the second pass in the oven. The parts might drop off or shift out of alignment. This is solved various ways in production, often by putting a tiny dot of epoxy glue under each component that will go on the 'bottom' face when the board is soldered a second time. The heat of the oven causes the epoxy to harden at the same time that the solder is molten, thus the part won't fall off even if the solder is melted the second time in the oven since the glue stays firm.

I have a different way to approach this, based on the availability of different solder pastes with different temperature profiles. My plan is to solder the 'bottom' face components using a paste with a higher melting point. When I flip the board to solder parts on the other side, I use a lower temperature paste. If the oven never gets hot enough to melt the solder on the bottom parts, then they will not fall off! Commonly available pastes have more than 200 degrees F difference in melting points, far more than the inaccuracies in my temperature control of the oven.

Now I am ready to solder the parts onto my 1130 printed circuit boards using the low temp paste, as soon as they arrive from the PCB manufacturer. With some practice learning how to use the normal (high) temperature paste, I can do the double sided component placement for my 'input controller' board since I put integrated circuits and other parts on both faces to simplify the wire routing.

My first PCB arrived today - it was a 2 day turnaround which skipped several steps that the others will include, such as the green colored 'solder mask' that insulates and protects all but the copper pads where parts are soldered, and it does not have the printed outlines and labels for parts (silkscreen layer). Functionally, it is fine for the purpose - it is a card that I will install inside the typewriter to interface it properly to my system. It has drivers that will activate the solenoids under control of the FPGA board.

My solenoid driver board as it came from the fab

I installed the parts (other than the connectors for the edge because they would make it hard for the PCB to sit evenly in the oven), using the solder paste, then put it into the reflow oven and presto - perfectly soldered board.
Parts pushed into solder paste and the board is being heated
Unfortunately I had miscounted my parts and was short two resistors, so when I pick some up tomorrow I will hand solder the remaining two onto the board. The resistors are about the size of a flea, very easy to lose and challenging to pick up with the tweezers and put into place - think about how hard it will be to solder it on.
Done with heating cycle, everything soldered down
The connectors were placed once the board cooled down, and soldered in, after which I did a full test of the connections. Everything looks great!

Board finished except for the missing two tiny resistors
After I bought some extra resistors, I hand soldered them and had a completed board. The resistors are the small rectangles you see to the side of the transistors which are the larger dark rectangles. They are normal size for surface mount resistors, a mere .08" long by .05" wide, but the parts can be as small as 1/100 of an inch long. Tweezers and a steady hand are necessary to work with them.

It was plugged into the typewriter and I am beginning to run some candidate logic sequences to verify that I am driving the operations correctly, such as carriage return, line feed, shift case, tab, and of course typing letters. So far, it is working exactly as I expected. Need to complete my fpga logic module then kick off detailed testing.

 Sometime this weekend I will spray the board with a protectant seal, using a silcone conformal spray, similar to the solder masks on modern boards but also protecting the components on the board. That will keep it in good order when mounted underneath the typewriter mechanism, subject to vibration, heat and grease/oil drippings.

Finished board with coating, ready for installation
The board is mounted on the underside below the space where the carriage travels, in the place that the IBM logic board was mounted before I began the conversion. Besides the cables connecting the board to the typewriter mechanism (most are connected in the picture below), there are several cables that will link this to the FPGA 1130 board and power feeds on the side delivering clean 5V and 12V power for use by other logic boards.
My board installed in place of IBM E50 logic board

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