SMT Solder Paste Printing Quality Control

Abstract: One of the key processes in the surface mount process is solder paste printing. Its control directly affects the quality of the assembled board. Through the solder paste characteristics, template design and manufacturing, as well as the optimization of printing equipment process parameters, etc., the solder paste printing quality control for preliminary discussion.

Keywords: solder paste stencil printer squeegee

Solder paste printing process is the key process of SMT. Its print quality directly affects the quality of printed board assemblies, especially for IC devices that have fine pitch pitches below 0.65mm, and has higher requirements for solder paste printing. These are all controlled by the function of the solder paste printer, the design and selection of the template, the choice of solder paste, and the parameters set by practical experience. This article discusses how to control the solder paste printing quality in terms of these aspects.

1, solder paste requirements
1,1 Good printability The viscosity and particle size of the solder paste are its main properties. The viscosity of the solder paste is too large, which can easily cause the solder paste not to be printed on the bottom of the template opening, and it can also stick to the scraper. Paste paste viscosity, it is not easy to control the deposition shape of the solder paste, after printing will collapse, it is easier to produce bridging, while the viscosity of the past in the use of soft scraper or scraper pressure, make the solder paste from the template Holes are scraped away to form concave solder paste deposits, causing insufficient solder to cause a false weld.

Excessive viscosity of solder paste is generally due to formulation reasons. Too low a viscosity can be adjusted by changing the printing temperature and the speed of the blade. The temperature and the speed of the blade reduce the size of the solder paste. It is generally considered that the optimum viscosity range for fine pitch printing paste is 800pa·s to 1300pa·s, while the common viscosity range is 500 to 900pa·s.

The particle shape, diameter size, and uniformity of the solder paste also affect its print performance. Generally, the solder paste particles are spherical and have a diameter about 1/5 of the opening size of the template, and the diameter of the particles should be uniform. The maximum and minimum size particles should not exceed 10%, so as to improve the uniformity of printing and Resolution. We can understand the relationship between the choice of solder paste and the device spacing from Table 1.

Table 1 Relationship between solder paste selection and device spacing Solder paste size range (um) Device pitch
75-45 -200/+325 0.60-1.30
45-25 -325/+500 0.40-0.60
35-25 -400/+500 0.20-0.40
<20 -500 <0.2

1,2 The adhesiveness of a good adhesive paste, in addition to the size and size of the solder paste, mainly depends on the composition of the flux system in the paste and the ratio of other additives. The good adhesive property of the solder paste makes the adhesion to the pad greater than the adhesive force of the opening side of the template when printing, so that the solder paste firmly adheres to the pad, improves the mold release property, and has good adhesiveness and energy. Keep enough time to reduce flying or falling pieces when mounting components.

1,3 Good Solderability Solder paste for printing, with a typical metal content of 90%. Solder paste solder balls must conform to oxide-free grades, ie, oxide content <0.1%, and total oxide content including surface-adsorbed oxygen = <0.04%. Solder paste is too long to be stored after printing. If the printing cycle is too long, the degree of oxidation will increase due to the volatilization of flux and other substances, affecting the wettability of the solder. Solder paste should be stored at 5-10°C and used at 22-25°C.

According to the performance and use requirements of the solder paste, the following points can be used to select the appropriate solder paste:
1) The activity of the solder paste can be determined according to the cleanliness of the surface of the PCB. The RMA level is generally used, and the RA grade is used when necessary.
2) Use different viscosity solder pastes according to different coating methods;
3) Use spherical fine particle solder paste for fine pitch printing;
4) When welding on both sides, high melting point solder paste is used on the first side, and low melting point solder paste is used on the second side to ensure a difference of 30-40°C between the two to prevent the first-side soldered component from falling off;
5) When soldering heat-sensitive components, low-melting solder paste containing bismuth should be used;
6) When using the no-clean process, do not use chlorine ions or other corrosive compounds.
7) It is also required that the solder paste has good cleanliness after reflow, and that solder balls are rarely generated and have sufficient soldering strength.

2, template

The template is the basic tool for solder paste printing. There are three main types: screen templates, all-metal templates, and flexible metal templates. The screen template is simple to manufacture and is suitable for small batches of products. The disadvantage is that the holes are not easily visible through the screen and the positioning is difficult, while the solder paste through the screen is only about 60% of the holes and is easy to plug. The opening size of the stencil is closely related to the stencil thickness. If the stencil thickness is too thick, it will lead to poor mold release of the solder paste and bridging of solder joints. If the thickness of the stencil is too thin, it will be difficult to meet the requirements of the assembly board with fine and fine pitch. See Table 2 for selection

Device Type Lead Spacing (mm) Pad Width (mm) Template Opening Size (mm) Template Thickness (mm)
Universal SMT Devices (Note) BGA 1.27 0.64 0.58 0.20-0.25
0.65 0.35 0.30-0.33 0.15-0.18
0.50 0.30 0.22-0.25 0.12-0.15
0.40 0.22 0.18-0.20 0.10-0.12
0.30 0.18 0.12-0.15 0.10
1.27 0.80 0.76 0.20-0.25
1.00 0.63 0.56 0.15-0.20
0.50 0.25 0.23 0.12-0.19
Flip Chip 0.25 0.12 0.12 0.08-0.10
0.20 0.10 0.10 0.05-0.10
0.15 0.08 0.08 0.03-0.08
Note: Includes general-purpose devices such as SOIC, PLCC, TSOP, and QFP

Template openings are generally manufactured by chemical etching, laser beam cutting and electroforming. In the manufacturing process, smooth and uniform opening sidewalls are targeted. The template can be made of tin bronze, beryllium bronze, stainless steel, foil and other materials with pad holes. Stainless steel is the most commonly used material for laser cutting to form stencils. The stencil opening obtained by cutting with a laser beam can naturally form a tapered inner wall, which is helpful for the release of the solder paste. This feature is particularly important for fine-pitch printing. In addition, the inner wall of the opening can be made smoother by electropolishing or nickel plating.

The flexible metal template is a combination of a non-straight wire mesh template and an all-metal template. The etched metal template is perforated around, and the size range is controlled between 15-20 mm. It is used for small joints fixed on the screen, and is fixed with tape around it. This kind of production process is simple, low cost, and can meet the production of small and medium-sized batches. At present, most of these templates are used at home and abroad. The distance between the metal template and the aluminum frame must not be too large, so as to ensure that the fiber tension exceeds the elastic point and has a certain degree of flexibility. Generally, the distance is controlled to 50-75 mm.

3, the choice of printing press <br> solder paste printing press to perform two major functions, the precise positioning of the template and PCB and scraper parameter control. At present, the printing press is developing rapidly, and most of the machines use the machine vision system to automatically locate the reference point on the PCB to complete the rapid adjustment before printing. MPM's Utraprint 2000 Solder Paste Press utilizes advanced up/down vision and optical technology to provide a sophisticated closed-circuit anti-corrosion system for fast and accurate positioning, and a 3D height detection system accurate to 12 to 14 pads per second The solder paste height was measured. The printable PCB was 0.381-12.7mm with a minimum pitch of 0.3mm. Suitable for users with large production scale and high product requirements. For users with small batches and many varieties, the flexibility of choosing a semi-automatic/manual printer is good, the price is more economical, and the operation process is simple.

4, the setting of printing process parameters
Types of 4 and 1 scrapers The types of scrapers used by different manufacturers are different. At the same time, differences in solder paste may require the use of different scrapers. There are three types of scrapers.
4, 2 blade adjustment
a) The quality of solder paste printing is best when the blade operating angle θ is generally 60-65o. The traditional printing method of solder paste is that the blade runs at an angle of 90o along the X or Y direction of the template, which often leads to different soldering training volume in different directions of the device. We have verified through multiple printing experiments that the printing of the squeegee in the direction of 45o can significantly improve the imbalance in the orientation of the different stencil openings of the solder paste, and at the same time can reduce the damage to the fine-pitch stencil opening.
b) The squeegee pressure does not depend only on the cylinder stroke to adjust to the optimum squeegee pressure, but also the squeegee parallelism. The pressure is generally 30N/mm2.
4、3 Printing speed The solder paste will roll forward on the template under the influence of the scraper. Fast printing speed is conducive to the rebound of the template, but it will also hinder the transfer of solder paste to the PCB pad, and the speed is too slow, the solder paste will not roll on the template, causing the solder paste printed on the pad resolution Poor, typically 25-30 mm/s for fine pitch printing speeds and 25-50 mm/s for coarse pitch printing speeds.
4,4 printing methods The most common printing methods are classified into contact printing and non-contact printing. The printing method with a gap between the template and the PCB is non-contact printing. General clearance value is 0.5-1.5mm, its advantage is suitable for different viscosity solder paste. The solder paste is pushed by the squeegee into contact with the PCB pad. After the squeegee is slowly removed, the stencil is automatically separated from the PCB. This reduces the risk of template contamination due to vacuum leaks.
The printing method with no gap between the template and the PCB is called contact printing. It requires the stability of the overall structure, is suitable for printing high-precision solder paste, the template and the PCB to maintain a very flat contact, only after printing and keeping with the PCB, so the method to achieve the printing accuracy, especially for fine pitch, ultra Fine-pitch solder paste printing. With the wide application of steel plates and the development of components in small and dense areas, contact printing is commonly used due to its high printing accuracy.
4、5 Evaluation of printing effect The ideal printing effect is the state shown in the figure below.
Factors Print Results Squeegee Hardness Harder Hard Too Soft Squeegee Velocity Slightly Faster Faster Slightly Slower Printing Pressure Is Too Large Too Small