A printed circuit board (pcb) mechanically supports and electrically connects electronic components using conductive tracks, pads and other features etched from copper sheets laminated onto a non-conductive substrate. pcb's can be single sided (one copper layer), double sided (two copper layers) or multi-layer. Conductor on different layers are connected with plated-through holes called vias. Advanced PCB's may contain components - capacitors, resistors or active devices - embedded in the substrate.
This article mainly deals with the soldering of through-hole components into printed circuit boards (PCBs). Through-hole components are those which have leads (meaning wires or tabs) that pass through a hole in the board and are soldered to the pad (an area with metal plating) around the hole. The hole may be plated through or not.
Soldering of other electrical items such as wires, lugs, have slightly different steps but the general principles are the same.
Steps
1. Select the correct component. Many components look similar, so read the labels or check the color code carefully.
2. Bend leads correctly if required, discuss stress relief. To be completed...
3. Clinching leads. Discuss whether to cut leads before or after soldering based on whether heatsinking effect is required. To be completed...
4. Melt a small blob of solder on end of the soldering iron. This will be used to improve the transfer of heat to your work.
5. Carefully place the tip (with the blob) onto the interface of the lead and pad. The tip or blob must touch both the lead and the pad. The tip/blob should not be touching the nonmetallic pad area of the PCB (i.e the fibreglass area) as this area can be damaged by excessive heat. This should now heat the work area.
6. "Feed" the solder onto the interface between the pad and lead. Do not feed the solder onto the tip! The lead and pad should be heated enough for the solder to melt on it (see previous step). If the solder does not melt onto the area, the most likely cause is insufficient heat has been transferred to it. The molten solder should "cling" to the pad and lead together by way of surface tension This is commonly referred to as wetting.
•with practice, you will learn how to heat the joint more efficiently with the way you hold the iron onto the work
•flux from the solder wire is only active for about one second maximum after melting onto the joint as it is slowly "burnt off" by heat
•solder will wet a surface only if:
•the surface is sufficiently heated and
•there is sufficient flux present to remove oxidation from the surface and
•the surface is clean and free of grease, dirt etc.
7. The solder should by itself, "run around" and fill in the interface. Stop feeding the solder when the correct amount of solder has been added the the joint. The correct amount of solder is determined by:
•for non plated through hole (non-PTH) PCBs (most home made PCBs are of this type) - stop feeding when the solder forms a flat fillet
•for plated through hole (PTH) PCBs (most commercially manufactured PCBs) - stop feeding when a solid concave fillet can be seen
•too much solder will form a "bulbous" joint with a convex shape
•too little solder will form a "very concave" joint.
Tips
•Most soldering irons have replaceable tips. Soldering iron tips have a limited working life and also are available in different types of shapes and sizes, to suit the a variety of jobs.
•It is easy to damage a component with too much heat. Some components (diodes, transistors, etc.) are quite susceptible to heat damage and must have a small aluminum clip (heat sink) clipped on to their leads on the opposite (non soldered) side of the PCB to prevent ruining the component. Use a 30 watt iron and practice soldering quickly so as to avoid excessive heating.
•The tip of a soldering iron tends to get stuck with time (if frequently used), due to oxides that build up between the copper tip and the iron sleeve. Plated tips do not usually have this problem. If the copper tip is not removed now and then, it will get stuck permanently in the soldering iron! It is then destroyed. Therefore: every 20 - 50 or so hours of use, when cold, remove the tip and move it back and forth and around so the oxide scales can come out, before locking it in place again! Now you soldering iron will last for many years of use!
•Keep handy a rubber-bulb or other suction desolderer (sucks up melted solder) or a spool of desoldering braid (fine copper mesh that absorbs melted solder) in case you mess up and need to disconnect something or remove excess solder from a joint.
Warnings
•Solders, especially lead-based solders, contain hazardous materials. Wash your hands after soldering, and be aware that items containing solder may require special handling if you dispose of them.
•Soldering irons are very hot. Do not touch the tip with your skin. Also, always use a suitable stand or holder to keep the tip up and off of your work surface.
Things You'll Need
A soldering iron. Soldering irons are usually either:
◦Fixed power - e.g 25W (small jobs) to 100W (large jobs, heavy cabling etc)
◦Variable temperature - tip temperature can be controlled to suit the size of the job
Tongs, needle-nosed pliers, or tweezers to hold the component.A clamp or stand to hold the board.Flux-cored solder wire.
◦Solder alloys.
■The most common solder alloy used in electronics is Tin-Lead 60/40. This alloy is recommended if you are new to soldering, but is hazardous.
■Various lead-free alloys are becoming popular recently. These require higher soldering temperatures and do not "wet" as well as Tin-Lead alloys. However they are safer and can be more effective. 96.5 Tin/ 3.5 Silver is the most successful and will produce a joint with less electrical resistance than any tin-lead alloy.
■Both lead and lead-free formulations are available online at places like solderdirect.com and in your local Radioshack/Homedepot store
◦Flux. Flux is an additive in solder that facilitates the soldering process by removing and preventing oxidation and by improving the wetting characteristics of the liquid solder. There are different types of flux cores available for solder wire.
■Rosin is most commonly used by hobbyists. After soldering, it leaves a brown, sticky residue which is non-corrosive and non-conductive, but can be cleaned if desired with a solvent such as isopropanol (also called isopropyl alcohol or IPA). There are different grades of Rosin flux, the most commonly used is "RMA" (Rosin Mildly Activated).
■No-clean fluxes leaves a clear residue after soldering, which is non-corrosive and non-conductive. This flux is designed to be left on the solder joint and surrounding areas, but should still be cleaned off as flux by its very nature is corrosive.
■Water-soluble fluxes usually have a higher activity (i.e is more aggressive) that leaves a residue which must be cleaned with water. The residue is corrosive and may also damage the board or components if not cleaned correctly after use.
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