I have been soldering the components on the board this past weekend. Soldering can easily become a nightmare when you have a lot of components, especially if they are surface mount parts.
Surface mount parts are much smaller than through-hole components and their footprints are usually much smaller. This makes it very hard to solder surface mount components when compared to their through-hole counterparts. One might choose to use surface-mount (SMD) components over through hole components when space is a limitation on the board. Also, because SMD components are on the "surface", they take up space on only the layer that they are on. Through hole components require "through holes" to be mounted so the hole goes through all of the layers. This means that you can't use the space underneath the component to route wires.
Space became an issue in some areas of my PCB therefore I had to choose SMD parts. However, I tried to use as much through hole components as possible to make soldering easier. There are some simple rules that I follow when I solder up a board. First, I solder the SMD components before I solder the through hole components. The reason is that I use a technique called "reflowing" to solder the SMD parts. This technique requires solder paste, which kind of speaks for itself. It is like solder but comes in a paste form. You apply the solder paste on all of the SMD component footprints first. Usually, you would use what is called a stencil to make this process easier. Stencil is basically a mask for your PCB with only the footprint areas exposed. Then you would use a spatula to apply the soldering paste which would automatically find its way to the footprints. However, stenciling is quite expensive so I did it the manual way. I applied the solder paste on the footprints using a toothpick being very careful and slow. You want to make sure you don't short footprints together by applying too much solder paste. Once this is done, you carefully place the SMD components on the solder paste-applied footprints. Using tweezers are highly recommended for this step.
Once all of the SMD parts are placed, next step is quite unusual: heat up the board. I know some people use heat guns, frying pans etc. I use frying pans. Turn on your stove and let it heat up the frying pan. Then add the main ingredient: the PCB. I really cannot tell you how long to keep the board on the PCB, but how I know is I watch the solder paste. The solder paste is usually gray colored, and when it is "done" it turns into a silver color just like regular solder. This is when I know my PCB is done cooking!! Check out the picture below:
When the soldering paste turns silver, remove the board and let it cool down. Once it is cool to touch, you can start soldering the regular through hole components. I am not going to talk about how to solder, that is very trivial. But I want to share the current status of the board with most of the components soldered.
You will notice that the 3.3V regulator is not populated on the board.... Unfortunately I messed up a little there. I now understand why people say "the first revision of a PCB never works". But mine will!!! It is a recoverable error. I miscalculated the pinouts of the 3.3 V regulators I ordered but I did find that RadioShack sells 3.3V regulators that will work with my board! So I ordered a new regulator right away and I will solder it when I get it.
Other than that, the only parts I am waiting for are the dual row female headers for the C2000 LP to sit on and SMD LEDs. I have obviously soldered many through hole components already, so I will not be reflowing the LEDs. It also turns out I am very short of through hole ceramic capacitors so I need to order a bunch of those. Overall, project is going really well so check back for more updates! I should be able to test the board in a couple of weeks!
Anil
