Abstract Nowadays, people generally use solder paste printing as a key process for controlling the quality of finished solder joints. If you want to get high-quality solder paste printing is not an easy thing to do, the solder paste printing process involves the template design, template manufacturing, template assembly, template cleaning and template life, these interactions affect each other. This paper has developed a guideline for the solder paste printing technology of the template, which aims to help technicians and production personnel solve some problems in the actual production to ensure the printing quality of the components. This article focuses on the solder paste printing of Ball Grid Array (BGA) and Chip Size Package (CSP) in SMT assembly and compares various technologies to lay the foundation for the development of the best printing process.

1 Formwork Manufacturing Technology

The template manufacturing process includes an addition method or a subtract method. In the additive process, such as electroforming, electroforming takes place by adding metal. In the subtractive process, the metal is removed from the stencil foil to form openings. Laser cutting and chemical etching methods are examples of typical subtractive processes.

1.1 Template

Template Type: There are four main types of templates that are commonly used: chemical etching, laser cutting, hybrid technology, and electroforming. The manufacturing process of the chemical etching template is mainly to cut the metal foil into a frame of a specific size, and use the photoresist to form an image on both sides of the metal foil. The double-sided optical tool is usually accurately aligned and positioned with a grating registration member, and the template aperture image can be exposed on the photoresist using a double-sided optical tool. The laser-cut template is made from the software Gerber(r) data running in the laser device. When a standard component and a fine-pitch component mixing technique are applied on the PCB, a laser cutting and chemical etching combined template manufacturing process should be used. The produced template is defined as a laser-chemical combination template or a hybrid technology template. Electroforming technology is a template addition method that uses lithographic imaging and electroplating processes. It is recommended that laser-cut or electroformed stencils be used in the most demanding applications for the uniform release of solder paste. However, the cost of these templates is high, and one study shows that the consistency of such templates is better than that of chemically etched templates.

Stencil opening design: A common problem with stencil design is the effect of hole design and hole design on printing performance. During the printing operation, when the squeegee is pushed on the stencil, the solder paste is pressed into the opening of the stencil. Then, the solder paste naturally flows onto the pads of the PCB while the printed board is detached from the stencil. It is ideal if the solder paste extruded into the opening can be completely released from the opening wall and adhered to the PCB pad to form a complete solder mass. The ability of solder paste to release from the inner wall of an opening depends on three factors:

1. Area ratio/aperture ratio of template design
2. Open hole sidewall geometry
3. The smoothness of the opening wall is outside, we will provide several different SMT template opening design to the public, as a reference in production, see Table 1.

Table 1 SMT Universal Opening Design Guide

Component Type Spacing Pad Footprint Hole Width Hole Length Template Thickness Range Aperture Ratio Range Area Ratio Range PLCC 50 25 23 100 8~10 2.3~2.9 1.07~1.17
QFP 25 14 12 60 6~7 1.7~2.0 0.71~0.83
QFP 20 12 10 50 5~6 1.7~2.0 0.69~0.83
QFP 16 10 8 50 4~5 1.6~2.0 0.68~0.86
QFP 12 8 6 40 3~4 1.5~2.0 0.65~0.86
0402 N/A 20×30 18 22 5~6 N/A 0.65~0.86
0201 N/A 10×20 8 16 3~4 N/A 0.65~0.86
BGA 50 32 Round 30 Round 30 Round 6~8 N/A 0.93~1.25
μBGA 40 15 Round 14 Square 14 Square 4.5~5.25 N/A 0.67~0.78
μBGA 30 12 14 Square 14 Square 4.5~5.25 N/A 0.67~0.78
μBGA 20 12 Round 11 Square 11 Square 3~4 N/A 0.69~0.92

Note: 1) Assume that the μBGA pad is not a solder mask. 2) The μBGA opening is a square hole. The 14mil hole opening radius should be 3mil, and the 11mil hole opening radius should be 2.5mil.
3) All specifications are in mils and round in metrics, ie, 0.65mm for 25 mils and 0.5mm for 20mils. The ratio is not a size.
4) N/A only as an area ratio.

Template opening shape: In terms of the effect of releasing the solder paste, the square opening is better than the round pad or the connected part (see Figure 1). The square template allows smooth release of solder paste. In order to reduce the occurrence of open clogging, the radius of the corner should be set at 0.010′′. For chemically etched stencils, the corner radius should be consistent with the stencil thickness.

Figure 1 How the size of the openings affects solder paste release and patterning. After the opening is expanded, it can be printed on the pad.

Template thickness: for BGA, the template thickness should be 0.005~0.006”. For the columnar ceramic grid array (CCGA), the template is required to have a thickness of 0.007′′, which can consume the amount of solder paste consumed. Restricted to the lowest limit, and for the CSP, the template thickness is required to be 0.004 to 0.005′′. Be careful when using the latter template because applying a solder paste on a larger opening template may " Solder paste, for example; 1206 capacitors or 0.050" pitch elements. Table 2 shows the recommended template thickness.

Table 2 Recommended template thickness
0.050′′ pitch
0.010~0.008′′
0.025′′ pitch
0.008~0.006′′
0.020′′ spacing
0.006~0.004′′
0.016′′ spacing
0.005~0.004′′
0.012′′ pitch
0.004~0.003′′

Aperture ratio and area ratio: For fine pitch components, the aperture or aperture ratio (width/thickness) should not be less than 1.5, which is important (see Figure 2). For CSP, the openings should be as small as 0.010" square. This is related to the area ratio. Solder paste adheres to the sidewalls with a very small surface area, rather than sticking to the PCB. This phenomenon is very likely to occur. Therefore, the area ratio (length x width/2 [length + width] x thickness) must be greater than 0.66.

The aperture ratio of fine pitch elements should not be less than 1.5, and the area ratio must be greater than 0.66.
Many manufacturers apply a 1:1 ratio when printing BGA and CSP. For the latter, the printing ratio is greater than the solder bump size 0.. 002 ~ 0.003′′, which is very common, this will make the paste after reflow slightly higher, so that more heat can meet the selected type III powder continuity, flexibility requirements. For CSP or μBGA packages using 0.012′′ diameter solder bumps, it is recommended to print 0.012 to 0.014′′ square openings. Some studies show that 0.014′′ openings are consistent and reproducible for solder paste printing for type III powders. The aspect may be the smallest opening. Printing of 0.010" or 0.012" square or round openings is likely to require the use of Type IV powders.

Table 3 lists the aperture ratios/area ratios of some practical examples of open hole designs of commonly used SMDs. The 20 mil pitch QFP uses a 5 mil thick template with an opening size of 10 x 50 mils and an aperture ratio of 2.0. The use of smooth stencil technology with open sidewalls enables excellent paste release and consistent print performance. A 16-mil pitch QFP can be used with a 5 mil template with an opening size of 7 x 50 mils and an aperture ratio of 1.4. This template is very difficult to release the solder paste. This is true even with high-tech templates.

Table 3 Aperture/area ratios for various surface mount devices

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