SMD soldering

This is a quick tutorial on how to solder an SMD board with 0603 and QFP sized components. This was the finished board:

‘0603’-sized components are fairly tiny; 1.6mm x 0.8mm. The image here shows some 0603 capacitors and the soldering iron. I broke the small soldering iron tip, so this is a larger tip which is non-ideal (I used this non-ideal tip for soldering these though, so it is just about feasible). Note the tip is clean and shiny (with solder and wet sponge). 0402-sized components are also possible, but I tend to standardize most components at size 0603 for ease of use at home.

Put some flux on the pads where the component is to be placed.
Picking up the components – note the tweezers should be reliable, otherwise this 0.8mm sized component may go flying or cannot reliably be held in place while soldering with your other hand

Use one hand to hold the tweezers _and_ the solder. Tweezers: held with thumb and forefinger, and grab the component. Then grab the end of the solder (leave a couple of inches of it extended) between the middle finger and forefinger, or between all three fingers (forefinger, middle finger and ring finger, so that just the little finger is idle). You can see the grip here. Use the other hand to bend the solder into the correct position.

Place the component at the correct location on the board, and slide fingers if needed, to move the solder into the correct position if it moved. See this image:
If you are having trouble with the grip, try swapping hands. Or get practice in whenever you eat Chinese food. Then, use the other hand to solder the component in place. If you don’t manage to solder it within a second or two (e.g. the solder slipped from your fingers), wait for it to cool a bit before trying again. You can see some soldered 0603 components here. There is too much solder (the result of using a large tip) but this was acceptable for my board. With a smaller tip, it should correctly look like a fillet instead of a blob. Notice the board looks smeary/sticky and this is due to the flux (it will be cleaned later).
0603-sized components can be visually inspected after soldering, and optionally you may wish to use a lens for the inspection.

Soldering QFP devices
Add a lot of flux to the board. The most important step is to align the component perfectly. The way I do it is to place the component, and then nudge from one side with a small instrument (e.g. tweezers) while observing through a lens, then gently slide the board around 90 degrees and nudge again to get the next side aligned, and then rotate another 90 degrees and repeat. Keep doing this (for a total of maybe 720 degrees or even more) until the component is now sitting perfectly. Use the lens and lamp to look at the component from all angles, to ensure it is spot-on aligned. Then, cut a piece of masking tape and use it to secure the component on the board (leave one edge bare so that it can be soldered from that side first). This is not as hard as it sounds; if you’re careful, the component will not move while taping it (the flux has some viscosity too).

As best as possible, ensure that the component did not move while it was taped down.

Now, carefully solder the pins. The thing to be aware of is how long you’re soldering for, because you don’t want the component to overheat. After you’ve soldered a few pins at each end, you can _gently_ remove the tape and solder a few pins on the other side. After a dozen or so pins, wait for it to cool before continuing. It doesn’t matter if the solder causes bridges; they will be removed later. The next thing (after the time) to be aware of is that the solder must be applied to the junction between the end of the pin and the board. Capillary action will suck it under.

Once all the pins look soldered at the junction between the end of the pins and the board, it is time to remove the bridges. Wait for the component to cool, then apply the desoldering braid (aka solder wick) and apply heat with the iron. Capillary action should cause the solder to be sucked up from the bridges into the wick. Trim the wick regularly to prevent it acting as a heatsink taking heat away from the iron tip. If you find the wick is not sucking, then apply some solder to the iron tip first. This acts like a ‘starter’, to help get the wick warmed and begin the capillary action.If that doesn’t work, then the wick is not touching the bridge well enough. Don’t apply force, because that will damage the QFP. Rather, actually apply more solder to the QFP, to make more of a bridge. Then the wick will suck it all off. As the wick is sucking off, gently drag the wick away from the pad, i.e. in the direction of the ends of the pins. The result will look like this (the smeary stuff is the flux). Each pin here is about 0.2mm wide, on a 0.5mm pitch:

You need to inspect every pin individually. There are two locations that need to be inspected per pin, and they are highlighted here in red.
The red highlights shows that the junction between pin end and board should look filleted as shown in the image, and also that the space between pins should not be bridged.

The inspection doesn’t end here. After the area has been de-fluxed (using isopropyl alcohol and an ESD-safe stiff plastic brush), you need to check that each pin is still attached. The inspection will need to be visual (inspecting the junction between pin ends and the board), but also mechanical: take the scalpel, and _gently_ press the side of the blade against each pin in turn. If you can feel the end of the blade flex by (say) 0.1mm then the join is good. Keep it gentle. If the pin was not correctly soldered but was only being held in place by some sticky flux, then it will detatch and you will feel and see it move slightly. That area of the IC can be reworked with solder, and then cleaned again and then inspected visually and with the scalpel again.

Author: admin

Leave a Reply