PCBA Tech - About SMT patch processing plus solder paste printing

Solder paste printing is a key process in the production of SMT chip processing plants, which affects the welding quality of PCB assembly boards. This article analyzes many elements of solder paste printing skills that affect print quality, analyzes its composition and mechanism, and provides solutions for these elements.

With the rapid development of component packaging, more and more PBGA, CBGA, CCGA, QFN, 0201, 01005 resistance-capacitance components have disappeared and become popular, and the detailed placement skills have also quickly opened up. In the process of consumption, The influence and dyeing of solder paste printing on the entire consumption process are becoming more and more popular among engineers. In the profession, the company also universally recognizes the good soldering to be obtained, and the quality of the product is always robust, and the most important thing is the printing of solder paste. Longhua SMT chip processing and consumption not only need to dominate and use solder paste printing skills, but also need to be able to analyze the difficult notices during the process, and apply the reformation to the consumer reality.

1. Skill control of solder paste printing process

Solder paste printing is a very technical process, which involves a lot of technical parameters, and the incorrect adjustment of each parameter will have a great impact on the quality of the placement product.

1. Viscosity of solder paste

The viscosity of the solder paste is the most important factor that affects the printing function. If the viscosity is too large, the solder paste cannot easily pass through the openings of the template, the printed lines are incomplete, the viscosity is too low, simple flow and edge collapse, affect? What is the sodium straw bath? The viscosity of solder paste can be measured with an accurate viscometer. What if the company purchases imported products in practice?

(1) In the process of returning to room temperature from 0 degree Celsius, the sealing and time must be ensured;

(2) The best use of special mixer for mixing;

(3) The output is small, and the solder paste is repeatedly used. It is necessary to formulate strict standards, and the use of solder paste outside the standard must be strictly stopped.

2. Tackiness of solder paste

The viscosity of the solder paste is not good, and the solder paste will not churn on the template during printing. The direct result is that the solder paste cannot fill up the template opening completely, and the composition of the solder paste will be insufficient. The viscosity of the solder paste for Nanshan SMT chip processing is too large, which will cause the solder paste to hang on the wall of the template hole and prevent all of it from being printed on the pad. The selection of the viscosity of solder paste generally requires its self-adhesive ability to be greater than its ability to bond with the template, and its adhesion to the template hole wall is less than its adhesion to the pad.

3. The uniformity and size of solder paste particles

The particle shape, diameter, and symmetry of the solder paste also affect its printing results. The printing characteristics of 3#, 4#, 5# solder paste issued by 01005 tools in recent occupations are really a lot of research. The author thinks that For the solder particles of a type of solder paste, the diameter of the largest particle in the typical limit is about 1/5 of the template opening standard, and the desired printing can be achieved by selecting a mesh plate suitable for thickness and skill. The same general small particles of solder paste will have very good solder paste print strip clarity, but rough edges will be generated, and the chance of being calm and safe by oxidized water is also high. As a general rule, the pin blocking distance is used as a serious selection factor in the process, and the results and prices are coordinated together.

Two, the elements of solder paste

Solder paste is much messier than pure tin-lead alloy, and its main components are as follows: solder alloy particles, flux, rheology modifier, viscosity control agent, solvent, etc. Indeed, we must grasp the relevant factors and choose different types of solder paste; together we must also choose large manufacturers with perfect product process skills and stable quality. Generally, the following elements should be paid attention to when selecting solder paste:

Three, the elements of the template

1. Information and engraving of the stencil

Generally, chemical etching and laser cutting are used. For high-precision screens, laser cutting should be used because the walls of the laser cut holes are straight, have a small roughness (less than 3μm) and have a taper. Some people have already tested and proved that the 01005 equipment of the size of the salt grains has higher precision requirements for solder paste printing. Laser cutting is no longer satisfactory. Special electroforming, also called electroplating, is required.

2. Some of the SMT stencils are related to solder paste printing

(1) Dimensions of the openings: The shape of the openings on the stencil and the shape of the pads on the printed board are very important for the fine printing of solder paste. In Nanshan SMT patch processing, high-end placement machines can accurately control the placement pressure, and the intention also includes trying not to knead and damage the solder paste picture, to avoid bridging and splashing in the reflow. The opening on the stencil is mainly determined by the size of the corresponding pad on the printed board. Generally, the size of the opening on the stencil should be 10% smaller than the corresponding pad. In practice, many companies have adopted a 1:1 ratio of openings and pads in the production of stencils. There are many welding techniques for small batches and various types of production. The author has tested and welded many QFN equipment. The method of spot solder paste, and strictly control the amount of solder paste at each point, but no matter how to adjust the reflow temperature, use X-RAY to detect, there are more or less solder balls at the bottom of the equipment. According to the actual situation, there is no condition to make the net plate, and finally the method of planting the ball with the equipment has achieved a better welding effect, but this is also satisfied with the special conditions, and can only be used in a small batch of production. SMT printing skills-how to do solder paste printing (2)

(2) The thickness of the stencil: The thickness of the stencil and the size of the opening have a great relationship with the printing of the solder paste and the subsequent reflow soldering. Specifically, the thinner the thickness, the larger the opening, which is more conducive to the release of the solder paste. . It has been proven that outstanding print quality requires that the ratio of the aperture size to the thickness of the stencil is greater than 1.5. Otherwise, the solder paste printing is not complete. Under normal circumstances, for the lead spacing of 0.3 ~ 0.4 mm, use a screen with a thickness of 0.12 ~ 0.15 mm, and use a screen with a thickness of 0.1 mm for the spacing below 0.3.

(3) SMT stencil opening direction and dimensions: the release of solder paste in the length direction of the PCB pad and the printing direction are the same, and the printing effect is better than when the two directions are straight. Specific stencil drawing skills can be implemented according to Table 2.