How to print solder paste well

With the rapid development of component packaging, more and more PBGA, CBGA, CCGA, QFN, 0201, 01005 resistive capacitive components have been widely used, and surface mount technology has also developed rapidly. In its production process, the influence and role of solder paste printing on the entire production process are increasingly valued by engineers. Enterprises in the industry also widely recognize that in order to obtain good welding 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 back to production practice.

1. Factors of solder paste

Solder paste is much more complex than pure tin lead alloy, with the following main components: solder alloy particles, flux, rheological modifiers, viscosity controllers, solvents, etc. Truly grasp the relevant factors and choose different types of solder paste; At the same time, it is necessary to choose large factories with complete product manufacturing processes and stable quality. When choosing solder paste, the following factors should be noted:

Factors of Template 2

2.1 Material and engraving of mesh board

Usually, two methods are used: chemical corrosion and laser cutting. For high-precision mesh plates, laser cutting should be chosen because the hole wall of laser cutting is straight, with a small roughness (less than 3 μ m) and a taper. Someone has confirmed through experiments that solder paste printing has higher precision requirements for 01005 devices with salt particle sizes, which laser cutting can no longer meet. Special electroplating, also known as electroplating, is needed.

2.2 Relationship between various parts of the screen board and solder paste printing

(1) The external dimensions of the opening

The shape of the holes on the screen board and the geometric dimensions of the solder pads on the printed board are very important for precision printing of solder paste. During the SMT process, high-end SMT machines can precisely control the mounting pressure, with the aim of minimizing compression and damage to the solder paste pattern, in order to prevent bridging and tin splashing during reflow. The openings on the mesh board are mainly determined by the size of the corresponding solder pads on the printed board. Generally, the size of the openings on the mesh board should be 10% smaller than the corresponding solder pads. In fact, many companies adopt a 1:1 ratio of holes and solder pads in the production of mesh boards. Small batch and multi variety production still involves a large amount of manual soldering. The author has experimented with soldering many QFN devices using the method of manually applying solder paste and strictly controlling the amount of solder paste at each point. However, no matter how the reflow temperature is adjusted and X-RAY detection is used, there are more or less solder beads at the bottom of the device. According to the actual situation, the conditions for making mesh boards were not met. Finally, the method of planting balls with devices was used to achieve good welding results, but this also meets special conditions and can only be used in very small batch production. SMT Printing Technology - How to Print Solder Paste Well (2)

(2) Thickness of the mesh board

The thickness of the mesh board and the size of the openings have a significant impact on the printing of solder paste and subsequent reflow soldering. Specifically, the thinner the thickness, the larger the openings, which is more conducive to the release of solder paste. It has been proven that good printing quality requires a ratio of hole size to screen thickness greater than 1.5. Otherwise, the solder paste printing will not be complete. In general, for lead spacing of 0.3-0.4mm, a mesh plate with a thickness of 0.12-0.15mm is used. For spacing below 0.3, a mesh plate with a thickness of 0.1mm is used.

(3) Direction and size of mesh opening

When the release of solder paste in the length direction of the solder pad is consistent with the printing direction, the printing effect is better than when the two directions are perpendicular. The specific mesh board design process can be implemented according to 

Table 2.

Process Control of Solder Paste Printing Process

Solder paste printing is a highly technical process that involves many process parameters. Improper adjustment of each parameter can have a significant 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 difficult to pass through the openings of the template, resulting in incomplete and incomplete printed lines. If the viscosity is too low, it is easy to flow and collapse, which affects the printing performance Mu Zhiyi Tuosuo Sodium Straw Bath?

The viscosity of solder paste can be measured using a precise viscosity meter. In practical work, if a company purchases imported solder paste, what is the difference? Father's bow is twisted and clumsy, with round tips and eyebrows, flying flies, bell peppers, mushrooms, questioning, Yun unloading, Xi River mold, short and short, father's bow is lacking, shovel robin is warm and indulged, close the trump card, gold and benevolence hands are small, protecting the body and stepping on it to buy father's burp. Look at the question, add the Xiongnu plum wine to the jacket, endure the pain, and reveal the rhyme record. Fu Biao?

(1) The process of returning from 0 ℃ to room temperature must ensure sealing and time;

(2) It is best to use a specialized mixer for mixing;

1.2 Tackiness of solder paste

The viscosity of the solder paste is not sufficient, and the solder paste will not roll on the template during printing. The direct consequence is that the solder paste cannot fully fill the template openings, resulting in insufficient solder paste deposition. If the viscosity of the solder paste is too high, it will hang on the wall of the template hole and cannot be completely missed on the solder pad.

The adhesive selection of solder paste generally requires its self-adhesive ability to be greater than its adhesion to the template, while its adhesion to the template hole wall is smaller than its adhesion to the solder pad.

1.3 Uniformity and size of solder paste particles

The particle shape, diameter size, and uniformity of solder paste also affect its printing performance. Recently, there have been many studies on the printing characteristics of 3 #, 4 #, and 5 # solder paste issued by 01005 device in the industry. The author believes that for a certain type of solder paste, the diameter of the largest particle within the model range is about or slightly less than 1/5 of the template opening size. By selecting a mesh plate with appropriate thickness and process punching, the ideal printing effect can be achieved. Usually, fine particle solder paste has better solder paste printing clarity, but it is prone to edge collapse and has a higher degree and chance of oxidation. Generally, pin spacing is one of the important selection factors, while considering both performance and price. The specific relationship between pin spacing and solder particles is shown in Table 1.

Nowadays, many units are considering the design specifications for the solder pads of 01005 components, but the first priority is to evaluate the impact of the corresponding design of the mesh board opening and the selected solder paste particles on the printing quality.

1.4 Metal content of solder paste

The metal content in solder paste determines the thickness of the solder after welding. As the percentage content of metal increases, the thickness of solder also increases. But at a given viscosity, as the metal content increases, the bridging tendency of the solder also increases accordingly.

After reflow soldering, it is required that the device pins be firmly soldered, with a full and smooth solder volume, and have a height climb of 1/3 to 2/3 in the direction of the device (resistive capacitive device) end height. In order to meet the requirements of solder paste quantity 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% for better performance.

3.1 Setting and Adjustment of Printing Parameters for Screen Printing Machine

(1) Scraper pressure

The change in scraper pressure has a significant impact on printing. Too little pressure can prevent the solder paste from effectively reaching the bottom of the template opening and from depositing well on the solder pad; Excessive pressure can cause the solder paste to be printed too thin and even damage the template. The ideal state is to scrape the solder paste clean from the surface of the template. In addition, the hardness of the scraper also affects the thickness of the solder paste. A too soft scraper can cause the solder paste to sink, so it is recommended to use a harder scraper or metal scraper.

(2) Printing thickness

The printing thickness is determined by the thickness of the template, and of course, there is also a certain relationship between the machine settings and the characteristics of the solder paste. The micro adjustment of printing thickness is often achieved by adjusting the scraper speed and scraper pressure. Appropriately reducing the printing speed of the scraper can increase the amount of solder paste printed onto the printed board. One thing is obvious: reducing the speed of the scraper is equivalent to increasing the pressure of the scraper; On the contrary, increasing the speed of the scraper is equivalent to reducing the pressure of the scraper.

(3) Printing speed

A fast scraper speed is beneficial for the rebound of the template, but at the same time, it will hinder the transfer of solder paste to the printed board pads, and a slow speed will cause poor resolution of the solder paste printed on the pads. On the other hand, the speed of the scraper is closely related to the viscosity of the solder paste. The slower the scraper speed, the greater the viscosity of the solder paste; Similarly, the faster the scraper speed, the lower the viscosity of the solder paste. Usually, the speed range for fine pitch printing is 12-40mm/s.

(4) Printing method

The printing methods of templates can be divided into on contact and off contact. Printing with gaps between templates and printed boards is called non-contact printing. When setting up the machine, this distance is adjustable, with a general gap of 0-1.27mm; The printing method of template printing without printing gaps (i.e. zero gaps) is called contact printing. The vertical lifting of the template for contact printing can minimize the impact on printing quality, and it is particularly suitable for fine pitch solder paste printing.

(5) Parameters of scraper

The parameters of the scraper include the material, thickness, and width of the scraper, the elasticity of the scraper relative to the tool holder, and the angle of the scraper to the template, all of which affect the distribution of solder paste to varying degrees. When the angle θ of the scraper relative to the template is between 60 ° and 65 °, the quality of solder paste printing is optimal.

When printing, the relationship between the opening size and the direction of the scraper should be considered. The traditional printing method for solder paste is to run the scraper along the x or y direction of the template at a 90 degree angle, which often results in different amounts of solder paste on the device in different directions of the opening. Experimental verification has shown that when the opening length direction is parallel to the direction of the scraper, the thickness of solder paste scraped out is about 60% greater than when they are perpendicular. Printing with a scraper at a 45 ° angle can significantly improve the imbalance of solder paste in different template opening directions, while also reducing damage to fine pitch template openings caused by the scraper.

(6) Demoulding speed

The detachment speed between the printed circuit board and the template can also have a significant impact on the printing effect. If the time is too long, it is easy to leave solder paste at the bottom of the template. If the time is too short, it is not conducive to the upright position of the solder paste and affects its clarity.

In fact, every manufacturing company of screen printing equipment conducts a large number of printing experiments during model development, and each design detail has its own characteristics. When you need to purchase screen printing equipment, you should consult the manufacturer in detail, make comparisons, and carefully study the manufacturer's experimental and demonstration process for the above parameters.

(7) Template cleaning

In the process of solder paste printing, it is generally necessary to clean the bottom of the template every 10 boards to eliminate any attachments at the bottom. Anhydrous alcohol is usually used as the cleaning solution.

3.2 Process control during solder paste use

(1) Strictly use solder paste within its validity period. Normally, solder paste should be stored in the refrigerator and kept at room temperature for at least 6 hours before use. After use, the solder paste should be stored separately and its quality should be confirmed before reuse;

(2) Before production, the operator uses specialized equipment to stir the solder paste evenly, and it is best to regularly use a viscosity tester or qualitatively measure the viscosity of the solder paste;

(3) After the first printed board or equipment adjustment on duty, the solder paste thickness should be measured using a solder paste thickness tester. The test points should be selected at five points on the test surface of the printed board, including the top, bottom, left, right, and middle, and the values should be recorded. The solder paste thickness should be within the range of -10% to+15% of the template thickness;

(4) During the production process, a 100% inspection is conducted on the quality of solder paste printing, mainly focusing on whether the solder paste graphics are complete, whether the thickness is uniform, and whether there is any solder paste sticking phenomenon;

(5) Clean the template according to the process requirements after completing the work on duty;

(6) After printing experiments or printing failures, the solder paste on the printed board needs to be thoroughly cleaned with ultrasonic cleaning equipment and dried to prevent solder balls from appearing after reflow soldering due to residual solder paste on the board during reuse.

3.3 Common Printing Defects and Solutions

Solder paste printing is a highly complex process that is influenced by materials and directly related to equipment and parameters. By controlling each small step in the printing process, it can be said that details determine success or failure. To prevent common defects in printing, the following will briefly introduce several of the most common defects that occur during solder paste printing and their corresponding prevention or solution methods.

3.3.1 Incomplete Printing

Incomplete printing refers to the absence of solder paste on certain areas of the solder pad. The possible reasons for this may be:

(1) Blocked openings or partial solder paste sticking to the bottom of the template;

(2) The viscosity of solder paste is too low;

(3) There are large-sized metal powder particles in the solder paste;

(4) Scraper wear.

Preventive solution: Clean the openings and the bottom of the template, choose a solder paste with appropriate viscosity, and ensure that the solder paste printing can effectively cover the entire printing area; Select solder paste with metal powder particle size corresponding to the opening size; Check and replace the scraper.

3.3.2 Pulling Point

Is the solder paste on the printed solder pad like a small hill? 遄, The possible reasons for this may be the gap between the scraper blades or the high viscosity of the solder paste. Prevention or solution: Adjust the scraper gap appropriately or choose a solder paste with appropriate viscosity.

3.3.3 Collapse

After printing, the solder paste collapses on both sides of the solder pad. Reason for occurrence:

(1) The pressure of the scraper is too high;

(2) The positioning of the printed board is not secure;

(3) The viscosity or metal content of the solder paste is too low.

Prevention or solution: Adjust pressure; Re fix the printed circuit board; Choose a solder paste with appropriate viscosity.

3.3.4 Thin solder paste

Reasons for occurrence:

(1) The template is too thin;

(2) The pressure of the scraper is too high;

(3) Solder paste flows and has poor properties.

Prevention or solution: Choose a template of appropriate thickness; Choose solder paste with appropriate particle size and viscosity; Reduce the pressure of the scraper.

3.3.5 Inconsistent thickness

After printing, the thickness of solder paste on the solder pads is inconsistent, and the reasons for this are:

(1) The template is not parallel to the printed board;

(2) The uneven stirring of solder paste results in inconsistent particle size.

Prevention or solution: Adjust the relative position between the template and the printed board; Stir the solder paste thoroughly before printing.

3.3.6 Edges and surfaces with burrs

The possible reasons for this may be the low viscosity of the solder paste and the rough walls of the template openings.