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This was the first time we had ever attempted surface mount soldering. Instead of pushing the leads of components through holes on a board and soldering them from the back (a.k.a. through-hole soldering), we only applied solder to the pads on the front and rested the component on top. In addition to the inherent difference between the layouts of the circuit boards, the soldering technique we used for surface mount soldering differed greatly from the technique we had been using for through-hole soldering. As a guide, we used CuriousInventor's Surface Mount Soldering 101 video.
Basics of surface Mount soldering:
Basics of surface Mount soldering:
- Flux- one of the most important materials in surface mount soldering. It can be applied in a number of ways including paste from a syringe or liquid that is brushed on, but either way, it cleans the metal so that solder can bond to it. Usually, solder contains flux, so when through-hole soldering, it does its job as the solder melts around the joint. However, surface mount soldering required that we put a lot of solder on the tip of the iron first and only touch the tip to the pads, so the flux had burned off before getting to the board. Thus, we applied the flux to the pads, which helped the soldered stick.
- Prepping the iron- As mentioned above, when surface mount soldering, you do not hold the solder at the joint, but you melt the solder on the tip of the iron, and the flux does the rest of the work. To the right is a picture of the tip of my soldering iron before touching it to the pads.
- Applying solder to the components- I will explain this process for a resistor here (however, further down on this page I will talk about the other types of components we soldered). Before placing the resistor down, on one of the two pads, we put down some solder. Then, using tweezers we held the component in place while reheating the solder we had just added. If things went well, solder connected that side of the resistor to the board and the resistor laid flat. After we had secured the part in place, we rotated the board 180°, reapplied flux to the other side, tinned the tip of the iron, and touched the tip to the pad. An ideal joint should look like this:
- Desoldering- It is possible to use the same desoldering techniques that you use for through-hole soldering (i.e. desoldering gun, solder sucker, etc.), but sometimes this will result in undoing all the joints, especially for components with multiple pins close to one another. Instead, we used a technique called wicking, which uses a copper braid to pull away the solder when heated. First, we melted solder onto the braid, and then held the braid to the joints while we heated it using the soldering iron. If done correctly, it should take away only the solder at the point.
Practice!
Practice!
We practiced surface mount soldering using a kit, but the goal wasn't to make anything that worked. The purpose of the exercise was to get experience surface mount soldering resistors (of various sizes) and chips. Starting with the biggest resistor, we worked our way down to the smallest resistor, and we practiced a few different techniques for soldering chips with multiple pins. The smaller resistors were definitely more difficult to solder than the bigger ones, and the chips were challenging to solder since the pins were so close together. The safest way to solder the chips was to touch the tip of the soldering iron to each pin separately, but a quicker method was to slide the iron across all the pins. However, this method was more likely to result in solder bridges. Below are pictures of my practice board.
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