This guide is intended to be an introduction to:
- What is a PCB?
- Our favourite solder alloys
- Soldering iron tips & temperatures
- How to make a good joint
- Close up soldering demo video
This is my personal opinion, based on my experience in the electronics industry, assembling prototypes and designing kits.
A printed what?
A Printed Circuit Board (PCB) is a lamination of conductive and insulating layers that are etched, drilled, routed, plated, coated and silk screened to create the desired electrical circuit with convenient mechanical (and sometimes artful, see Boldport) properties. The PCBs we use at Rakit are simple 1.6mm thick 2 layer boards with white soldermask and black silkscreen. Exposed copper on pads and holes are plated or coated to stop the copper oxidising and ensuring the ready adhesion of solder during assembly. Our boards are usually coated with lead free solder in a process called HASL (Hot Air Solder Levelling), but sometimes we use gold plating for decorative/tactile purposes.
Solder, why are there so many different types? What should I use?
Some important qualities of a good solder for assembling kits:
- Low melting point
- Good/fast flow
- Clear no clean flux
- (bonus) Low fuming or inoffensive smell
- Eutectic alloy… wait, what!?
Solder made using a eutectic ratio (such as 63% Tin 37% lead ) has a sharp transition from solid to liquid at its melting point, conversely it also sets quickly when heat is removed. This helps quite a bit during hand assembly as any movement in the joint during the liquid to solid transition can cause bad joints e.g cracks, disconnects.
Solder made using a non-eutectic ratio are widely present in the market to cater for other needs, applications, working environments etc. 60/40 SnPb (60% Tin, 40% lead) has been used for years because it is a nice compromise betweenmelting point, price and mechanical strength.
In addition to setting up fast, eutectic alloys have a lower liquid point than other ratios of the same metals.
So which solder then?
Solder is a tricky one, especially in the EU with the ROHS directive in full force. All of our pre-assembled kits are built using lead free solder. My personal favourite for lead free applications is a SAC3 alloy (3% silver, 0.5% Copper and the balance Tin) made by Warton Metals. This solder is literally 3% silver by weight and hence quite expensive. For a lead free solder its melting point is a quite reasonable 217-219ºC
However, for personal/hobbyist use we favour a 63/37 tin/lead alloy. It has a melting point of 183ºC (~34ºC lower than SAC3), flows nicely and makes beautiful shiny joints. Being 63% lead you just have to be sensible, wash your hands afterwards and don’t eat it.
63/37 Sn/Pb – [LINK RapidOnline – 63/37 Warton Metals]
Soldering Tips (pun intended)
Soldering tips for general purpose through hole components come in a few shapes, our favourites are the round and chisel shaped type. Given the choice I would opt for the chisel type as it allows the user to get much better contact with the flat area for thermal transfer, but in a pinch the round type tips should work nicely too.
TIP: Use the largest iron tip you can get away with, the thermal mass in the tip helps greatly!
Our irons are set to 350ºC for lead soldering and 380-420ºC for lead free solders. The tip temperature has to be a lot hotter than the melting point of the solders themselves to properly activate the flux and wet the pad and component leads properly. The tip temperature also helps to compensate for the poor thermal contact between tip and board and the thermal losses involved.
FLUX!? After finishing a soldering joint there is often a residue, this is flux. Solder typically has a flux core, the flux is there to increase adhesion of the solder to oxidised/corroded surfaces. Flux is corrosive at soldering temperatures and works to strip away a thin layer of metal oxide that is always present on solder-able surfaces. Flux residues can be cleaned off with isopropanol alcohol or circuit board cleaner.
Wet/Wetting!? Wetting of the solder is achieved when the solder has melted sufficiently to adhere properly to the pad and lead. When the solder becomes liquid it coats and penetrates the copper surfaces and forms a new alloy at the junction.
Soldering Iron + Solder + Kit = Joints
Simple maths, but unfortunately making your first solder joint can be a miserable experience without a bit of direction.
The whole soldering process, with practice takes around two seconds for a typical joint. The process I use has been broken down into 5 steps.
Introduce the iron to the joint to be soldered and apply light pressure.
The irons tip should be making contact with both the pad and the component leg.
Apply a small amount of solder between the iron and pad and then continue feeding into the joint, but away from the iron.
Solder is being freely accepted by the joint.
Solder has fully wicked into the joint and there is a smooth concave surface between pad and pin.
Remove the solder from the joint.
Quickly after removing the solder, remove the iron.
You are now the owner of a well formed, shiny solder joint.
Here is a short video of a joint made using an iron set to 350 degrees C, 63% tin 37% lead eutectic solder with 2% flux core.
Notice that each joint takes about 2 seconds.
And a hi-resolution image of the completed solder joint as shown in the video.