Tag Archives: Soldering

What is Soldering?

Although soldering electronic components in place is a complex activity, most people involved with the soldering process do not realize it. Complicated chemical and thermal processes occur within a very small space when soldering. To make a good solder joint, it is necessary to follow a few basic rules.

Apart from just making good electrical contacts, solder joints should also be mechanically strong and must not oxidize. Additionally, there should not be chemical residues in the joint. Usually, chemical residues come from flux, which can corrode plastic and metallic surfaces both.

Manufacturers offer solder in three categories—consumer, industrial, and high-end. The automotive and health industry makes use of the third category. Consumer and industrial grades are more common for manual, automated, and other construction purposes.

For several years, the standard was the leaded solder. With a relatively low melting point of around 183 °C, leaded solder has good flow and wetting characteristics. For proper melting and formation of a good solder joint, the recommended temperature at the tip of a soldering iron is 120 °C above the alloy’s melting temperature. This corresponds to a tip temperature of about 300 °C.

Manufacturers provide flux inside the hollow of the solder wire. The flux helps to dissolve oxides of the metals at the solder joint. General purpose leaded solder is typically an alloy of tin and lead in the ratio 63:37. Typically, the tin in the alloy amalgamates with the metal (typically copper), producing an alloy of the two metals, as an intermetallic diffusion zone. This helps to form a good solder joint, well-formed, mechanically strong, and durable.

However, an ideal solder joint does not happen in all cases. Sometimes, the solder forms a cold solder joint. Reasons for the formation of a cold solder joint are the presence of highly oxidized metals and dirt, inadequate heating, or fast cooling after the melting process. Inadequate wetting is common in cold solder joints, leading to easy detachment of components.

It is easy to recognize a cold solder joint with leaded solder. The joint has a dull matte surface against a shiny, glossy surface of a good solder joint. With lead-free solder, this is no longer the case. Newer alloys of lead-free solders usually form a matte surface. However, this depends on the specific composition, and it remains matte whether the solder is establishing a good or a cold joint.

New lead-free solders are RoHS compliant, meaning they do not contain certain hazardous substances, as specified by the EU Directive and the Restriction of Hazardous Substances.

The lead content in lead-free solder cannot cross a 0.1% limit. The intention is to prevent the operators from inhaling toxic vapors. Earlier, the use of suitable extraction systems prevented the risk of such inhalation, provided they were in actual use.

The absence of lead in lead-free solders has resulted in an increase in their melting point. The presence of about 95% tin raises the melting point of the alloy from ~217 °C to ~227 °C. This also changes the flow characteristics. Higher temperatures mean the actual soldering time must be small to prevent damage to the components.

A Bench-Top Reflow Soldering Machine

You can easily solder a board with leaded components if you have a hand held soldering iron. Another method of soldering several leaded components in a short time is to pass the board through a wave soldering machine. However, component manufacturers are moving away from leaded components to making more of leadless components, and the soldering technology has had to follow through.

Soldering leadless components or surface mount components requires a different technique than solder wire and soldering iron. Usually, this needs a reflow oven. The process of soldering surface mount components involves applying solder paste to the pads on the board, carefully placing the components in their designated places on the solder paste, and passing the mounted boards through a reflow soldering machine.

Using surface mount technology has its own advantages over the use of through-hole components. Apart from the several electrical, mechanical, cost, and size benefits of using surface mount components, the use of a reflow machine for soldering the whole board within about three minutes is enough reason for switching over totally to surface mount technology.

Selecting a bench-top reflow soldering machine requires looking at different aspects. A practical wide drawer type design makes it easy to load and unload boards. A large window on the side should allow you to see inside the machine when soldering. Modern reflow machines usually feature digital controls along with a touch controlled display panel. Some even offer a USB port allowing you to connect to a computer for a more detailed control.

Reflow soldering machines are available in different sizes, the larger ones allowing a large enough surface to solder several boards at a time. However, larger surfaces are only useful if the machine can distribute the heat evenly over it, since you would like to have all the boards soldered properly, and not just the ones in the middle. This is true for large boards also, and an even heat distribution helps to solder all the parts in the same way.

For evenly distributing the heat, reflow soldering machines use full-width quartz infrared lamps, followed up with an air circulation system. They are set up in a special way to enable a minimum temperature difference over the soldering area. Most manufacturers of soldering machines include PCB holders with brackets that do not influence board heating.

An important factor influencing soldering of surface mount components is the thermal profile. As the board passes through different regions of the reflow machine, it must gradually heat up to the proper temperature to enable soldering, and subsequently, cool down at a defined rate. As the board remains within the machine for only a definite time, the temperature variation over time defines its thermal profile.

Most modern reflow soldering machines are computer controlled and allow creation of elaborate thermal profiles by adjusting the speed at which the board traverses the entire length of the machine and the temperatures of different zones during its journey. Users can define up to three preheating zones, a reflow peak, and a cooling down phase. You can also store a few profiles so the machine can be operated in a stand-alone manner.

How to solder like a pro

Manual soldering is a skill that gets better with practice. For those who are starting out for the first time, manual soldering can be risky, unless they take proper care and follow safety instructions. Manual soldering involves application of heat locally by a soldering iron, whose tip may reach temperatures as high as 400°C. Soldering materials are sometimes toxic, especially if using lead based solders. Some very useful information about soldering can be found here.

One of the best tips to follow for both starters and experienced people is to don safety glasses before starting to solder. This is something that should become a habit for everyone who is soldering, because hot solder entering the eye can be dangerous. A wire, bent the wrong way, can easily flick hot solder into the air. When cutting a component leg, the cut piece can travel at high speeds. Safety glasses will save the eyes from all these flying missiles.

For most people, soldering skills will not be very good at first, but will certainly improve over time. There is no magic in making perfect solder joints every time, you will simply get used to how to hold things, when the iron is hot enough and the feel for how long you need to apply the heat on.

One basic question that comes up often is whether people ought to use leaded solder or unleaded solder. Leaded solder is composed of Tin (Sn) and Lead (Pb), typically in the ration 60:40, with lead being added to bring down the melting point of the composition to about 180°C. However, Lead being unsuitable to human health, has led to creation of unleaded or lead-free solders. The composition of lead-free solder varies, but in general, these have a higher melting point, nearer to 240°C.

The lower melting point of leaded solder makes it easier for soldering work, and beginners find it easier to practice with. Unleaded solder also has a more corroding effect on the tip of the soldering iron, so you need to change the tip more frequently if you are doing a lot of soldering with unleaded solder. For fine electronics soldering, it is preferable to use a thin gauge of solder wire such as 0.7mm in diameter. Thicker solder is intended for heavier electrical work.

For solder to melt and flow easily, a chemical compound is used; this is called Flux. Usually, the solder wire has a hollow core, in which flux is filled. As you heat the solder wire, flux melts first and helps solder to melt and flow. However, melting flux releases fumes that although not harmful in small quantities, it is advisable to avoid breathing in.

Soldering should preferably be done in a large, well-ventilated room. If that is not possible, a fume extractor or even a fan should be used to draw the fumes away. The tip of the soldering iron should be kept clean and well wetted with solder. This keeps the tip in good condition for a longer time, preventing pitting. For cleaning the tip, use a wet sponge or some wire wool. Wire wool can be used to clean the surfaces to be soldered, resulting in faster and better-soldered joints.

How to solder – an illustrated guide

Guide to learning to solderWe love when we come across electronics info and guides that others are sharing freely – and especially those that encourage others to share their knowledge and work.

For example…here is a fully illustrated guide to learning how to solder which was done by the fine folks at http://mightyohm.com. They’ve created a super guide with all the basics covered as well as some interesting tips and tricks that can make your soldering experience a little better. This would be a great staple for some basic electronics classes.

To see the full soldering guide, click on the image above.

Thank you to the creators of this comic book: Mitch Altman, Andie Nordgren and Jeff Keyzer. Great work!

What causes oxidation on surface mount pads?

perf boardSolder contacts must meet minimum contact rating specifications to effectively be used. But sometimes you’ll begin to solder an electronic component to a board and realize that oxidation has occurred. Why does this happen?

Typically, there are two reasons that oxidation occurs on boards.

The main reason is improper storage and handling. Boards should be received from your supplier in a vacuum pack, which would render them clean and ready to accept solder. Once the vacuum seal has been broken, the boards are exposed to the environment. Boards that are stored in facilities with high humidity will experience accelerated deterioration. When handling boards, use only gloved hands and handle by the edges whenever possible.

Another way PCBs become contaminated is at the factory level. If your boards have just been removed from their packaging, chances are the problem is not oxidation. Much of the contamination is at the manufacturing level which is found after the bag has been opened. The main reasons this happens is because of incorrect ph levels, careless handling of the PCBs or if the boards are poorly washed with the surface film.

How can this be avoided? Handle the boards properly including using proper storage methods. Keep the boards in dry storage area, free from humidity. Make sure that anyone that handles them wears gloves to keep body oils from the pads and instruct everyone to touch only the edges of the boards. If your problems are from the factory, address the factory about the issues to avoid the same problems with future orders.