Practical tips to solve the problem of soldering solder beads thoroughly

1. [Industry background] Under the wave of "slimming revolution" of electronic products (the pursuit of lighter, thinner and more power-saving), modern electronic manufacturing widely uses high-precision SMT technology. However, on this fingernail-sized solder joint, there is a hidden danger that cannot be ignored - solder beads. This small point, which can only be seen under the microscope, may lead to major quality accidents such as circuit shorts and even equipment fires. Here's a summary of 20 years of experience in the industry, giving you an in-depth look at this small but critical process challenge.

2. [Formation principle of solder beads] Imagine a miniature metal rain 🌧️: when the circuit board is soldered at high temperature, the extruded trace amount of solder paste particles will splash and solidify in the furnace at 300°C, and finally form a metal ball with a diameter of 0.2 - 0.4 mm (similar to a micro marble). These "metal raindrops" often hide in chip pin gaps or on the sides of chip components, and are as dangerous as blood clots in electronic blood vessels.

Solder beads are caused by a variety of factors, such as raw materials, solder paste, stencils, mounting, reflow soldering, environment, etc. Therefore, it is important to study the causes of this and control it in the most effective way.

 Raw material problems lead to the production of solder balls and their control methods

3. [Key Causes and Solutions] Substrate treatment √ Soldering point design: The pad overflow value is the key, and it is recommended to use a fitting tolerance of plus or minus 5%. √ three steps of material control: solder mask compliance testing→ board surface cleaning treatment→ humidity monitoring (as shown in Figure 1). √ field example: A mobile phone motherboard caused a micro-short circuit due to the solder mask falling off, and the product failure rate dropped by 68% after repair. It is recommended to insert a schematic diagram of the comparison between qualified and unqualified solder mask //

▶ Field example: Ripple effects caused by substitute components. When non-standard components are used (as shown in Table 1 for example), the incidence of solder beads increases by 35% for every 10% decrease in matching. It is recommended to establish a hierarchical evaluation mechanism for substitutes.

Model Comparison Standard Tolerance Substitute Tolerance Risk Value 0402 Capacitor ±0.1mm ±0.15mm High Risk BGA Chip 0.05mm 0.12mm Medium Risk

Raw Material Management ‖ Gold standard for solder paste‖ • Metal composition: 50±2% by volume (table tennis stacking principle) • Degree of oxidation: For every 0.5% increase, the soldering defect rate is doubled. • Particle distribution: 20 - 45 μm gradation preferably (see Figure 2 particle size analysis model) • Active shelf-life: Extended shelf life to 8 months in refrigeration.

▶ Process control black technology: The vacuum anti-oxidation feeding system developed by Nofil Company reduces the oxidation rate of solder paste to 1/3 of the industry level.

Equipment Parameter Optimization "Three Elements of Printing Adjustment" → pressure: gold value 0.4kg/cm² (analogous piano key force) → speed: sinusoidal compensation algorithm at 25mm/s standard speed → template: Precise matching system for thickness parameters (see formula derivation).

Manufacturing environment Note: Every 5°C fluctuation in temperature = 15% change in viscosity, and the linkage environmental compensation system can stabilize the yield. It is recommended to use a multi-point temperature and humidity monitoring scheme.

<!--[if !supportLists]-->1.<!--[endif]-->PCB quality, components The pad design of the PCB is not reasonable, and if the component body is pressed too much on the PAD and the solder paste is extruded too much, solder beads may be generated. When designing a PCB, it is necessary to select the right component package and the right PAD. If the PCB solder mask is not well printed and the surface is rough, resulting in solder beads during reflow, the PCB incoming material must be tightened for inspection, and when the solder mask film is seriously bad, it must be returned or scrapped. Solder beads must be carefully removed from the PCB if there is moisture or dirt on the pads, which can lead to the formation of solder balls. In addition, it is often encountered that customers come in to substitute requirements for devices with different package sizes, resulting in a mismatch between the device and the PAD, and it is easy to produce solder beads, so the substitution should be avoided as much as possible.

<!--[if !supportLists]-->2.<!--[endif]-->Moisture in the PCB Too much moisture in the PCB, when it passes through the reflow oven after mounting, the moisture expands sharply and produces gas, resulting in the formation of solder beads. It is required that the PCB must be dry vacuum packed before being put into SMT production, and if it is wet, it needs to be baked in an oven before use. For organic solder pack (OSP) boards, baking is not allowed. According to the production cycle, the OSP board can be put into production before 3 months, and it needs to be refueled if it exceeds 3 months.

<!--[if !supportLists]-->3.<!--[endif]-->Solder paste selection Solder paste has a significant impact on the soldering quality, and the metal content, oxide content, metal powder particle size, and solder paste activity in the solder paste all affect the formation of solder beads to varying degrees. Metal content, viscosity. Under normal circumstances, the volume ratio of metal content in solder paste is about 50%, the mass ratio is about 89% - 91%, and the rest is flux, rheology modifier, viscosity control agent, solvent, etc. If the proportion of flux is too large, the viscosity of the solder paste decreases, and in the preheating area, the force generated by the flux vaporization is too large, and it is easy to produce solder beads. The viscosity of solder paste is an important factor affecting the printing performance, usually between 0.5 - 1.2 KPa·s, and the best viscosity of solder paste is about 0.8 KPa·s when stencil printing. When the metal content increases, the viscosity of the solder paste increases, which can more effectively resist the forces generated by vaporization in the preheating zone, and can also reduce the tendency of the solder paste to collapse after printing, thereby reducing the solder beads. Oxide content. The oxide content in the solder paste also affects the soldering effect. The higher the oxide content, the greater the resistance to the metal powder during the bonding process after melting, and the oxide content on the surface of the metal powder will increase during the reflow soldering stage, which is not conducive to the "wetting" of the pad and the production of tin beads. Therefore, vacuum operation is required in the metal powder (Powder) process to prevent oxidation of Powder. Particle size and uniformity of metal powders. Metal powders are very fine spherical particles, and their shape, diameter, size and uniformity all affect their printability. The oxide content in the finer particles is higher, and if the proportion of fine particles is large, there will be better printing clarity, but it is easy to collapse the edge, so that the tin beads increase; The large proportion of larger particles will increase the number of tin, and the difference in uniformity will lead to the increase of tin beads. Solder paste activity. If excessive diluent is added, the force generated by the diluent vaporization in the preheating area is too large, and it is easy to produce tin beads. If you encounter a solder paste with poor activity, it is best to stop using it immediately and replace it with a good one.

Template issues lead to the generation of solder beads and their control methods

4. [Whole Process Control System] Build a closed loop of "prevention-detection-improvement" (as shown in Figure 4 fishbone diagram): Prevention system: 55 standardized operating specifications. Real-time monitoring: laser detection equipment + AI image recognition. Retrospective improvement: Establishment of a weld quality database (example: Factory Z reduced solder ball defects by 92% through SPC system).

5. [Innovation and Breakthrough] ▶ The current industry frontier: • Nano-level solder joint CT scanning technology • Self-healing solder research and development (laboratory stage) • AI dynamic temperature control algorithm.

If the stencil is too thick, the solder paste printing will be thick, and it is easy to produce solder beads after reflow soldering. Principle of template thickness selection: / If the template is too thick and there are many solder beads, remake the template as soon as possible. The opening of the template is not treated with anti-solder beads, which is easy to produce solder beads. Regardless of whether there is lead or lead, the opening of the chip part of the tin printing template needs to prevent the opening of the solder bead. Improper, excessively large, and offset formwork openings will lead to the generation of solder beads. The size of the PAD determines the size of the formwork opening, and the most critical elements of the formwork opening design are size and shape. In order to avoid excessive printing of solder paste, the opening size is designed to be less than 10% of the contact area of the corresponding pad; The lead-free template opening should be designed to be larger than the leaded one, so that the solder paste covers the pads as completely as possible.

/Improper adjustment of printing machine parameter setting leads to the generation of solder balls and its control method

After printing, the solder paste has a collapsed edge, and solder beads may form after the reflow oven. The solder paste has a collapsed edge, which is related to the pressure, speed and demoulding speed of the printing machine. If there is a solder paste collapse phenomenon, it is necessary to readjust the scraper pressure, speed or demoulding speed to reduce the collapse edge and reduce the occurrence of solder balls. Printing starts before the right bit, and the printing is offset, so that part of the solder paste sticks to the PCB, and solder beads may be formed. The printed solder paste on the PAD is too thick, and the excess solder paste overflows after the component is pressed down, and it is easy to form solder beads. The printing thickness of solder paste is a major parameter in production, usually equal to the thickness of the stencil (1 + 10% ±15%), and too thick will cause the collapse of the edge to easily form solder beads. The printing thickness is determined by the thickness of the stencil, which is related to the setting of the machine and the characteristics of the solder paste. Minor adjustments to the printing thickness are often achieved by adjusting the squeegee pressure and printing speed.

/Mounting pressure problems lead to the generation of solder balls and their control methods

If the pressure of the chip center is set too much, when the component is pressed on the solder paste, a part of the solder paste may be squeezed under the component, and during the reflow soldering stage, this part of the solder paste will melt and easily form solder beads, so the appropriate placement pressure should be selected.

/Furnace temperature problems lead to the production of solder beads and how to control them

The reflow soldering curve can be divided into four stages: preheating, insulation, reflow, and cooling. The temperature is raised to between 120 and 150 degrees in the preheating stage, which can remove the volatile solvent in the solder paste and reduce the thermal vibration of the components; At the same time, gasification will occur inside the solder paste, if the cohesion between the metal powder in the solder paste is less than the force generated by gasification, a small amount of solder paste will overflow from the PAD, and some will hide under the sheet resistor and form tin beads after reflux. It can be seen that the higher the preheating temperature and the more rapid the heating of the preheating zone, the spatter of gasification will be increased, and the easier it is to form tin beads. Therefore, it is necessary to adjust the temperature of the reflow oven, reduce the speed of the conveyor belt, and adopt a more moderate preheating temperature and preheating speed to control the solder beads.

/ Process control issues leading to the generation of solder balls and how to control them

The precautions for the use of solder paste mainly include the following aspects: under the specified storage conditions, the storage period of solder paste is generally 3-6 months; The use of solder paste should follow the principle of "first-in, first-out", and should be stored in a sealed form in a constant temperature refrigerator at a temperature of 2°C - 10°C; Before use, take it out of the refrigerator, seal it and put it at room temperature for at least 4H, open the cap when the solder paste reaches room temperature, take out part of the solder paste, squeeze all the air between the cap and the solder paste inside, and immediately cover the outer cap. If the bottle is opened at low temperature, it is easy to absorb water vapor, and it is easy to produce tin beads during reflow soldering; After opening, it should be stirred evenly as required to reduce the viscosity of the solder paste, and in principle, it should be used up once on the same day after opening; When the solder paste is placed on the template for more than 30 minutes and is not used, the stirring function of the printing machine should be used before stirring, if the interval is long (more than 1H), the solder paste should be put back into the can and the cap should be tightened, and the reuse should be stirred evenly; When removing the solder paste from the template, replace it with another empty can to prevent contamination of the fresh solder paste, and the solder paste scraped back from the template should also be sealed and refrigerated. Process management also has its corresponding requirements: the optimal temperature during printing is 25°C±3°C, the relative humidity is 45% - 65%, the temperature and humidity are too high, the solder paste is easy to absorb water vapor, and it is easy to produce solder beads during reflux; After the printing fails, it is best to clean the PCB thoroughly with ultrasonic cleaning equipment, dry it and then put it in the oven to bake according to the specifications to prevent tin beads from appearing when reuse; If the template is not cleaned in time, or the wiping is not thorough, there is residual solder paste on the back of the template, which contaminates the PCB surface and produces solder beads. It must be operated in strict accordance with the template cleaning specifications, and a special person must be determined to clean and inspect; After printing solder paste, the PCB should be placed in the shortest possible time to prevent solvents such as flux from volatilizing and easily generating solder balls.

In SMT production, there are many factors that produce solder balls, and it is not enough to focus on one aspect or adjust one parameter. We need to carefully control every detail in the preparation stage and during the production process, and study how to control the factors that affect the solder balls, so that the welding can achieve the best results and meet the high quality requirements of electronic connection technology.