With the rapid development of component packaging, more and more PBGA, CBGA, CCGA, QFN, 0201, 01005 resistance-capacitance components have been widely used, and surface mount technology has also developed rapidly. In its production process, solder paste The influence and function of printing on the whole production process are getting more and more attention from engineers. Enterprises in the industry also generally agree that in order to obtain good soldering and long-term reliable products, the first thing to pay attention to is the printing of solder paste. In production, it is not only necessary to master and apply solder paste printing technology, but also to be able to analyze the causes of problems and apply improvement measures to production practice.
1 Factors of Solder Paste
Solder paste is much more complex than a simple tin-lead alloy. The main components are as follows: solder alloy particles, flux, rheology modifier, viscosity control agent, solvent, etc. Really grasp the relevant factors, choose different types of solder paste; at the same time, choose a large factory with perfect product manufacturing process and stable quality. Generally, the following factors should be paid attention to when selecting solder paste:
1.1 Viscosity of solder paste
The viscosity of the solder paste is the most important factor affecting the printing performance. If the viscosity is too high, the solder paste will not easily pass through the openings of the stencil, and the printed lines will be incomplete. If the viscosity is too low, it will easily flow and collapse, which will affect the resolution and resolution of printing. The smoothness of the lines.
The viscosity of solder paste can be measured with an accurate viscometer. In actual work, if the company purchases imported solder paste, it can use a simple method to make a qualitative judgment for the viscosity during normal production: pick up the solder paste with a scraper to see if it is gradually Paragraph down, reach the moderate viscosity. At the same time, we also believe that to make the solder paste have good viscosity characteristics every time it is used, the following points need to be done:
(1) During the process of returning from 0°C to room temperature, the sealing and time must be guaranteed;
(2) It is best to use a special stirrer for stirring;
(3) The production volume is small, and the solder paste is used repeatedly. It is necessary to formulate strict specifications, and the use of solder paste outside the specifications must be strictly stopped.
1.2 Tackiness of solder paste
The viscosity of the solder paste is not enough, and the solder paste will not roll on the template during printing. The direct consequence is that the solder paste cannot completely fill the openings of the template, resulting in insufficient solder paste deposition. If the viscosity of the solder paste is too high, the solder paste will hang on the wall of the stencil hole and cannot be completely missed on the pad.
The viscosity selection of solder paste generally requires that its self-adhesive ability is greater than its adhesion ability to the template, and its adhesion to the template hole wall is smaller than its adhesion to the pad.
1.3 Uniformity and size of solder paste particles
The particle shape, diameter and uniformity of solder paste solder also affect its printing performance. Recently, there are many studies on the printing characteristics of 3#, 4#, and 5# solder paste printed and issued by 01005 devices in the industry. The author thinks that a certain Solder particles like solder paste, the diameter of the largest particle in the model range is about or slightly smaller than 1/5 of the opening size of the template, and then the ideal printing effect can be achieved by selecting a stencil with appropriate thickness and process. Usually, the solder paste with fine particles will have better solder paste print definition, but it is prone to sagging, and the degree and chance of oxidation are also high. Generally, pin spacing is used as one of the important selection factors, taking into account both performance and price. The relationship between specific pin spacing and solder particles is shown in Table 1.
Now many units are considering the pad design specifications of 01005 components, but the first thing is to evaluate the influence of the correspondingly designed stencil opening and the selected solder paste particles on the printing quality.
1.4 Metal Content of Solder Paste
The amount of metal in the solder paste determines the thickness of the solder after soldering. As the percentage of metal increases, the thickness of the solder also increases. However, at a given viscosity, as the metal content increases, the bridging tendency of the solder increases accordingly.
After reflow soldering, it is required that the pins of the device are welded firmly, the solder volume is full, smooth, and there is a 1/3 to 2/3 height climb in the height direction of the end of the device (resistance device). In order to meet the requirements of the amount of solder paste for solder joints, solder paste with a metal content of 85% to 92% is usually selected. Solder paste manufacturers generally control the metal content at 89% or 90%, which is more effective.
2 factors of the template
2.1 The material and engraving of the stencil
Usually, chemical etching and laser cutting are used. For high-precision stencils, laser cutting should be used, because the laser-cut hole wall is straight, with small roughness (less than 3 μm) and a taper. Some people have proved through experiments that for 01005 devices with the size of salt particles, solder paste printing has higher precision requirements, and laser cutting can no longer meet the requirements. Special electroforming, also called electroplating, is required.
2.2 The relationship between each part of the stencil and solder paste printing
(1) Dimensions of the opening
The shape of the hole on the stencil and the shape and geometry of the pad on the printed board are very important for the precise printing of solder paste. During placement, the high-end placement machine can accurately control the placement pressure, and the purpose also includes trying not to squeeze or damage the solder paste pattern, so as to avoid bridging and tin splashing during reflow. The openings on the stencil are mainly determined by the corresponding pad size on the printed board. Generally, the size of the hole on the stencil should be 10% smaller than the corresponding pad. In fact, many companies adopt a 1:1 ratio of openings and pads in the production of stencils. There are still a lot of manual soldering in small batches and multi-variety production. The author has soldered many QFN devices in experiments, using manual The method of dispensing solder paste, and strictly controlled the amount of solder paste at each point, but no matter how to adjust the reflow temperature, there are more or less tin beads on the bottom of the device when detected by X-RAY. According to the actual situation, there are no conditions for making stencils. Finally, the device ball planting method is used to achieve a better soldering effect, but this also meets special conditions and can only be used in small batch production.
Here are a few pictures for your reference.
(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, the more conducive to the release of the solder paste. It has been proved that good printing quality must require a ratio of opening size to stencil thickness greater than 1.5. Otherwise the solder paste printing is incomplete. In general, yes, for a lead spacing of 0.3 to 0.4mm, use a stencil with a thickness of 0.12 to 0.15mm, and for a spacing below 0.3, use a stencil with a thickness of 0.1mm.
(3) Stencil opening direction and size
When the release of solder paste in the length direction of the pad is consistent with the printing direction, the printing effect is better than when the two directions are perpendicular. The specific stencil design process can be implemented according to Table 2.
1 Process control of solder paste printing process
Solder paste printing is a highly technological process, which involves a lot of process parameters, and improper adjustment of each parameter will have a great impact on the quality of the mounted product.
1.1 Setting and adjustment of printing parameters of screen printing machine
(1) Squeegee pressure
The change of squeegee pressure has a great influence on printing. If the pressure is too small, the solder paste will not reach the bottom of the stencil opening effectively and will not be well deposited on the pad; if the pressure is too high, the solder paste will be printed too thin and may even damage the stencil. The ideal is to just scrape the solder paste off the stencil surface. In addition, the hardness of the scraper will also affect the thickness of the solder paste. A squeegee that is too soft will dent the solder paste, so a harder squeegee or metal squeegee is recommended.
(2) Printing thickness
The printing thickness is determined by the thickness of the stencil. Of course, the setting of the machine is also related to the characteristics of the solder paste. The slight adjustment of the printing thickness is often realized by adjusting the speed of the scraper and the pressure of the scraper. Appropriately reducing the printing speed of the squeegee can increase the amount of solder paste printed to the printed board. One thing is obvious: reducing the speed of the scraper equals increasing the pressure on the scraper; conversely, increasing the speed of the scraper equals reducing the pressure on the scraper. the
(3) Printing speed
The fast speed of the scraper is conducive to the rebound of the template, but at the same time it will hinder the transfer of the solder paste to the pad of the printed board, and the slow speed will cause poor resolution of the solder paste printed on the pad. On the other hand, the speed of the scraper has a great relationship with the viscosity of the solder paste. The slower the speed of the scraper, the greater the viscosity of the solder paste; similarly, the faster the speed of the scraper, the smaller the viscosity of the solder paste. Usually, the speed range for fine-pitch printing is 12-40mm/s.
(4) Printing method
Template printing methods can be divided into contact (on-contact) and non-contact (off-contact). Printing with a gap between the template and the printed board is called non-contact printing. When the machine is set, this distance is adjustable, and the general gap is 0-1.27 mm; while the printing method of stencil printing without printing gap (that is, zero gap) is called contact printing. The vertical lifting of the stencil for contact printing can minimize the impact on printing quality, which is especially suitable for fine-pitch solder paste printing.
(5) Parameters of the scraper
The parameters of the scraper include the material, thickness and width of the scraper, the elasticity of the scraper relative to the knife holder, and the angle of the scraper to the template, etc. These parameters affect the distribution of solder paste to varying degrees. The quality of solder paste printing is the best when the angle θ of the scraper relative to the template is 60°～65°.
At the same time of printing, the relationship between the size of the opening and the direction of the scraper should be considered. The traditional printing method of solder paste is that the squeegee runs along the x or y direction of the template at an angle of 90°, which often leads to different amounts of solder paste on the device in different directions of the opening. It has been verified by experiments that when the long direction of the opening is in the same direction as the direction of the squeegee The thickness of the solder paste scraped when parallel is about 60% more than that when the two are perpendicular. The squeegee prints in the direction of 45°, which can significantly improve the imbalance of the solder paste in different stencil openings, and can also reduce the damage of the stencil to the fine-pitch stencil openings.
(6) Demoulding speed
The detachment speed of the printed board and the template will also have a greater impact on the printing effect. If the time is too long, it is easy to leave solder paste on the bottom of the template, and if the time is too short, it is not conducive to the uprightness of the solder paste and affects its clarity.
In fact, every screen printing equipment manufacturing company will do a lot of printing experiments when developing the model, and the design details also have their own characteristics. When you need to purchase screen printing equipment, you should consult the manufacturer in detail, make more comparisons, and carefully study the manufacturer's experiments and demonstration processes for the above parameters.
(7) Stencil cleaning
In the solder paste printing process, the bottom of the stencil needs to be cleaned every 10 boards to eliminate the attachments on the bottom. Anhydrous alcohol is usually used as the cleaning solution.
1.1 Process control when using solder paste
(1) Strictly use the solder paste within the validity period. The solder paste is stored in the refrigerator on weekdays. Before use, it must be kept at room temperature for more than 6 hours, and then it can be opened for use. The used solder paste should be stored separately, and the quality must be determined before use. ;
(2) Before production, the operator uses special equipment to stir the solder paste to make it even, and it is best to regularly use a viscosity tester or qualitatively measure the viscosity of the solder paste;
(3) After printing the first printing board on duty or adjusting the equipment on the same day, use a solder paste thickness tester to measure the thickness of the solder paste printing. Numerical value, the solder paste thickness range is required 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 content is whether the solder paste pattern is complete, whether the thickness is uniform, and whether there is any sharpening of the solder paste;
(5) Clean the formwork according to the process requirements after the work on duty is completed;
(6) After the printing experiment or printing failure, the solder paste on the printed board is required to be thoroughly cleaned and dried with ultrasonic cleaning equipment to prevent solder balls after reflow soldering caused by residual solder paste on the board when it is used again.
1.2 Common printing defects and solutions
Solder paste printing is a very complicated process, which is not only affected by materials, but also directly related to equipment and parameters. Through the control of various small links in the printing process, it can be said that details determine success or failure. Defects that often appear, the following briefly introduces the most common defects and corresponding prevention or solutions during solder paste printing.
1.2.1 Incomplete printing
Incomplete printing means that part of the pad is not printed with solder paste. The cause may be:
(1) The opening is blocked or part of the solder paste sticks to the bottom of the template;
(2) The viscosity of the solder paste is too small;
(3) There are large-sized metal powder particles in the solder paste;
(4) The scraper is worn out.
Preventive solutions: clean the opening and the bottom of the stencil, select a solder paste with a suitable viscosity, and make the solder paste printing can effectively cover the entire printing area; select a solder paste with a particle size of the metal powder corresponding to the size of the opening; check and replace the scraper.
1.2.2 Sharpening
Sharpening means that the solder paste on the pad after printing is in the shape of a small peak. The reason may be that the gap between the scraper blades or the viscosity of the solder paste is too high. Prevention or solution: Properly reduce the scraper gap or choose a solder paste with a suitable viscosity.
1.2.3 Collapse
After printing, the solder paste collapses toward the sides of the pad. cause:
(1) The pressure of the scraper is too high;
(2) The printed board is not firmly positioned;
(3) The viscosity or metal content of the solder paste is too low.
Prevention or solution: adjust the pressure; re-fix the printed board; choose a solder paste with a suitable viscosity.
1.2.4 Solder paste is too thin
Causes:
(1) The template is too thin;
(2) The pressure of the scraper is too high;
(3) The fluidity of the solder paste is poor.
Prevention or solution: choose a template with a suitable thickness; choose a solder paste with a suitable particle size and viscosity; reduce the pressure of the scraper.
1.2.5 Inconsistent thickness
After printing, the thickness of the solder paste on the pad is inconsistent. The reasons are:
(1) The template is not parallel to the printed board;
(2) The solder paste is not stirred evenly, making the particle size inconsistent.
Prevention or solution: adjust the relative position of the template and the printed board; fully stir the solder paste before printing.
1.2.6 Burrs on edges and surfaces
The reason may be that the viscosity of the solder paste is low and the walls of the stencil openings are rough.
Prevention or solution: choose a solder paste with a slightly higher viscosity; check the etching quality of the template opening before printing.
2 Conclusion
In order to ensure the quality of surface mount products, it is necessary to analyze and study the key factors in each link of production, and to formulate effective control methods. As a key process, solder paste printing is the most important thing. Only by formulating suitable parameters and mastering the rules between them can we obtain high-quality solder paste printing quality.
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