What is the difference between flux and solder paste?
What is the difference between flux and solder paste?
Blog Article
What is the difference between flux and solder paste?
1. What is the difference between flux and solder paste?
Although solder paste and flux work together, they are not the same thing.
Solder paste is a product used to bond different parts of metal alloys to each other.
Flux is an ingredient added between solder pastes or added to solder pastes. Flux acts as a cleaner during the soldering process, making solder paste more effective.
When you mix solder paste and flux, it will be more durable because the flux allows it to be perfectly fixed.
Apex-New Material flux manufacturer will introduce solder paste in detail.
2. What is solder paste?
Solder paste is also called solder paste, tin paste, which is mainly a mixture used to bond metal parts together. The composition of solder paste is relatively complex, and it is a mixture of solder powder, flux and other additives. Of course, the composition of additives still depends on the needs of solder paste.
Solder paste has a certain viscosity, so it can be attached to electronic components at a given position. As the temperature rises, the electronic components and PCB will be welded to form a permanent connection as the solvent and some additives evaporate.
It can be said that solder paste is glue. Its function is to bond two independent objects together. Therefore, solder paste is the key point of PCBA manufacturing.
Three, the role of solder paste
Solder paste connects two metals together
Solder paste can make wires and other components permanently connected to the circuit board
In template printing, solder paste can be used for electrical connection
Solder paste can connect mechanical parts together, especially for adjacent parts, it is the best choice
Four, how to use and store solder paste
1. Storage method
The storage of solder paste should be controlled at 1-10℃℃; the service life of solder paste is 6 months (unopened); it should not be placed in the sun.
2. How to use (before opening)
The solder paste temperature must rise to the ambient temperature (25+2°C) before it can be turned on, and the temperature recovery time is about 3-4 hours. It is forbidden to use other heaters to heat up instantly. Stir thoroughly after reaching ambient temperature. The mixing time is 1-3 minutes, depending on the type of mixer.
3. Usage (after opening)
Add about 2/3 of the solder paste to the stencil, and try to keep the amount on the stencil to no more than 1 can.
Depending on the production speed, the solder paste on the steel mesh should be added in small amounts and multiple times to maintain the quality of the solder paste.
The solder paste that is not used up on the day should not be placed together with the unused solder paste, but should be stored in another container. It is recommended that the solder paste be used up within 24 hours at room temperature after opening.
When using it the next day, the newly opened solder paste should be used first, and then the unused solder paste and the new solder paste should be mixed in a ratio of 1:2 and added in small amounts and multiple times.
After the solder paste is printed on the substrate, it is recommended to put the parts into the reflow oven within 4-6 hours to complete the soldering.
Please scrape the solder paste on the steel plate first, put it in the solder paste tank and change the line for more than 1 hour.
Due to dust and other pollution in the air, please follow the method of "Step 4" after the solder paste is continuously printed for 24 hours to ensure product quality.
To ensure printing quality, it is recommended to manually wipe the openings on both sides of the steel plate every 4 hours.
Maintain the indoor temperature at 22-28℃ and the humidity RH30-60% to obtain the best working environment
To wipe the wrongly printed substrate, it is recommended to use industrial alcohol or industrial cleaner.
V. Types of solder paste
Solder paste is divided into three categories according to post-processing: ordinary rosin cleaning, no-cleaning, and water-soluble solder paste
1. Ordinary rosin cleaning
Ordinary rosin cleaning is divided into two categories: activated rosin and lightly activated rosin
Classification of solder paste
During the soldering process, this solder paste has a better "tinning speed" and a good "welding effect". After the work is completed, there is more rosin residue on the surface of the PCB. The worker can use a detergent to clean it, and the PCB will glow without any residue, ensuring that the PCB has good insulation resistance and passes various electronic performance technical tests.
2. Clean-free type
Through welding, the surface of the PCB is relatively smooth, with less residue, and no need to re-clean. It can be tested through various electrical performance technologies to grant welding quality, while shortening the manufacturing process and increasing speed.
3. Water-soluble paste
Due to technical reasons in early manufacturing, there were too many residues on the surface of the PCB, which seriously affected the product quality and the electronic performance was not ideal. At that time, the cleaning process produced mostly used CFC, but it was not environmentally friendly and many countries had banned its use. In response to the needs, water-soluble solder paste came, which can be cleaned after welding, reducing product costs and meeting environmental protection requirements.
Water-soluble solder
Secondly, solder paste has different categories according to function, product or manufacturing, which can be complicated. Therefore, it can be selected according to the alloy composition mesh and viscosity.
Sixth, how to choose the right solder paste?
Details determine success or failure. Choosing solder paste seems to be a trivial matter, but in fact, solder paste determines the success or failure of the PCB assembly process.
There are many solder pastes available on the market. Although it does not seem to be so different according to the classification, not all solder pastes are the same. Special solder pastes are generally more expensive, but they usually have additional advantages.
Choosing the right solder paste will avoid many problems. Many factors affect the choice of solder paste, such as: wetting characteristics, gap control, flux residue, alloy strength, alloy flexibility and other performance indicators.
Changes in these performance indicators will affect the choice of solder paste. For example:
Quality can be maximized by selecting a paste that has the best performance with the materials, geometries and heating processes used to manufacture the product.
Throughput can be maximized by selecting solder products that are suitable for the best deposition and heating methods.
Cost targets depend on quality and output, and can be maximized by considering materials, direct labor, inspection, rework and scrap value.
Here are some factors to consider when choosing the right solder paste,
1. Lead or lead-free solder paste
The biggest difference between lead and lead-free is toxicity and non-toxicity, and the other is melting temperature. Lead-free solder paste has a higher melting point. Here you can choose according to the actual requirements. Regarding leaded and lead-free solder pastes, there is a detailed summary in the previous article. You can click on the title to jump directly.
2. Water-soluble or no-clean solder paste?
Water-soluble solder paste contains high molecular compounds, such as polymers, which are not as effective as rosin/resin in preventing reoxidation. After the circuit board passes the reflow stage, water-soluble solder paste makes the circuit board look cleaner, and the flux residue is burned off and easily washed off in the board washer.
No-clean solder paste functions the same as water-wash paste, but the residue is left on the board. No-clean chemicals are usually rosin/resin-based materials. Rosin/resin forms an excellent oxide barrier and protects the "clean" surface from reoxidation during reflow, which may damage the appearance. The specific one can be selected according to actual requirements.
3. Melting point temperature of solder paste
As shown in the figure below, each alloy has a temperature at which it changes from solid to liquid. The phase change from solid to liquid begins when the solidus is reached and ends when the liquidus is reached.
Below the solidus, the alloy is 100% in the solid state.
Between the solidus and liquidus, a region called the plastic range, some parts of the alloy are solid but most of it is liquid.
When the solidus and liquidus are equal, the alloy is called a eutectic alloy.
Although wetting begins at the solidus temperature, optimal wetting is achieved at a peak temperature 15°C or higher above the liquidus. If the solder joint needs to maintain physical integrity in subsequent operations (such as a second reflow process), the peak temperature of the subsequent operation needs to be lower than the solidus temperature of the alloy.
4. Alloy composition
Generally speaking, you can choose Au80Sn20, Au88Ge12, Au96.85Si3.15 solder alloy composition, which can match the welding requirements.
For Ag or Pd electroplated products, you can choose Au88Ge12, Au96.85Si3.15 solder paste:
For non-thermal shock products, you can choose bismuth-containing solder powder.
5. Particle size (mesh number)
Also known as particle size, it refers to the mesh number of the sieve per square inch; in the actual tin powder production process, most of the tin powder is collected with several layers of sieves with different mesh numbers. Because the mesh size of each layer of sieve is different, the mesh size of the tin powder passing through each layer is also different, and the particle size of the tin powder particles collected in the end is also a regional value.
Therefore, the larger the mesh number of the solder paste, the smaller the particle size of the tin powder in the solder paste; the smaller the mesh number, the larger the particles of the tin powder in the solder paste.
That is, when the solder paste manufacturer selects the solder paste according to its mesh index, it should be determined according to the distance between the smallest solder joints on the PCB: if there is a larger distance, a solder paste with a smaller mesh size can be selected. On the contrary, when the solder joint spacing is small, a solder paste with a larger mesh should be selected, and the general particle size is about 1/5 of the SMT steel mesh opening:
6. Viscosity
In the SMT workflow, from the laser template printing (or spotting) of the solder paste to the attachment of the components to the reflow heating process, there is a process of moving, placing or carrying the PCB in the middle; in this process, in order to ensure that the printed (or dotted) solder paste does not deform and the components attached to the PCB solder paste do not shift, the solder paste is required to have good viscosity and retention time before the PCB enters the reflow heating.
7. Storage stability
We need the solder paste to have stable quality, but in actual applications, the stability of the solder paste will change from purchase to warehousing and storage for a period of time.
8. Printing stability
In the actual production process, the performance of reflow soldering is also very important and has a great impact on quality.
9. Final welding effect
Mainly includes the following four aspects: good wettability, less BGA fusion failure, preheating collapse, and stop printing recovery ability.
Solder wetting is part of the process where the metal in the solder combines with the metal on the printed circuit board (PCB) or component. In the comparison with different solder pastes, you will find that A is more wet than B.
Comparison between different solder pastes
In the BGA process, due to conditional limitations, the fusion ability of solder paste is more important.
The higher the preheating collapse, the lower the incidence of poor bridging. You can find that there is no collapse in the left picture, that is, 0.3mm/0.2mm, but there is collapse in the right picture, that is, 0.4mm/0.3mm.
Excellent shutdown recovery can improve printing production efficiency to a certain extent. From this photo, you can see the difference curve after one hour of parking.
Selecting the right solder paste should not only consider the characteristics of the solder paste itself and its impact on quality and production, but also various factors in mass production. Therefore, it is necessary for us to constantly record and summarize the impact of solder pastes from various suppliers on products in actual production. Report this page