Solder paste is a mix of different components used to solder SMD components. Unlike solder wire, that is commonly utilized with solder pencils, solder paste is used when reflow techniques are performed. You can find it typically in jar or syringes depending on the quantity you will need.
Unlike solder wire, other precautions have to been taken. For example, the solder paste has to be stored in a refrigerator since the properties will be compromised at ambient temperatures for long times. Therefore, the idea is to take out of the fridge the solder paste when it is used and then return it as soon as possible. But on the other hand, it is recommended that the solder paste is applied once it reaches ambient temperature for a better soldering.
So, the best idea is to take out the solder past from the refrigerator, extract the quantity you will need, return it to the refrigerator and let the extracted solder paste reaches ambient temperature before soldering. It also has to be considered that once it is opened it has an expiration date (and also even it is not open it will have it). Typical life of the solder paste is around 6 months.
Initially, the composition of solder paste was tin, lead and flux, but with the new RoHS regulations there are lead free compositions in which the lead is replaced by other materials, as copper. In a high number of applications, especially civil applications, RoHS compositions are now the typical used since it is mandatory according regulations, but for military or space applications solder paste with lead are the preferred one due to its reliability in worst case conditions.
As commented before, solder paste is used typically when reflow techniques are used to solder SMD components, like hot air stations, hot plates or solder reflow ovens. It is complicated to find what is the ideal quantity to use, and several studies have been carried out. You will finally find what is the best quantity based on your experience.
If you use low quantities of solder paste, the strength of the joint will not be optimum and there will be possibilities that the soldering is broken with time, especially with temperature changes. Besides the electrical conductivity can be compromised. On the other hand, if too much solder paste is used, when the solder is melt there are possibilities to generate short-circuits if some components are near to other ones, and also, when dealing with high frequency electronic circuits, the performance can be deteriorated.
In order to apply correctly the solder paste, it is important how the PCBs are designed. When it is possible, it is recommended to manufacture the printed circuit board with a solder mask layer, in which the whole PCB will be protected from soldering, with the exception of the pads in which the SMD components will be placed. Also, a silkscreen mask will be very useful to quickly identify which component will be place in each location. In this way you will have to place manually the solder paste in each pad.
When you are going to be dealing with many copies of the same PCB it is very recommendable to manufacture a stencil. The stencils can be built with different materials, for example, there are plastic or metallic ones. When looking for best accuracy, the stencil is created with laser machines that allows to have very small windows for the smallest pads as 0201 packages of BGA pads.
Once you have the stencil, you will have to align it over the PCB, and stick the stencil to the board, for example using Kapton in order to avoid, when applying the solder paste, the stencil is displaced. To apply the solder paste, the best way is to use a spatula to distribute the solder paste over the board trying to have the same quantity over all the PCB pads. Finally, you will have to remove the stencil carefully and start placing all the components. You will have to take in mind that, once the solder paste is placed, you can not have longer times without soldering to avoid the solder paste will not lose its properties.
When all the components are placed you can start to solder the components, you can do it one by one by using a solder stencil, at different regions using a hot-air station, in this case you will have to take care the components are not moved with the air, or you can place the PCB in the hot plate or solder reflow oven with the recommended temperature profiles.
Solder paste can be used also for rework tasks, for example, when you want to disolder a component, you can apply a small quantity of solder paste to the solderings, in order to melt the solder paste together with the old soldering and improve the disoldering using a solder pencil. Other example is, when you have your PCB inside a mechanical box, it is complicated to heat the PCB to do a correct soldering due to the heat dissipation and when you use a solder pencil, the solder tends to go to the pencil instead of to the pcb/component. For these tasks is better to apply solder paste in the pads of the component and using the stencil solder apply the heat to the PCB with the solder paste already applied.
Solder paste is also used when pick and places machines are used. Depending on the pick and place machine, the solder paste will have to be placed manually, using the stencil, or they can have a dispenser and, automatically, they will be able to place the solder paste in the positions that have been programmed.
An that's all for this post. If you liked it, please share it on the social networks. Also, we recommend you this other post in wich we explained how to solder SMD components properly.