As a solder user you know that you need certain solder for different situations. The same is true for solder paste. If you haven't had the chance to check out our article about solder paste take a second to do so here. Choosing solder paste can be confusing. What is solder paste used for? Well, there are 2 main categories: industrial work and electronics. Let's talk about electronics today, more specifically SMD soldering. The most common use of solder paste is in surface mount soldering because it is better to solder multiple pins at once then doing so separately. There are three main types of solder pastes available but what would you use for electronics?
The main solder pastes used for electronics are Type 3, Type 4 or Type 5. The type is defined by particle size of the solder. The recommended type for pitched components is Type 5. Types 3 and 4 are common solder pastes but they can become difficult to use in projects with fine pitch components. Take a quick look below at the particle size differences in these magnified views:
This is a magnified image of the size the solder particles inside of the paste. You see how the particles in Type 5 are much smaller than in Type 3? Using a Type 3 paste while trying to solder a CPU chip would make the task troublesome. The particle size is significantly large enough and would cause bridging between leads. But, with Type 5 solder paste, the job is easier because of the fine particle size. The trade off is cost as type 5 is more expensive.
Generally, for components like resistors, capacitors, and other small SMD components you want to have less flow or spread. This is where the Type 3 paste comes in handy. Type 4 solder paste is perfectly in the middle but is usually used for smaller components like dip switches and TSOP components. Type 4 pastes are not as common as 3 & 5.
Particle size is important but there is more to know. For instance, the solder can be made of different alloys. The most commonly used solder alloys are SAC 305 for lead free and 63/37 for leaded. There are other types of solder alloys like 96/4 and low temp Bismuth alloy. The only thing about these alloys is that you need to know how to use them. Or should I say when to use them, since different alloys are used to solder different things. The most commonly used are the SAC 305 and the 63/37, since they work well on circuit boards. The low temp is great for temperature sensitive devices or those with small components because it is a very easy to solder to work with.
So now we know about the solder in the paste but what about the rest? There is more than just solder in there. Flux is a crucial part of solder paste. Why? It keeps the solder balls together, cleans and helps the metal accept solder which leads to solid joints. Be careful because there are multiple types of flux available. The most common is no clean flux but other options include rosin flux and water soluble flux. The reason why no clean flux is most common is in it's name. It allows the user to complete their job without adding an extra cleaning step. Rosin flux based solder paste may sound familiar since it has been around longer, but most users prefer no clean over rosin now. The water soluble flux is not as commonly used in electronics because water is used to clean a freshly made PCB board.