The Correct Way to Use Solder Paste in PCB Assembly Projects
One of the most dependable methods for joining two metals without fusing them is soldering which is why solder paste is used to integrate electronic parts in a PCB. Before integrating the components on the board, it should be ensured that there are no impurities or oxidation on the board because it may lead to failure or circuit board short. Solder paste is used for this purpose. Metal fragments and flux are the ingredients in solder paste. Flux removes dirt from the board’s surface and stops further oxidation. Solder paste does more than only provide the protection, it also hardens the joints and permits current flow.
“A solder paste is a metal alloy that melts to create a permanent bond between two metals. It is a combination of metal solder particles and sticky flux.” A solder paste must meet several important criteria, one of which is that it has a lower melting point. Only then will it start melting before the workpieces do, forming a junction. Another reasonably effective conductor of electricity is solder paste. It permits the assembly of PCBs and the connecting of electrical wires using solder paste. Solder paste makes an electrical connection between them in addition to a mechanical one.
At first, tin and lead were combined to create the solder pastes. Lead and tin made about 40% of the solder pastes used during electrical soldering. However, due to the negative impact of lead and the harm to the environment, most PCBA factories complete orders through lead-free PCB assembly, and the solder paste used by FS Technology does not contain lead. Next, FS Technology will explain in detail how to solder components with solder paste.
Use the correct type of solder paste in soldering
There are many ways to ensure the quality of PCBA, and using premium solder paste in SMT assembly is also a way to improve soldering quality. In order to meet various soldering needs, FS Technology can provide various levels and types of solder paste for SMT assembly. Depending on the materials which are planning to solder and the kind of soldering, which is required to be done, there must be one selected. Depending on the size of the solder balls, different classes of solder paste are separated. It is impossible to rate the balls based on their exact sizes. As a result, the solder paste is rated so that 80 percent of the pellets fall within a certain size range. The various grades and the size of the solder balls for each grade are listed below.
Grade of Solder Paste
Size of Particle
75mm to 150mm
45mm to 75mm
25mm to 45mm
20mm to 38mm
10mm to 25mm
5mm to 15mm
2mm to 11mm
2mm to 8mm
In addition to choosing the size of the particles, it is also a good way to choose the appropriate solder paste for PCBA projects according to the type of PCB flux:
- Water Soluble Flux based Solder paste
They are made up of organic components and glycol bases. Utilizing cleaning products that are easily accessible on the market, you can remove the water-soluble fluxes.
- No-Clean solder paste
The solid residues and resins used to create the no-clean fluxes. Utilizing no-clean fluxes has the primary benefit of saving you money on cleaning supplies.
- Rosin-Based Solder Pastes
These are the type of solder pastes which are made up of rosins which can be cleaned by using solvents after soldering.
However, not every environment will allow you to employ this flux. To use this flux, you must have a clean environment with an inert flow.
How to apply solder paste?
You must carefully follow certain processes whether you’re using solder paste for high volume PCB assembly or prototype PCB assembly.
Applying solder paste to the circuit boards is the first step. You must ensure that you are putting this only where you’ll be soldering while doing this. With a syringe, solder paste can be individually applied to pins, but as you might guess, this is a labour-intensive manual method that would never be practical for regular production operations. Instead, the solder paste will be applied by circuit board assemblers using either a stencil or jet printing.
Stencil: The manufacturer will create a stencil with holes or apertures that are the same size and shape as the surface – mounted pads on the design to use the CAD data from the PCB design tools. Based on the parts to be bonded and the quantity of paste needed, these stencils can be constructed from a wide range of materials and are frequently laser cut for precision. Materials that will smooth out the aperture openings and aid in the even distribution of the paste can be used to plate and coat the stencils. It normally takes between 15 and 45 seconds to fill each aperture equally with solder using a squeegee to distribute solder over the stencil. To determine the quality of the deposited solder paste, the board is optically scanned when the print process is finished. After cleaning the stencil, the printing procedure is then repeated for the following board.
Jet Printing: Although stencil printing is excellent for big production runs, building a stencil takes time, and it can also be challenging to precisely control the quantity of solder for each part. These problems are addressed by jet printing solder paste, which is created using a modified recipe to make the paste more liquid. The jet printer applies the appropriate amount of solder to each interface mount pad on the board using the same PCB design data from which a stencil is made. Depending on the kind of solder required for each application, the printer effortlessly switches in and out a cartridge from which tiny drops of solder paste are ejected. PCB assemblers can accurately regulate how much solder is applied and are given a great deal of flexibility by jet printing.
Correct storage method for better application of PCB solder paste
Depending on the temperature and humidity of the PCBA factory, store these ready-to-use solder pastes in specially designed packaging that keeps the product’s temperature between 0° and 30 °C (86 °F and 84 °F) for at least 48 hours.
Solder paste should be delivered overnight or through chilled transportation. In summer, transport at night as much as possible to reduce time and avoid high temperatures. Paste packaging may include cold packs that arrive melting. If the 48-hour travel time is not exceeded, this is typical and not a sign of damage or improper handling. Upon arrival, solder paste must be moved right away to controlled storage. The use of refrigeration is advised but not always necessary.
At normal temperatures (22 °C/72 °F), solder pastes usually have a storage life of between three and six months. Unrefrigerated: 25 °C (77 °F) / Refrigerated: 0°-12 °C (32 °F-55 °F). The performance of solder paste may be impacted by exposure to temperatures above 30 °C (85 °F). Keep solder paste away from heat sources that are hotter than 32 °C (90 °F). When feasible, store solder pastes in syringes or cartridges vertically, tip down. Packages should occasionally be turned 180 degrees if they are horizontally stored.
Paste must be used before the date indicated on the product-specific TDS, which is the Date of Manufacturing (DOM) on the label. On a FIFO system with the date and time of removal from storage recorded, the oldest solder paste lots should be utilized first. Cold usage of solder paste is not permitted. Before opening the paste package, filler material should be allowed to attain room temperature, which is 68°–77 °F (20°–25 °C). Normally, this takes between four and six hours. DO NOT HEAT THE PASTE FORCEFULLY.
The solder paste in jars ought to be lightly and thoroughly mixed with a plastic tool for at least one minute after it has reached room temperature. Syringes and cartridges for solder paste don’t need to be mixed. It is not advised to use automated solder paste mixing equipment since it could harm the solder paste.