SMT Fine pitch Stencil design

Stencil  design and solder paste selection in SMT high density and  fine pitch assembly

　　Preface　　

With the development of SMT towards fine spacing components, the density of SMD package pins is becoming more and more dense, and the trend of decreasing package size has posed a severe challenge to solder paste printing. Statistics show that more than 50% of the assembly defects are caused by solder paste printing 3, therefore, solder paste printing has become a major factor affecting the quality of SMT assembly.

As we all know, there are 3 main factors that affect the quality of printing in solder paste printing: equipment, solder paste selection, and stencil selection. Equipment is mainly purchased according to the overall requirements of the production line, this article does not do research. The main factors considered in the selection of the template are template design, opening design and template printing. The template manufacturing technologies include chemical etching, laser cutting, electropolishing, electroplating and electroforming. Focus on the selection of solder paste, solder powder size, alloy can provide a certain content to paste supplier requirements, select several suppliers of solder paste samples in the experiments, the best judgment paste suitable for the enterprise products on the basis of experimental data.

Template selection

Formwork thickness and opening design

When the solder paste printing, there is a certain correlation design template template thickness and opening size, the relationship with the area ratio (AR. ratio of the opening area and template open side wall area) to measure, it is the most important parameter in the design of template. The general laser template AR is larger than 0.66, and the electroforming template is larger than 0.6. This design balances the amount of printing clarity with the amount of solder paste, avoids solder defects and reduces bridge formation. When the thickness of the template is selected, the suitable width and too small formwork opening are chosen to make the solder joint open or the solder joints lack of solder; too large openings are easy to bridge. Table 1 provides design guidance between the stencil thickness and the opening width 2.

Opening design of template and pad

The thickness of the stencil must be carefully designed for a printed solder joint to allow sufficient solder paste to be delivered to a non fine spacing pad while avoiding excessive solder paste deposition on a fine pitch pad. In order to adapt to the minimum size of the pad and take into account the amount of solder paste on the large pad, there are several ways to deposit the solder paste on the pad to a proper amount:

Local thinned formwork

This template has templates of different thickness in fine pitch and non fine pitch pads, size usually is: 0.008in. (0.2mm) for fine pitch, 0.006in. (0.1 5mm) for fine pitch; or 0.006in. (0.1 5mm) for fine pitch, 0.004in. (0.10mm) for fine pitch.

The fine spacing opening dimensions on the template are reduced by 10%-30% compared to the size on the pad. This reduces the amount of solder paste deposited on the pad. It also provides some space for solder paste bonding and solder paste collapse in solder paste printing. The sides of the opening are evenly reduced

Staggered printing

The template only half the length of the pad, and arranged in a staggered manner, for a tin lead solder paste coating, when melts during welding, the molten solder is considered to be the other half flow toward the pads, achieve full coverage. For bare copper or nickel surfaces, the molten solder may not flow to the other half of the pad area without printing solder paste, so the PCB of bare copper or nickel surface shall be carefully considered.

Other shapes

The shape of the template opening is in a selected form, such as a triangle, a teardrop shape, etc., to achieve the purpose of reducing the amount of solder paste deposition on a fine spacing pad.

Compromise template thickness

Choose a compromise template thickness to fit both the fine pitch pad and the non fine spacing pad to replace the various template thicknesses required for different pads.

Template material selection

Under the condition of the selected thickness and the opening size, the performance of the template is mainly affected by the opening process of the template, metal plate and template. At present, there are 5 kinds of formwork on the market: brass, stainless steel, molybdenum, alloy 42 and electroformed nickel. The processes for making templates include chemical etching, laser cutting, electropolishing, electroplating and electroforming. Each kind of plate or manufacturing process has its inherent advantages and limitations. The key items to evaluate the performance of the template are vertical wall opening, wall smoothness, and dimensional accuracy. Besides, durability, chemical resistance, good opening ability and cost are also important factors. Table 2 compares all kinds of template materials. Table 3 summarizes the relative performance characteristics of the template manufacturing technology.

Solder paste is a paste form of solder, because of its unique advantages, for industrial importance of particular importance. The solder paste has a deformable viscoelastic form that can be selected for use in shape and size. The viscosity of the solder paste provides an adhesive power that allows the element to remain on the pad without attaching additional glue until the element forms a permanent metallurgical connection with the pad. The metallic properties of solder paste provide relatively high electrical conductivity and thermal conductivity. Therefore, solder paste is the most viable material for surface mount manufacturing. It is not only suitable for automated production, but also has a certain viscosity and high conductivity. It provides electrical, thermal and mechanical interconnections for electronic packaging and assembly.

Solder paste selection

Solder paste selection usually consists of solder powder size, alloy content, and viscosity. Usually, the alloy content is about 90%, and the viscosity is about 1000Pa.s 4. The size, shape and uniformity of solder alloy powder particles are important parameters that affect solder paste properties

Solder paste printing of fine particles is better, especially for high density and narrow spacing products, because of the fine pitch stencil aperture size is small, must use the small particles of alloy powder. Otherwise it will affect the printing and release. Generally, the diameter of solder particles is about 1 / 5 of the opening size of a template, so the 25-45 m is generally used for narrow spacing components.

Evaluation experiment of solder paste and template

Although according to the above theories, the choice of parameters of template and paste, but the market template and solder paste manufacturer of many, which are the most suitable for the enterprise, but also the need for the experiment. In order to decide; the following 2 experiments, for reference only.

Experiment 1: how to evaluate the printing performance of each manufacturer under the same process parameters?

Using the same GB file, we evaluate the performance of templates processed by several manufacturers. All the printing parameters, equipment, materials, and even the environment are the same in the evaluation experiment, and the template processing methods are the same, such as laser processing. Through the analysis and measurement of various printing size (such as solder paste deposition area and volume, etc.) and printing defects (such as bridging, excessive or insufficient solder volume) to evaluate the performance of the template.

The template design includes a wide range of component types, and the experiments presented in this paper have selected 38 components, including 0201, 0402, 20-30mil, pitch, mu, BGA, 16, 20, 25mil, pitch, QFP, and 40mil pitch CBGA.

Experiment two: the best combination of solder paste for different templates and different suppliers is used.

Simulate the actual solder paste printing process, printing stopped suddenly for a period of time, and then continue printing, testing the ability to restore solder paste. In the experiment, 5 templates and 3 solder pastes were used to test and stop for a period of time. The deposition data of solder paste before and after stopping were compared and analyzed.

experimental condition

Templates: 5 templates are selected for study. #5 is an electroforming template, and other templates (#6, 11, 21, and 22) are laser cutting templates. All templates use the same GB file, so all template openings are exactly the same size. Use a compromise template thickness of 5 mil.

Solder paste: experimental use of 3 different solder paste. The solder paste "a" and "B" are all the size of Sichuan solder powder, Sn63 / Pb37 alloy and 90% alloy content, but the supplier is different. The solder paste "C" is the size of the V solder powder (usually 30 micron of solder powder), Sn63 / Pb37 alloy and 90.8% slightly higher alloy content.

Printing times and stopping times: Experiment 1, 26 continuous solder paste printing (6 trial printing and 20 official test printing). For experiment two, 6 consecutive solder paste printing, and 6 printing times after the stop time. Deposition data were compared before and after cessation of solder paste. The stopping time was 15 minutes and 90 minutes respectively.

Printed board: the test program consists of 4 printed plates (A, B, C, D) and 6 test printing plates (1, 2, 3, 4, 5, 6). The PCB size for testing: 254 * 406.4 * 1.6mm, the test board is bare copper, no solder resist and lead. Each board has 8 base points for the alignment of the screen machine and the solder paste tester.

Component: although not all components are selected for solder paste inspection, the template design has a wide range of components, as follows:

1.16 mil (0.4mm) pitch QFP:2 (128 solder paste total deposits)

2.20 mil (0.5mm) pitch QFP:2 (200 solder paste total deposits)

3.31 mil (0.787mm) pitch mu BGA:2 (98 solder paste total deposits)

4.0201 chips:18 (36 solder paste total deposits)

5.0402 chips:10 (20 solder paste total deposits)

6.20 mil (0.5mm) pitch mu BGA:2 (800 solder paste total deposits)

7.40 mil{1.0mm) pitch BGA:2 (total of 992 solder paste deposits)

8.25 miI (0.635mm) pitch SOIC:5 (100 solder paste total deposits)

So, each plate has 2374 solder paste, the total number of deposition is tested, each template printed 12 plates, a total of 28488 solder paste deposition total.

Use equipment: use DEK maximum press and GSI / Lumonics 8100 solder paste tester.

DEK printing machine setting parameters such as:

1. printing speed =25mm / S

2. =15kg (0.9375lb / inch blade pressure of blade length)

3. scraper angle =60 degrees

4. scraper type = metal

5. scraper length =406mm

6. separation speed =0.5mm / S

7. separation distance =2.Omm

Printing conditions: each type of printing, whether it is to stop 15 or stop for 90 minutes, all 5 templates are printed with new solder paste. The printing order is random, and the printing (4 fictitious boards and 12 test boards) uses the plate exactly the same. Before using solder paste, stir for 30 seconds. In all the studies, the temperature and humidity in the room were kept under control. Control each print interval to ensure consistent intervals. After the 4 fictitious board, the first 6 boards were printed, then stopped printing, and stopped for 15 or 90 minutes. This time the press was free. After stopping, continue the printing of the board. The test continues, during which the template is not clean.

Test content: after each board is printed, the solder paste tester generates a document for each board. Test the amount and deviation of solder paste for printing.

Calculation and result

The data of solder paste deposition were collected before and after the stop. For each template and solder paste combination, the average solder paste per component was calculated before and after the cessation of the solder paste in order to investigate the restoration of solder paste. Recovery is the amount of solder paste deposited after waiting time, which can be defined as the average solder paste after stopping