Solder paste printing is a key process in Guangzhou SMT surface mount processing production, affecting the welding quality of the PCB assembly board.

This article analyzes the many factors affecting printing quality in solder paste printing technology, analyzes their causes and mechanisms, and provides solutions for these factors.

　　With the rapid development of component packaging, more and more PBGA, CBGA, CCGA, QFN, 0201, 01005 resistor-capacitor components and other components are widely used. Surface mount technology has also developed rapidly. In its production process, the influence and effect of solder paste printing on the entire production process are increasingly valued by engineers.

Companies in the industry also widely agree that to obtain good welding and long-term reliable products in terms of quality, the first thing to pay attention to is solder paste printing.

Nanshan SMT surface mount processing production not only needs to master and use solder paste printing technology, but also needs to be able to analyze the causes of problems that occur and apply improvement measures back to production practice.

　　1. Solder paste factors Solder paste is much more complex than pure tin-lead alloy, the main components are as follows: solder alloy particles, flux, rheology modifier, viscosity control agent, solvent, etc.

Truly grasp the relevant factors and select different types of solder paste; at the same time, it is also necessary to select large factories with perfect product process technology and stable quality.

Generally, the following factors should be considered when selecting solder paste: 2. Stencil factors 2.1 Stencil material and engraving Generally, two methods of chemical etching and laser cutting are used. For high-precision stencils, laser cutting should be used. Because the laser-cut hole walls are straight, the roughness is small (less than 3μm), and there is a taper.

Some people have already proved through experiments that for 01005 components with salt grain size, solder paste printing has higher precision requirements, and laser cutting is no longer sufficient, and special electroforming, also called electroplating, needs to be used.

　　2.2 Relationship between various parts of the stencil and solder paste printing (1) Aperture size The shape and geometric dimensions of the apertures on the stencil and the pads on the printed board are very important for the precise printing of solder paste.

During Nanshan SMT surface mount processing, high-end mounting machines can accurately control the mounting pressure, and the purpose also includes trying not to squeeze or damage the solder paste pattern, avoiding bridging and solder splashing during reflow.

The apertures on the stencil are mainly determined by the corresponding pad size on the printed board.

Generally, the size of the apertures on the stencil should be 10% smaller than the corresponding pads.

In practice, many companies use a 1:1 ratio of apertures to pads in stencil manufacturing. Small-batch, multi-variety production still has a lot of manual soldering. The author has soldered many QFN components, using manual solder paste spotting, and strictly controlling the amount of solder paste at each point, but no matter how the reflow temperature is adjusted, using X-RAY detection, there are more or less solder balls at the bottom of the components.

Based on the actual situation, the conditions for making stencils are not available, and finally, the method of component ball planting is used to achieve better welding results, but this also meets special conditions and can only be used in very small-batch production.

SMT printing technology-how to do a good job of solder paste printing (2) (2) Stencil thickness The thickness of the stencil and the size of the aperture are closely related to the printing of solder paste and subsequent reflow soldering, specifically, the thinner the thickness, the larger the aperture, the more conducive to solder paste release.

It has been proved that good printing quality requires the ratio of aperture size to stencil thickness to be greater than 1.5.

Otherwise, the solder paste printing is incomplete.

Generally, for lead spacing of 0.3～0.4 mm, a stencil with a thickness of 0.12～0.15 mm is used, and for spacing below 0.3, a stencil with a thickness of 0.1 mm is used.

　　3) Stencil aperture direction and size The release of solder paste in the length direction of the pad is consistent with the printing direction, and the printing effect is better than when the two directions are perpendicular.

Specific stencil design technology can be implemented according to Table 2.

　　3. Technical control of solder paste printing process Solder paste printing is a highly technical process, involving many technical parameters. Improper adjustment of each parameter will have a great impact on the quality of the mounted product.

　　1.1 Viscosity of solder paste The viscosity of solder paste is the most important factor affecting printing performance. If the viscosity is too high, the solder paste is not easy to pass through the apertures of the stencil, and the printed lines are incomplete. If the viscosity is too low, it is easy to flow and collapse, affecting the printing quality. The viscosity of solder paste can be measured with an accurate viscometer. In practice, if the company purchases imported solder paste, it should pay attention to the following points: (1) The process of recovering from 0 ℃ to room temperature must be ensured in terms of sealing and time; (2) Stirring should use a dedicated stirrer; (3) If the output is small and the solder paste is reused, strict standards need to be formulated, and solder paste outside the standards must be strictly prohibited from use.

　　1.2 Tackiness of solder paste If the tackiness of the solder paste is insufficient, the solder paste will not roll on the stencil during printing, and the direct result is that the solder paste cannot completely fill the stencil apertures, resulting in insufficient solder paste accumulation.

If the tackiness of the solder paste in Nanshan SMT surface mount processing is too large, the solder paste will hang on the stencil hole wall and cannot be completely printed on the pad.

　　The selection of solder paste tackiness generally requires that its self-adhesive ability is greater than its adhesion to the stencil, and its adhesion to the stencil hole wall is less than its adhesion to the pad.

　　1.3 Solder Paste Particle Uniformity and Size The shape, diameter, and uniformity of solder paste particles also affect its printing performance. Recently, there has been a lot of research in the industry on the printing characteristics of #3, #4, and #5 solder pastes caused by 01005 components. I think that for a certain type of solder paste, the diameter of the largest particle within the size range is approximately or slightly less than 1/5 of the stencil opening size. By selecting a stencil with appropriate thickness and skillfully punched holes, ideal printing results can be achieved.

Generally, solder paste with fine particles will have very good solder paste printing clarity, but it is prone to collapsing edges, and the degree and chance of oxidation are also high.

Generally, the pin spacing is used as one of the important selection factors, while taking into account both performance and price.

The specific relationship between pin spacing and solder paste particles is shown in Table 1.

　　Currently, many units are considering the design standards of solder pads for 01005 components, but the main focus is on evaluating the impact of the corresponding stencil opening design and the selected solder paste particles on printing quality.

　　 　　1.4 Metal Content of Solder Paste The metal content in solder paste determines the thickness of the solder after soldering.

As the percentage of metal content increases, the solder thickness also increases.

However, at a given viscosity, as the metal content increases, the tendency of solder bridging also increases accordingly.

　　After reflow soldering, the component pins need to be firmly soldered, with full and smooth solder volume, and a climb height of 1/3 to 2/3 of the height in the direction of the component (resistor component) terminal.

In order to meet the demand for solder paste volume at the solder joints, solder paste with 85% to 92% metal content is generally used. Solder paste manufacturers generally control the metal content at 89% or 90%, which has excellent application effects.

3.1 Setting and Adjustment of Screen Printing Machine Printing Parameters (1) Squeegee Pressure Changes in squeegee pressure have a significant impact on printing.

Too little pressure will prevent the solder paste from effectively reaching the bottom of the stencil openings and will not accumulate well on the pads; too much pressure will cause the solder paste to be printed too thinly, or even damage the stencil.

The ideal situation is to just scrape the solder paste clean from the stencil surface.

In addition, the hardness of the squeegee will also affect the thickness of the solder paste.

A squeegee that is too soft will cause the solder paste to be recessed, so it is recommended to use a harder squeegee or a metal squeegee.

　　 　　(2) Printing Thickness The printing thickness is determined by the thickness of the stencil, of course, the machine settings and the characteristics of the solder paste also have a certain relationship.

Fine adjustments to the printing thickness are often achieved by adjusting the squeegee speed and pressure.

Slightly reducing the squeegee printing speed can increase the amount of solder paste printed onto the printed circuit board.

One point is very obvious: reducing the squeegee speed is equivalent to increasing the squeegee pressure; conversely, increasing the squeegee speed is equivalent to reducing the squeegee pressure.

　　(3) Printing Speed A fast squeegee speed is beneficial for stencil rebound, but it will also prevent the solder paste from being transferred to the printed circuit board pads, while a speed that is too slow will result in poor resolution of the solder paste printed on the pads.

On the other hand, the squeegee speed and the viscosity of the solder paste are closely related. The slower the squeegee speed, the greater the viscosity of the solder paste; conversely, the faster the squeegee speed, the smaller the viscosity of the solder paste.

Generally, for fine-pitch printing, the speed range is 12 to 40 mm/s.

　　(4) Printing Method Stencil printing methods can be divided into contact (on-contact) and non-contact (off-contact).

Printing with a gap between the stencil and the printed circuit board is called non-contact printing.

During machine setup, this gap is adjustable, generally 0 to 1.27 mm; while stencil printing without a printing gap (i.e., zero gap) is called contact printing.

Contact printing with vertical stencil lifting can minimize the impact on printing quality, and it is particularly suitable for fine-pitch solder paste printing.

　　(5) Squeegee Parameters Squeegee parameters include the material, thickness, and width of the squeegee, the elasticity of the squeegee relative to the blade holder, and the angle of the squeegee relative to the stencil, all of which affect the solder paste distribution to varying degrees.

Among them, when the angle θ of the squeegee relative to the stencil is 60° to 65°, the quality of solder paste printing is optimal.

　　During printing, the relationship between the opening size and the squeegee direction should be considered.

The traditional printing method of solder paste is that the squeegee runs at a 90° angle along the x or y direction of the stencil, which often leads to different amounts of solder paste on the component at different opening directions. Experiments have confirmed that when the long direction of the opening is parallel to the squeegee direction, the thickness of the scraped solder paste is about 60% more than when the two are perpendicular.

Printing with the squeegee at a 45° angle can significantly improve the imbalance phenomenon of solder paste on different stencil opening directions, and can also reduce the damage of the squeegee to fine-pitch stencil openings.

　　(6) Demolding Speed The separation speed of the printed circuit board and the stencil will also have a significant impact on the printing effect.

If the time is too long, solder paste residue may remain at the bottom of the stencil; if the time is too short, it is not conducive to the solder paste standing upright, affecting its clarity.

　　In fact, each screen printing equipment manufacturing company conducts numerous printing tests during type development, and the details of the design also have their own characteristics.

When purchasing screen printing equipment, you should consult the manufacturer specifically, make more comparisons, and carefully study the manufacturer's experiments and verification processes for the above parameters.

　　(7) Stencil Cleaning During the solder paste printing process, the bottom of the stencil generally needs to be cleaned every 10 boards to remove the attachments on its bottom. Anhydrous alcohol is generally used as the cleaning liquid.

　　3.2 Technical Control of Solder Paste Usage (1) Strictly use solder paste within its validity period. Store solder paste in the refrigerator daily. Before use, it needs to be placed at room temperature for more than 6 hours before opening the lid for use. Store the used solder paste separately. When using it again, determine whether the quality is qualified; (2) Before production, the operator uses special equipment to mix the solder paste to make it uniform. It is best to regularly use a viscosity tester or qualitative test to check the solder paste viscosity; (3) The first printed board of the day's shift or after equipment adjustment, use a solder paste thickness tester to determine the solder paste printing thickness. The test points are selected at the top, bottom, left, right, and center of the printed board test surface (5 points), and the values are recorded. The solder paste thickness range needs to be -10% to +15% of the stencil thickness; (4) During the production process, 100% inspection of the solder paste printing quality is carried out. The main contents are whether the solder paste pattern is complete, whether the thickness is uniform, and whether there is a solder paste tailing phenomenon; (5) After the shift is completed, clean the stencil according to technical requirements; (6) After printing tests or printing failures, the solder paste on the printed circuit board needs to be completely cleaned with an ultrasonic cleaning device and dried to avoid solder balls after reflow soldering due to solder paste residue on the board when used again.

　　3.3 Common Printing Defects and Solutions Solder paste printing is a very complex technology. It is affected by materials and is directly related to equipment and parameters. By controlling each small aspect of the printing process, it can be said that details determine success or failure. To avoid common defects in printing, the following briefly analyzes several of the most common defects that occur during solder paste printing and their corresponding prevention or solutions.

　　3.3.1 Incomplete Printing Incomplete printing refers to the solder paste not being printed in some areas on the pad.

Possible causes: (1) Aperture blockage or some solder paste sticking to the bottom of the stencil; (2) Solder paste viscosity is too low; (3) There are large metal powder particles in the solder paste; (4) The scraper is worn.

　　Prevention and solution: Clean the apertures and the bottom of the stencil, select solder paste with appropriate viscosity, and ensure that the solder paste printing effectively covers the entire printing area; select solder paste whose metal powder particle size corresponds to the aperture size; check and replace the scraper.

　　3.3.2 Tailing Tailing is when the solder paste on the pad after printing is in the shape of a small hill. The cause may be the scraper gap or the solder paste viscosity is too high.

Prevention or solution: Appropriately reduce the scraper gap or select solder paste with appropriate viscosity.

　　3.3.3 Collapse After printing, the solder paste collapses to both ends of the pad.

Causes: (1) Too much scraper pressure; (2) The printed circuit board is not firmly positioned; (3) The solder paste viscosity or metal content is too low.

　　Prevention or solution: Adjust the pressure; refasten the printed circuit board; select solder paste with appropriate viscosity.

　　3.3.4 Solder Paste Too Thin Causes: (1) The stencil is too thin; (2) Too much scraper pressure; (3) Poor solder paste fluidity.

　　Prevention or solution: Select a stencil of appropriate thickness; select solder paste with appropriate particle size and viscosity; reduce scraper pressure.

　　3.3.5 Uneven Thickness After printing, the solder paste thickness on the pad is uneven. Causes: (1) The stencil and the printed circuit board are not parallel; (2) The solder paste is not mixed evenly, resulting in uneven particle size.

　　Prevention or solution: Adjust the relative position of the stencil and the printed circuit board; mix the solder paste thoroughly before printing.

　　3.3.6 Burr on Edges and Surface The cause may be that the solder paste viscosity is too low, and the aperture wall of the stencil is rough.

　　Prevention or solution: Select solder paste with slightly higher viscosity; check the etching quality of the stencil apertures before printing.

　　4 Conclusion To ensure the quality of surface mount products, it is necessary to analyze and study the key factors in each stage of production and formulate effective control methods.

As a key process, solder paste printing is even more crucial. Only by formulating appropriate parameters and mastering the rules between them can high-quality solder paste printing quality be obtained.