How do I solder in the oven using the reflow method?

The following materials are important for assembly:

• Soldering paste for SMD soldering processes, with a soldering temperature of optimally approx. 140 ° C
• Syringe head for easier application of the solder paste on the pads.
• Adhesive tape (also works with stencil)
• Components that need to be soldered
• Flux
• Soldering iron
• Normal solder
• Ultrasonic cleaner or isopropyl alcohol

Not everyone has an infrared oven lying around for PCB assembly in the household, much less a solder bath where the circuit board can run through. Nevertheless, it is possible with simple means to assemble SMD boards yourself and even to provide the sufficient amount of solder paste without a stencil.

Of course, this method only works with simpler PCBs, where not too much has to be covered and only for smaller series or individual pieces. In all other cases, a stencil is helpful. But when you buy a circuit board online, you often don't get a stencil, or it is already worn out. That is why we use normal adhesive tape here, because yes, you can!

Step 1: The bare board

Don't get confused, PCB stands for printed circuit board and when we talk about PCB, board or mainboard we always mean the same thing, the board as shown below. The advantage of these printed circuit boards is that they always have a so-called solder mask. This means that the solder cannot adhere there, or only very weakly. So it is relatively easy to solder such a board in a reflow process using a commercially available oven, since the solder can only connect where it should. In this example we will solder a memory board for the MiSTer. The plugs are very easy to solder, they are soldered by hand in a conventional manner. We solder the memory chip in the reflow process. To do this, the PCB must first be completely cleaned. Dust and grease prevent clean soldering. So it's best to take with isopropyl alcohol and rub off the entire surface thoroughly and let it dry.

The bare board (PCB)

Step 2: The tape on the PCB

The memory chip should only be soldered to the legs of the IC and therefore we have to attach the adhesive tape so that only a very narrow channel of approx. 1mm remains open at the pins. It would have been better here if adhesive tape had also sealed the edge on the left and right, but I was simply too lazy, which is not a good thing! So it does not do exactly the same as in the picture, but tapes everything off, except for the pins that have to be soldered. Not every pin has to be cut out separately (that would be insane), but simply a strip across all pins.

The PCB with tape

Step 3: The tape in detail

Here you can see that the tape is neither straight nor really evenly stuck on. But that doesn't matter at all, because in the end there is so little solder paste on it that everything is put into perspective again. It is important, however, that unlike me, you also have to cover the edges on the left and right here. Ultimately, a strip should simply remain visible and open on all pins to be soldered.

The PCB with adhesive tape for SMD assembly

Step 4: Apply the solder paste

The solder paste is usually a little difficult to get out of the potty. I prefer to use the solder paste that comes in the syringe, where usually a thick needle is also attached. You can then simply use this syringe head to apply a thick sausage of solder paste to the pins. If you only have one potty, use a screwdriver or a thin plastic knife to lift out some paste and spread it over the tiller. It really takes very little solder paste! Anything you apply too much here can be thrown away afterwards. Please note that the solder paste has not yet passed its expiry date and it is best to keep the paste in an airtight container in the refrigerator, this will keep it longer. I already had solder pastes that dried out after 2 weeks, despite being sealed, it was just too warm. So never put the needle into the cooler or syringe, but put the original airtight lid back on!

Roughly apply the PCB with solder paste

Step 5: We flatten the solder paste

I often use a credit card, or a bonus card, or whatever sometimes ends up in advertisements. These cards are made of plastic, do not clean anything, have a straight edge and are very easy to wash off. With such a card, you can now simply press the solder paste flat. You will scrape a fair amount of paste off the PCB again, but that doesn't matter. It is simply important that in the end there is still some of this solder paste on it. In this example, I was relatively generous with leaving solder paste on. Much less would have been enough. But it shows that a slightly thicker layer also works.

Flatten the PCB with solder paste

Step 6: Carefully peel off the tape again

After the paste is flattened, we can remove the tape. Make sure that you don't get the solder paste on the tape either yourself or other areas of the board. There should only be solder paste on it where there is soldering. As already described above, I should have attached the tape on the left and right. Now you see that it was awkward not to do this. The solder paste will now also connect to the vias if we do this in the oven. So it has to be carefully removed by hand on the left and right. Finally, a thin strip of solder paste should remain on the pins.

The PCB with solder paste without tape

Step 7: The SMD assembly

There are probably many advantages to putting the memory on the board in addition to the solder paste before everything is baked. It is now very important to ensure that the SMD parts are correctly placed on the circuit board. With some boards, like this one, the orientation of the component is only visible in the unassembled state. You can see the point on the top right, then the point must also be on the memory. However, once the memory has been placed, you can no longer see whether it is correctly placed. So make sure that it has the right orientation. Furthermore, you have to place the component so that the legs of the component are placed as well as possible on the pins of the board. If these are not properly aligned, then solder bridges and even worse, short circuits will automatically occur. So take your time and place the component as precisely as possible on the board.

The PCB with solder paste and assembled

Step 8: Baking in the non-reflow oven

I have found that reflow soldering can be done in a normal furnace without any problems. However, I would not take the normal oven that you also use for food, since the soldering process could possibly form toxic fumes and residues. Better buy a cheap stove like me. Mine here only has an analog temperature control, i.e. I can roughly set how hot it should be. However, that the temperature is very important, I bought a separate temperature meter for meat, which I have outside the oven and which I guided the probe over the door into the interior of the oven. This temperature meter shows me the temperature down to a tenth of a degree, which is absolutely sufficient. Now it is important to know which solder paste you have used. In my case, it is a solder paste that becomes active at 140 degrees Celsius, so it melts and liquefies. So I set the oven to 140 degrees and put the board in the oven from the start! This is very important because we don't want to stress the components. It is important that the components heat up slowly and the baking process, I mean the reflow process, only takes as long as absolutely necessary. So off with the board in the oven and the temperature set to 140 degrees. Now the temperature display monitors and ensures that the oven reaches 140 degrees, otherwise nothing melts and also ensures that it does not go over it massively. As soon as my oven reaches 145 degrees, I screw back until the heating lamp on the oven goes out. At the same time, you still have to look at the watch, because once the temperature has been reached, no more than 30 seconds should pass. If this time is up, the oven switches off immediately and the oven door opens so that the temperature can drop quickly but not too quickly.

Oven with thermometer for reflow soldering

Step 9: Check whether the baking process was successful

When the oven has cooled down and the PCB is at a temperature you can touch, it gets exciting. Now the board has to be examined in detail. The more precisely you looked beforehand whether the memory with its legs was on the pins, the more beautiful the print will look. In my example, I was not very precise, but it is sufficient for trouble-free operation. The legs are not absolutely on the pins, but they don't make a short circuit either. This step is really, really, really important! If after the reflow soldering you have a short course somewhere, or something didn't work, then this can completely destroy everything if you put it into operation. So always check carefully. Also very important: you cannot put the whole thing in the oven again. The solder paste will hardly liquefy again and the components would not necessarily withstand the stress. If something is not good, the conventional soldering iron must be used and corrected by hand.

The PCB with populated and baked

Step 10: Excess solder cleaning

It is also very important to look for any possible course conclusions, also to remove excess solder. If you applied a little too thickly during the application or took away too little, then solder can also be in places where it shouldn't be. It often forms small balls that are then somewhere on the board. If you are unlucky, the PCB works at the beginning, but suddenly these solder balls loosen and later short-circuit in the system, which can then destroy more than just your handicrafts. So keep your eyes open and look for such remains, remove and clean.

The PCB with assembled and baked in detail

Step 11: Apply other SMD components

Very professional hobbyists have two different solder pastes that become liquid at different temperatures. So you can bake the first side with the solder paste that melts at a higher temperature and then bake the back with the solder paste that melts at a lower temperature. However, most SMD components are subject to additional stress if they have to be put in the oven twice. I would always see that the complicated and delicate parts are reflowed in the oven and then do the rest by hand. Here, too, I soldered on the consensors by hand. With a little practice, this can be done without problems and the memory takes away the stress.

The PCB back with capacitors

Step 12: Solder the SMD by hand

If you want to solder SMD by hand, it is advisable to provide the first pin with some solder. The PCB is already warmed up a bit and soldering is easier. Then you use tweezers to take the SMD part and place it on the circuit board so that you can reheat the solder that has already been applied. As soon as this solder has connected to the SMD component, you don't actually need the tweezers anymore. You can then simply solder the other side of the component and it is good. Just always use good solder that already has flux in it, or otherwise use enough flux. A cold solder joint (i.e. one that does not shine) is a potential problem. Either right from the start or later. Make sure that the solder joints look as shown below, then it works with the contact.

The PCB back with capacitors in detail

Step 13: Solder the remaining components

As a rule, the components are soldered in height. This way, they do not get in the way of other soldering processes. In addition, parts that contain plastic, such as this connector here, should only be soldered in at the end. If these are already on at the beginning, they can possibly melt in the oven and then the plug can no longer be used. If you have THT components (Through Hole Technology) like this connector here, which is also subject to mechanical stress, then make sure that your solder is not only on the bottom, but that the solder is clean through the hole on the other Side flows. In the picture you can see that the solder has flowed well upwards from below and thus gives the connector sufficient stability. This pays off if you want to unplug the module from time to time.

The PCB with a socket on top

Step 14: final exam

Checks again whether all tiller connectors and components are really soldered on. Checks whether there are any short circuits. Checks that any solder balls or any cut wire remnants have been removed from the board. Check whether you have applied the components in the correct orientation. Checks whether there are no cold solder joints. Checks that the THT components are well soldered.

The PCB with base soldered below

Step 15: Now it's getting exciting!

In our case we built a memory board for the de10 nano on which we want to operate the MiSTer. So if everything is cleaned and the board has been checked as well as possible, then we can put it on the de10. Here again make sure that the board has been correctly plugged in, so that it does not move a row or a pin! That would also be an absolute death blow for the board or the de10.

The PCB on the de10 nano Cyclone V board

Step 16: One last check on the other side

Take a good look at the board from all sides. Sometimes it looks absolutely perfect on one side and you have the feeling that everything is right. But if you look at it again from a different angle, a pin in the air or a row suddenly slips. So, take your time and take a close look at it from all sides. You can only switch on the power when you are absolutely sure.

The PCB on de10 nano MiSTer board

Step 17: Did we do it well?

Well, this step is not really part of reflow soldering, but it doesn't matter. So we connected everything and switched on the device. No magic smoke, no blue flame, no flash bang, success! Good, but how good? Now it's time to test. In our case there is a memory test that checks the memory at a defined speed. He then writes and reads out the written values. If this worked without problems, he counts up the green number. If he has found a difference, the red number is counted up. So it is clear that the red number should always show 0, because only a single 1 is a problem and makes the module unreliable. In addition, such tests should not only be taken for 5 minutes, but much longer. It's like a burn-in test. This means that the components become warm and that must not lead to failure or malfunction.

The PCB on the high-speed test bench

Step 18: Have fun!

Yes, now you have soldered your first PCB yourself in the reflow process and you will have fun and enjoy it. Because what you do yourself is always better than what you bought! Now have fun and enjoy your success!