EncoderCAT PCB - soldering SMD devices
- A fine tipped thermostatic soldering iron
- a good desoldering braid ... some are not very good. Chem Wick (eBay) is excellent.
- Rework Solder Flux spray from RS Components (or similar)
- A flux removal spray ... for when the board is completed. eg: Electrolube ULS (outdoors!!)
- A good magnifying glass.
- I use 60/40 lead/tin solder - its nicer to use than lead-free - and has a lower melting point.
(It doesn't comply with ROHS
... unless you interpret as Real Old-fashioned Heavy-metal Solder !! )
I first coat the whole component side of the board in the spray flux and let it dry.
Spray a small amount into a small container and apply to the areas for SMD soldering.
Place the PCB on a heating radiator to dry it until it becomes sticky.
Then when you place components on the board they stick to the sticky syrupy flux.
You can drag them around in a controlled manner.
I would suggest the following order of assembly:
- Crystal (X1), choke (L1) and micro USB connector
- Resistors, capacitors, LED, voltage regulator.
- The the ICs
- Finally the through-board mounted connectors.
This is possibly the most tricky component to solder.
It has contact pads underneath only.
- Apply solder flux and then tin the four pads -
- remove excess solder ... but aim to leave a very thin blob of solder on each pad
(if it is not very thin then you will have different heights.)
- Apply a blob of solder flux and dry until sticky
- Position X1 on the pads ensuring that a thin area of pad is visible all round.
- Wipe the iron and then get a blob of fresh solder on the tip.
- Carefully hold X1 fixed with fine screwdriver tip and then apply the blob of solder to one pad
holding the iron in place for 2-3 seconds.
- Check that X1 is now fixed by this pad. You now no longer need to hold it in position.
- Repeat the process on the other four corners.
- Remove excess with desoldering braid and inspect.
- simillar to X1 but a bit easier as the contacts are at the side as well as underneath.
Micro USB connector
- This has four ground pads and 5 contacts.
- The good news is that it has four tiny lugs which locate it on the holes in the PCB.
- Fix it in position by soldering the grounded casing first.
.making sure that the five contacts are aligned with their PCB tracks
- Be very careful not allow solder to run inside the connector (or else the USB plug wont go in !)
- Then solder the five contacts .. they are slightly recessed inside the casing
. but are easily soldered by poking the solder iron (with a solder blob) and then and stroking outwards.
- Then clean up with desoldering braid.
- Then inspect with a magnifier
The ATxmega192A3 is the easiest because of relatively wide pin spacing. (0.8 mm )
Be careful with orientation
- It has a small location dot at pins 1 and 64 but may have a larger dot at other corners!!
- The pin 1 /64 corner of the device is angled.
All the ICs
- Use sticky solder flux. Place the device on the PCB.
If the flux has a treacle-like viscosity, then you can slowly drag the IC into position.
- Hold with a screwdriver tip and apply a solder blob (2-3 seconds) to 2 or 3 pins of one corner.
- this will fix the device - check with a magnifier that it hasn't moved.
- If ok - then fix another corner with a solder blob.
- If ok then flood solder all sides starting away from the fixing points.
... dont worry about the total short circuiting of all pins - the desolder braid will remove the excess.
.. You need to apply heat to all areas for 3 seconds or so to ensure good joints.
.. Gently stroke outwards with the fine soldering tip.
- Then lay clean desolder braid along one side at a time and apply the iron to soak up excess solder
- inspect carefully - identify solder bridges and reapply the braid to remove them.
Be very careful when soldering and particularly when using the braid so that:
- You do not bend the pins by dragging them sideways.
- You do no lift the pins by pulling off the braid when the solder has cooled to the pins
I often finally neaten the pin joints by stroking them outward with a fine soldering iron tip
with a tiny amount of solder.
(This may cause a recurrence of inter-pin solder bridges - which I can remove with the braid)