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Solder paste selection

In the SMT manufacturing process, solder paste is an indispensable auxiliary material, choice high-quality solder paste is related to the quality of the product. This article will briefly analyze and introduce how to evaluate a suitable solder paste.

Metal particle size selection

Solder paste can be divided into 6 types according to the size of metal particles, of which the size of No. 1 powder is the largest, and the size of No. 6 powder is the smallest (see the table below). The pad spacing is large, and the size of the stencil mesh opening is large, then use solder paste with larger metal particles. On the contrary, for the assembly of fine-pitch components, solder paste with small particles is required. In principle, in terms of cost and soldering quality, a large particle size solder paste has a low cost and a small oxidation probability, and has a good application effect. Conversely, solder pastes with smaller metal particles are more expensive and have a higher chance of oxidation. On the other hand, for small stencil opening sizes, large particles of solder paste may cause problems such as poor printing and poor mold releasability, while small-sized metal particles can improve the mold release performance of solder paste, but printing slump etc. problem may occur.

Therefore, what type of solder paste is used is particularly important. How to choose the solder paste that suits us in actual production, we need to follow an important principle, that is, the "five-ball principle", that is, in the stencil mesh opening or the smallest pads can place five solder balls. The diameter of this solder ball is based on the maximum size of the type.

For example, for QFP components with 0.4mm pitch, the width of the pad is generally 0.2mm, and the maximum width of the stencil mesh opening can only be 0.2mm, otherwise there is a risk of distance. If you choose No. 3 powder, the diameter of the metal particles is 0.025-0.045mm , Then the maximum solder ball diameter of 0.045X5 is equal to 0.225mm, which is larger than the opening width of the stencil mesh, and the diameter range of No. 4 solder paste is 20-38 microns, and the largest solder ball diameter is 0.038*5=0.19mm, which satisfies the "five balls principles" requirements, therefore, for 0.4mm QFP components need to choose No. 4 powder solder paste and it is not recommended to choose No. 3 powder solder paste.

Slump test

Slump test is to evaluate the ability to retain the shape (round or square) of the solder paste after printing. If slump occurs after the solder paste is printed, the lateral area (width) of the solder paste expands, and the solder paste of adjacent pads will adhere together or the gap between them will reduce. When the components are mounted, the solder paste is squeezed, and then short circuit after reflow will occur. So this test is very important, and it is an important feature of solder paste. The slump test includes cold slump and hot slump.

Cold slump

Check the solder paste immediately after printing to determine the printed shape of the solder paste. Then, place the solder paste in an environment with an ambient temperature of 20-30°C and a space relative humidity of 40%-60% (SMT workshop environment) for an hour and check to confirm that the shape of the solder paste has not changed, and the edge area is not solder ball separation is allowed.

Hot slump

Place the PCB with solder paste printing on the heater, heat the solder paste, wait for the PCB surface temperature to reach about 150℃ (140-160℃), then keep it for 10-15 minutes, observe the shape of the solder paste on the pad variety. During the entire heating process and after cooling, the shape of the solder paste on the pad should be kept in its original state without any change. Although the part of the volatile components in the solder paste evaporates, the state of the solder ball particles is more obvious, and the surface has pores, but the shape of the solder paste cannot change, and there must be no solder ball separation at the edge.

Printability and working life test

Solder paste needs to have good printability and long working life during use. The working life of solder paste mainly refers to the continuous printing time of solder paste on the stencil. The solder paste needs to fill all the openings on the stencil mesh well, especially some tiny openings. Moreover, when the stencil and the PCB are separated, the solder paste should have a good mold release performance, to ensure that the solder paste deposited on the pad has a good shape and a sufficient volume, and there will be no abnormal conditions such as splashing.

This evaluated process can be combined with the normal production process without additional preparation or investment. Perform SMT solder paste printing normally, observe the solder paste status on the stencil, the shape of the solder paste after printing, SPI detect the volume or coverage area of the solder paste, and the state of the solder paste on the stencil after long time printing. There was a case: the solder paste completely adhered to the squeegee after continuous printing for a period of time, and there was no solder paste visible on the stencil. This situation will seriously affect the filling of the solder paste in the stencil mesh hole. Printability and working life certainly cannot meet normal production requirements.

Wetting test

Solder paste is printed onto the PCB pad, and in accordance with the normal reflow process, the solder paste will wet the PCB pad after melting. Observe the area coverage of the solder paste on the pad after printing. The solder paste melts and wets the pad area. In theory, the larger the diffusion range, the better (relative to the solder paste). The solder must obviously wet the pad, and there should be no obvious phenomenon of non-wetting and dewetting.

Tackiness test

Tackiness is the ability of the solder paste to fix the component. The component placed on the solder paste will not move due to the movement on the track or stop. If the adhesion force is insufficient, when the PCB with the component on the track or stop, the component may move, causing deflection and short circuit.

The components are usually mounted on the test board or PCB board of the printed solder paste, and maintained for a period of time, and the crash test is performed at intervals of two hours. Let the PCB run on the track, and there will be no component displacement at the normal stop position.

Whether a solder paste is suitable or not depends on the soldering quality. All the above evaluation elements are for good soldering results. Through the statistics of the soldering quality data after reflow, the defects generated are analyzed and determined whether they are related to the solder paste quality. The production site can use some simple comparison tests to verify, through the production of different brands, different types of solder paste production comparison, from which to choose one suitable for the production characteristics of the solder paste.