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Step 8Preparing for Soldering
Now that we can design surface mount circuit boards we want to solder, let's move on with our Hot Plate Soldering System. Before we can to solder with it, we must stencil on solder paste and place parts on our circuit board. Let's go through the process with some pictures of an actual board I soldered. (I also created the board using the tools and process I just described.)
The board must be held securely while paste is applied. I use two pieces of thick brass shim stock with notches cut in them, and duct tape them onto a wooden base. See Picture 15 for details. The circuit board is held very firmly, but can still be removed without untaping the shims.
Place a board in position within the holder. Place the stencil over it and line up the openings as accurately as possible, then tape the stencil securely at the top. See Pictures 16 and 17. Picture 18 shows the holes in the stencil. Note the irregular opening on the DFN footprint I made the openings in the
stencil too small and had to hack it to get enough solder on. I don't recommend this!
With the board in position and the stencil in place over it, apply a bead of solder paste along the top edge of board (Picture 19). The solder paste needs to be at room temperature for application, so be sure to take it out of the refrigerator at least 8 hours before you plan to use it. I'm using lead-free paste and it seems to be lasting just fine if I keep it refrigerated when I'm not using it. Just keep it tightly sealed and away from food especially if you get paste with lead in it.
Use the razor blade as a squeegee and spread the solder paste. Ideally, you'll make just one pass across the board. Don't panic if you have to make an extra pass or two. That can cause a bit of smearing on your board, but that's not too critical in my experience. Picture 20 shows the stencil after the solder paste is applied.
Now lift the stencil off the board and tape it back so it can't flop back down while you're moving the circuit board. Use rubbing alcohol and a tissue to clean the paste off the stencil. Don't try to reuse leftover paste. Picture 21 shows the board after stenciling. You can also practice applying solder to the same board by simply removing the paste from the board with a tissue and some rubbing alcohol.
After applying paste it's time to place your parts. Try to arrange a static-free work space for parts placement. Static can destroy sensitive parts, so try to keep it minimized. If you're real concerned or if static is a problem in your house, then use a metal plate for a work surface and connect a wire from it to the ground pin of a nearby electrical socket. Be sure to touch the metal plate first before touching any parts to be placed. Create a parts placement diagram in your PCB design software and print it out. It's a good idea to enlarge this diagram for easy reading. One of mine is shown in Picture 22. Use a pair of tweezers the parts are tiny! (See Picture 23.) Set them in place carefully with just a bit of force, but you don't have to mash them into place. If you're visibly squishing the solder paste, you're pushing too hard. Some parts are placed in Picture 24 and all of the parts are placed in Picture 25.
Once the parts are placed you're ready to solder! Handle your board carefully so you don't disturb the parts. I slide it onto a flat plate of some sort metal, wood or perf board seem to work. Carry your board to your hot plate and slide it carefully onto the plate. The plate should be preheated and ready for the solder cycle, but I haven't told you how to do that yet! So let's turn our attention back to the hot plate and our control system.
The board must be held securely while paste is applied. I use two pieces of thick brass shim stock with notches cut in them, and duct tape them onto a wooden base. See Picture 15 for details. The circuit board is held very firmly, but can still be removed without untaping the shims.
Place a board in position within the holder. Place the stencil over it and line up the openings as accurately as possible, then tape the stencil securely at the top. See Pictures 16 and 17. Picture 18 shows the holes in the stencil. Note the irregular opening on the DFN footprint I made the openings in the
stencil too small and had to hack it to get enough solder on. I don't recommend this!
With the board in position and the stencil in place over it, apply a bead of solder paste along the top edge of board (Picture 19). The solder paste needs to be at room temperature for application, so be sure to take it out of the refrigerator at least 8 hours before you plan to use it. I'm using lead-free paste and it seems to be lasting just fine if I keep it refrigerated when I'm not using it. Just keep it tightly sealed and away from food especially if you get paste with lead in it.
Use the razor blade as a squeegee and spread the solder paste. Ideally, you'll make just one pass across the board. Don't panic if you have to make an extra pass or two. That can cause a bit of smearing on your board, but that's not too critical in my experience. Picture 20 shows the stencil after the solder paste is applied.
Now lift the stencil off the board and tape it back so it can't flop back down while you're moving the circuit board. Use rubbing alcohol and a tissue to clean the paste off the stencil. Don't try to reuse leftover paste. Picture 21 shows the board after stenciling. You can also practice applying solder to the same board by simply removing the paste from the board with a tissue and some rubbing alcohol.
After applying paste it's time to place your parts. Try to arrange a static-free work space for parts placement. Static can destroy sensitive parts, so try to keep it minimized. If you're real concerned or if static is a problem in your house, then use a metal plate for a work surface and connect a wire from it to the ground pin of a nearby electrical socket. Be sure to touch the metal plate first before touching any parts to be placed. Create a parts placement diagram in your PCB design software and print it out. It's a good idea to enlarge this diagram for easy reading. One of mine is shown in Picture 22. Use a pair of tweezers the parts are tiny! (See Picture 23.) Set them in place carefully with just a bit of force, but you don't have to mash them into place. If you're visibly squishing the solder paste, you're pushing too hard. Some parts are placed in Picture 24 and all of the parts are placed in Picture 25.
Once the parts are placed you're ready to solder! Handle your board carefully so you don't disturb the parts. I slide it onto a flat plate of some sort metal, wood or perf board seem to work. Carry your board to your hot plate and slide it carefully onto the plate. The plate should be preheated and ready for the solder cycle, but I haven't told you how to do that yet! So let's turn our attention back to the hot plate and our control system.
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Trying this procedure on a board without solder mask is more risky, but might work. I can't say it won't; only that the risk goes up with each move away from using a stencil and solder mask. The risk also increases as pad spacing decreases. So the tinier the part, the more important solder mask and stencil become.