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How to Select Solder Paste for SMT Printer

Time:2019/03/21 丨 Source:未知 丨 Browse times:

SMT printing process selection of solder paste is a very complex process, if not paid more attention, it will probably cause waste of components.
Usually,the evaluation of the advantages and disadvantages of a solder paste and process performance is based on its printability, mountability,
solderability and other properties to make corresponding judgments. How to choose solder paste? It is impossible for us to try out all of them before choosing them.
Therefore, when choosing a product, we should first look at its packaging to see if it is tight. Then look at its product introduction, because its role is different,
so the choice of solder paste is also different, when purchasing must choose their own model.
SMT solder paste
 
Solder paste
Generally speaking, solder paste is the first choice for SMT printing press, and then it is selected according to alloy composition, particle size, viscosity and other indicators.
 
Classification of SMT solder paste:
 
 
A. Ordinary rosin cleaning solder paste [divided into RA (ROSIN ACTIVATED) and RMA (ROSIN MILDLY ACTIVATED)]:
This type of solder paste shows better "tin loading speed" and can ensure good "welding effect". After welding,
PCB surface rosin residue is relatively large, and can be cleaned with appropriate cleaning agent, after cleaning,
the surface of the plate is smooth and no residue, which ensures cleaning. The rear panel has good insulation impedance and
can pass various technical tests of electrical performance.
 
B. Clean-free solder paste [NC (NO CLEAN)]: After soldering,
PCB plate surface is relatively smooth and less residual, which can be tested by various electrical performance technology without need of re-cleaning.
This solder paste shortens the production process while guaranteeing the welding quality and speeds up the production progress.
 
C, water-soluble solder paste [WMA (WATER SOLUBLE PASTES)]: Because of technical reasons,
the solder paste produced in the early stage generally had too much residual PCB board surface, and its electrical performance was not ideal,
which seriously affected the product quality. At that time, CFC cleaner was used to clean it, because CFC was not environmentally friendly,
many countries had banned it. In order to meet the needs of the market, water-soluble solder paste came into being.
After the welding work is completed, its residues can be cleaned by water,
which not only reduces the production cost of customers, but also meets the requirements of environmental protection.
 
2. Selection criteria of solder paste for SMT printing press:
 
 
1. Composition of solder paste alloy:
In general, the Sn63/Pb37 solder alloy component can meet the welding requirements; for the welding of silver (Ag) or palladium (Pd) coated devices,
the solder paste with Sn62/Pb36/Ag2 alloy component is generally selected;
for the PCB welding of non-heat shock resistant devices, the solder powder containing Bi is selected.
 
2. Viscosity of solder paste (VISCOSITY):
In SMT workflow, there is a process of moving,
placing or transporting PCB from printing (or dotting) solder paste and pasting components to feeding into reflow soldering heating process.
In order to ensure that the printed (or dotted) solder paste does not deform and the components pasted on PCB solder paste do not shift,
solder paste should be used before PCB enters reflow soldering heating process. Good stickiness and retention time.
A. Viscosity index of solder paste (i.e. viscosity) is usually expressed in terms of "Pa.S". Among them, 200-600 Pa.S
solder paste is more suitable for needle-type point injection or production equipment with higher automation. Printing process requires relatively high viscosity of solder paste,
so the viscosity of solder paste used in printing process is generally about 600-1200 Pa.S, which is suitable for manual or mechanical applications. Printing;
B. The solder paste with high viscosity has the characteristics of good solder joint pile forming effect and is more suitable for fine space printing,
while the solder paste with low viscosity has the characteristics of fast drop, tool wash-free and time-saving in printing.
C. Another characteristic of solder paste viscosity is that its viscosity will change with the stirring of solder paste, and it will decrease when stirring;
when stirring is stopped a little, its viscosity will return to its original state; this is very important for how to choose solder paste with different viscosity.
In addition, the viscosity of solder paste has a great relationship with temperature. Under normal conditions,
the viscosity of solder paste will gradually decrease with the increase of temperature.
 
3. Number of solder paste and tin powder (MESH):
In domestic solder paste manufacturers, different solder pastes are classified by "particle size" of solder powder,
while many foreign manufacturers or imported solder pastes use the concept of "stencil number (MESH)" to classify different solder pastes.
The basic concept of mesh number (MESH) refers to the number of meshes per square inch of screen area. In the actual tin powder production process,
several different meshes are mostly used to collect tin powder. Because the mesh size of each mesh layer is different,
the particle size of tin powder through each stencil layer is also different. Finally, the particle size of tin powder collected is also a regional value.
A. From the above concepts, the larger the number of solder paste, the smaller the particle diameter of solder powder in the solder paste,
and the smaller the number of solder paste, the larger the particle size of solder powder in the solder paste.
B. If the solder paste manufacturer chooses the solder paste according to the index of solder paste number,
it should be determined according to the distance between solder joints with the smallest distance on PCB:
if there is a larger distance, the solder paste with a smaller number of eyes can be selected; otherwise,
when the distance between solder joints is small, the solder paste with a larger number of eyes should be selected;
generally, the particle size is less than 1/5 of the opening of the template.
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