How To Solder
Soldering is an essential process in the electronics industry, plumbing, and jewelry industry, and it is a vital skill for electronics hobbyists working on home projects. Soldering helps hold a component in place and make tight connections between various components and wires on a printed circuit board (PCB).
This is achieved with the help of a low melting point metal called solder and a handheld tool known as a soldering iron. This article covers the basics of soldering, the main tools used, and a step-by-step process of how to solder a component to an electronic board.
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What is soldering
Soldering uses metal with a low melting point (solder) to join two or more metal connections. A soldering iron is a hand tool that supplies the heat needed for soldering. The iron delivers heat to the solder, which melts and flows into the joint between the workpieces. The solder cools in a few seconds, creating the soldered joint. The chosen solder should have a lower melting point than that of the components being connected in the process; otherwise, parts of the components may melt.
Figure 2: The soldering process
Tools used in soldering
The following components are commonly used in soldering:
Soldering iron supplies the heat to melt the solder and make an electrical connection between two components/wires. The iron consists of a tip that comes in contact with the solder, heating it, and an insulated handle used to hold the tool. Soldering irons can be gas-powered, electric, or externally heated, but electric types are the most common. The soldering iron gets heated up when plugged in. The iron tip can transfer heat to melt the solder to make the connection. Some soldering irons come with inbuilt temperature control, allowing the user to set a predefined temperature for operation. This is useful while working with different solders of different melting points. Excessive heat on the tip can overheat and damage the solder and other components on the printed circuit board.
Soldering iron tips
The soldering iron tip transfers heat to the joint by thermal conduction from the metal-to-metal contact between the joint and the tip. Soldering iron tips are versatile and come in different shapes and sizes like conical, chisel, needle, and knife. The tip selection is an important element to proper soldering to ensure a proper solder and to prevent hitting nearby components or wires. Also, the soldering iron tip should be big enough to ensure proper heat transfer into the joint. This ensures the solder melts and flows properly. The selection of the tip type is a personal choice, and it depends primarily on the application. For example, a conical tip is used for general purpose soldering, whereas a needle tip performs extremely detailed and concentrated soldering works. See our article about soldering iron tips to learn more.
A soldering station is a temperature-controlled soldering device designed for soldering the components on tiny chips which can get damaged by excessive heat or temperature. A soldering station has soldering tools connected to a control unit (CU) that includes automatic/manual temperature controllers, an electrical transformer, and displays for temperature and timer.
A soldering station is broadly classified into analog and digital types. In an analog soldering station, a heating element heats the soldering iron tip to the recommended temperature and then goes off. The iron cools to a specific limit with time, and at this point, the heating element is triggered to heat the iron tip again. A digital soldering station has a Proportional-Integral-Derivative (PID) controller within its microprocessor to regulate its temperature. A digital soldering station controls the temperature automatically, and it is more precise, unlike the analog soldering station where the temperature has to be adjusted using an adjustment knob. If the accuracy of operation is the primary concern, a digital soldering station is the best option.
Figure 3: Soldering station
The solder used in electric circuits is broadly classified into two types: tin/lead solders and lead-free solders. Tin/lead solders are easy to use; hence this type of solder has been more popular than the lead-free solders. However, as exposure to lead leads to serious health issues in humans and adversely affects the environment, these solders have been phased out of commercial use. See our article on the types of solder for more information.
Soldering iron stand
It is valuable and handy to have a soldering iron stand for placing the soldering iron for short breaks. This stand prevents the hot iron tip from coming in contact with flammable materials or accidentally injuring the user’s hands. Most soldering stations have a built-in iron stand along with a sponge or brass wool to clean the tip.
A standard method to clean the iron tip is to use a tip cleaner like a wet sponge or a brass wire wool. When the damp sponge is wiped against the tip, the water in the sponge cools the solder, and the mechanical rubbing action removes the contamination on the tip, leaving a thin coating of clean solder on the tip. The contaminated solder sticks to the brass when the brass wire wool is rubbed against the tip. The tip is pushed into the brass wool, and upon taking it back, the tip is left with a thin solder coating.
Flux helps clean the surfaces being soldered and prevents oxidation of the hot solder tip. The flux is placed at the core of the solder or used separately as a paste. Most fluxes produce fumes when the solder is heated, which can pose a danger to the user’s health in the long run. Hence it is advisable to have a well-ventilated area for occasional soldering and use a fume extractor in case of repeated exposure. Read our article on soldering iron tip care guide for more information.
Figure 4: Soldering tip cleaning sponge
How to solder a component or wire
Tinning the soldering iron tip
The tip of a soldering iron gets oxidized rapidly, acting as an insulator prohibiting proper heat transfer to melt the solder while working on an electronic board. Tinning is the process of covering the tip with a thin amount of solder. It should be done ideally at the beginning and end of the soldering process and regularly during the process to extend the life of the soldering iron tip.
Perform the following steps for tinning the iron tip before starting the actual soldering work:
- Attach the tip tightly to the iron and screw the tip tightly in place.
- Heat the soldering iron by turning on the power supply, and wait for the tip to get heated.
- Use a damp sponge to clean the soldering iron tip. Wait a few seconds till the tip heats again.
- Holding the solder in one hand and soldering iron in the other, touch the iron tip with the solder until the solder flows evenly across the tip.
How to solder a component to the printed circuit board (PCB)
Figure 5: Good soldering joint
Once the soldering iron tip has been tinned, perform the following steps to solder a component to an electronic board:
- Place the component in the hole on the board as per the marking, and fix it by flipping the board over and bending the leads outward at a 450 angle.
- Touch the tip of the heated iron on the copper pad, and the component lead simultaneously. Hold the position for 3-4 seconds to heat the pad and the leads.
- Continue holding the soldering iron tip in the position as explained in Step 3, and touch the solder to the joint. Most solder melt around 1800C; hence make sure to get the solder joints hotter than this temperature.
- Remove the soldering iron and wait for the solder to cool down naturally. This might take a few seconds. Once cool, cut off the extra wire from leads.
- Check whether the joint formed by the solder is proper. A good solder joint will be conical in shape. If the shape is not formed correctly, cut off the component lead and perform the process again.
- If the solder forms sticky chunks instead of getting melted completely, it might be due to oxidation at the soldering iron tips. Tinning the iron is highly recommended, at least before and after the soldering process.
How to solder wires
Figure 6: Soldering two wires
Perform the following steps to solder wires together:
- Remove the insulation from the wire ends that are going to be soldered. Twist the strands with fingers in case of a stranded wire.
- Heat the soldering iron and touch the tip to the end of one of the wires and hold for 3-4 seconds.
- Keeping the iron in place as discussed in Step 2, touch the solder on the wire until it melts and coats the entire wire. Repeat the process for the other wire too.
- Mount both tinned wires on top of each other and touch the heated iron tip on both wires. The process melts the solder and evenly coats both wires.
- Remove the soldering iron, and wait for a few seconds till the soldered connection cools and solidifies. A heat shrink may be used to cover the connection.
Figure 7: Using a soldering iron and solder sucker for desoldering
The solder and components can be removed from a circuit board for repair and troubleshooting purposes. A solder wick known as desoldering braid can desolder a joint. Perform the following steps to desolder a joint:
- Place a section of the desoldering braid on top of the joint that is to desoldered.
- Touch the tip of a heated soldering iron on top of the braid. This melts the solder below, which gets attached to the braid.
- Remove the braid. The solder would be attached to the braid.
Alternatively, use a solder sucker to remove solder from a board. A solder sucker is a handheld vacuum that can suck up solder while it is hot just by pressing a button.