EncoderCAT PCB  - soldering SMD devices

You need:

• 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:

1. Crystal (X1), choke (L1) and micro USB connector
2. Resistors, capacitors, LED, voltage regulator.
3. The the ICs
4. Finally the through-board mounted connectors.

Specific comments:

Crystal  X1

This is possibly the most tricky component to solder.

It has contact pads underneath only.

My plan:

• 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.

Choke (L1)

- 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

ICs

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)