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What are the composition of solder paste for SMT?
As an indispensable part of the SMT production process, the size of the particles of the solder powder in the solder paste, the proportion of metal content, the content of flux, mixing time, temperature recovery time, as well as the placement of the solder paste and storage time will affect the final printing quality of the paste.
SMT special solder paste composition is generally divided into these two categories, namely, solder powder alloy and flux.
Solder Paste
The combination of alloy powder material and role:
Solder powder alloy is the core combination of solder paste, accounting for at least 85% to 90% of the total quality of the paste. More common alloy combinations are the following categories: tin-lead alloy (Sn-Pb), tin-lead-silver (Sn-Pb-Ag) alloy, tin-copper alloy (Sn-Cu), tin-silver-copper alloy (Sn-Ag-Cu), tin-bismuth alloy (Sn-Bi), tin-bismuth-silver alloy (Sn-Bi-Ag) and so on.
Alloy powder material of the effective composition, ratio, particle size are oxidized layer relative to the solder paste in terms of direct impact.
When the alloy content is relatively high, can ease the collapse of the solder paste, conducive to promote the solder paste composition of full bright solder joints, and because the flux content with the alloy content continues to improve and change, so the post-solder residue is also reduced, you can effectively prevent the presence of tin beads, however, the disadvantage is that the printing and soldering process requirements related to the increasingly stringent requirements.
When the alloy content is relatively low, the printing performance will be very good, solder paste is not easy to stick scraper, good wettability, leakage board durable, production and processing will be easier, but the disadvantage is easy to collapse, easy to produce tin beads or bridging and other defects.
Flux combination and role:
1. Active agent: the active ingredient is the most important is to play to get rid of PCB circuit board copper film pad surface and electronic devices welding parts of the important role of oxides, and it can also reduce the liquid surface tension of tin, lead and other metals;
2. Organic solvents: the active ingredient is mainly concentrated in the solder paste stirring process to play a role in regulating the proportionality of the solder paste has a greater impact on the service life of the solder paste;
3. Thixotropic agent: the substance is mainly used to adjust the viscosity of the solder paste and printing performance, can be avoided in the printing process, often the tail, sticky and other problems exist;
4. Rosin: rosin can increase the adhesion of solder paste, and has a systematic protection against post-solder PCB circuit board oxidation again important role; will also play a vital role in the fixing of electronic devices.
The solder paste composition will directly affect the quality of SMT soldering, and should be correctly selected before soldering.