SPI: Solder Paste Inspection
Why Do Technicians Need To Do Solder Paste Inspection?
Solder paste is composed mainly of alloy powder, flux, and additives. It is an upgraded alternative to flux paste, and technicians widely use it in SMT. The quality of the solder paste will directly affect the post-soldering quality of electronic products, so the solder paste should be strictly checked and measured.
What Are The Three Parts Of Solder Paste Inspection?
Professionals divide solder paste inspection into basic performance tests, Operation on the machine, and Reliability tests.
1. Basic Performance Test
The basic performance test of solder paste mainly refers to IPC-J-STD-005 (Requirements for Soldering Pastes) and JIS-Z-3284 (Japanese Industrial Standard))
Tin powder size and particle size distribution
70% to 90% of the solder paste is composed of tin powder, and the performance index of tin powder largely determines the performance of solder paste.
Technicians test the metal ratio using the quality inspection method, see IPC-TM-650, 2.2.20. The test value should not exceed ±1% of the set value.
Technicians perform viscosity testing using Malcolm (in Pa-s) or Brookfield (in cp) and require that the test value does not exceed 10% of the set value. When using the Malcolm test, technicians also need to pay attention to the Ti and R-value of the paste. Ti value is the thixotropic index, reflecting the speed of the paste under the action of external force to change the form. Generally good Ti value is 0.4 ~ 0.6 or so. R-value in the viscosity does not recover rate, reflecting the ability of the paste to return to the initial state when the external force is reduced.
The test is concerned with hot and cold collapse. Cold collapse represents the ability of the solder paste to maintain its printed form when printed. For some delicate pitch pin components, if the cold collapse is poor, it will directly appear before the soldering of even tin, which will inevitably cause a large area of short circuit phenomenon after reflow.
IPC has developed a standard for testing tin beads according to the powder diameter range of tin powder for the tin beads. For example, for T1-T4 tin powder, there should not be more than 75μm tin beads, while for T5-T6, there should not be more than 50μm tin beads.
The printed circuit board (PCB) substrate mainly contains OSP copper, tin-plated, nickel gold, and other substrates. IPC-TM-650 requirements for wetting cannot appear obvious back or non-wetting phenomenon; the standard can meet the SMT manufacturers of the final product judging.
Flux paste granularity
The flux detection mainly includes granularity, viscosity, solid content, acid value, halogen, etc. The flux paste formula design determines the density, solid content, halogen content, and an acid value of flux paste and sets the value. The performance parameters of the paste are specific. Still, the granularity of the flux paste can primarily affect the performance of the paste. Such as the viscosity of the paste, under the tin, stability, and so on. When the granularity of a flux paste is large, the hard particles will act as the role of tin powder, thus increasing the viscosity of the solder paste. In addition, the appearance of the paste state coarse, the overall poor rolling when printing indirectly affects the amount of tin, resulting in soldering defects.
2. Run On The Machine
After the basic performance test of the solder paste is passed, technicians can test it on the machine. The purpose of the machine test is to detect the under tin, demold, and the process window width of the printing speed of the solder paste and to detect the physical and chemical stability of the solder paste through a long time of printing.
Whether the rolling of the solder paste will speed up the evaporation of the solvent and the reaction speed of the active material with the tin powder, once the solvent evaporates too fast, the solder paste will appear dry and affect the rolling of the solder paste. In addition to the form of the solder paste that technicians cannot change, the cold collapse value within eight h is not against the IPC test requirements. The fluctuation of the solder paste under the tin six within the allowable range, the adhesion of the solder paste to the stencil is much smaller than the adhesion of the solder paste to the solder tray, etc.
The soldering process is the chemical reaction between flux paste and tin powder. Once the rapid reaction between tin powder and flux paste occurs in the printing process, it consumes part of the flux paste activity.
Excellent performance of the solder paste needs to ensure that it consistently maintains suitable performance parameters in the eight hours printing process.
3. Reliability Testing
Whether it is the actual performance test of the solder paste or the up-run, all the testing items serve for soldering reliability. The conventional ones are surface insulation impedance (SIR), electromigration, void rate, etc.
Solder Paste Daily Inspection and Measurement Steps
Before Production And Processing:
- A routine inspection of outsourced parts, commonly known as IQC.
- Tin powder inspection and measurement, where technicians use a high-power microscope to compare the granularity of the tin powder.
After Production And Processing:
- Technicians inspect and measure microelements, usually assessing and measuring Pb, Hg, and other hazardous metal materials. If it is a halogen-free product, they also need to routinely check the content of halogens and use the atomic absorption spectrometer here.
- Viscosity testing the viscosity of the solder paste in the use of the process is critical, so each batch should be strictly checked and measured. Technicians usually use the density measuring instrument in the viscosity test.
- Printability, printing, solder paste is an indispensable step, so it is very critical. Each shipment needs to be sampled and tested with a printing stencil to test the printability.
- Test after welding six usually involves lubrication and solderability to see whether the solder paste dries after welding firm, with the scrap PCB board and no electrical components for testing.
- Solder joint gloss, the test is to point the solder paste on the glass piece with a simple stove unfolded heating. After looking at the solder joint is not glossy; in addition, you can also see whether the solder paste residue is clean and neat.
- The last is the tin bead test.
The formulation system determines the performance of a solder paste, the usage of the process environment, and other factors. Therefore, in determining the performance parameters of a solder paste, the perfect evaluation system and supporting testing equipment are used to find the proper product process window effectively.