PCB News - Causes and control methods of tin beads in SMT production

Causes and control methods of tin beads in SMT production

Over the past two decades, with the continuous development of electronic information products that are lighter, thinner, energy-saving, miniaturized, and planar, electronic products with different uses must adopt surface mount (SMT) technology. The tin bead is a serious hazard to electronic products, so how to reduce the tin bead is one of the key management and control content of SMT enterprises.

According to related cases, during reflow soldering in SMT production, due to the splash of tin paste metal particles, it is easy to form tiny spherical solder beads or irregularly shaped solder particles, which are called tin beads. Tin bead is one of the main defects in SMT production. The diameter is about 0.2～0.4mm. It mainly appears on the side of the SMT component or between the IC pins. It not only affects the appearance of the PCB product, but also may cause a short circuit during use, which seriously affects The quality and life of electronic products may even cause personal injury.

Tin beads are caused by many factors. Raw materials, solder paste, templates, mounting, reflow soldering, environment, etc. may all cause tin beads. Therefore, it is very important to study the causes of it and strive for the most effective control.

1. PCB board quality, components

The PCB pad (PAD) design is unreasonable. If the component body is pressed on the PAD too much and the solder paste is squeezed out too much, solder balls may be produced. When designing a PCB, it is necessary to select a suitable component package and a suitable PAD. The PCB solder mask is not well printed and the surface is rough, resulting in tin beading during reflow. The incoming PCB inspection must be tightened. When the solder mask is seriously defective, it must be returned or scrapped. There is moisture or dirt on the pad, which leads to the production of tin beads. The moisture or dirt on the PCB must be carefully removed before it is put into production. In addition, customers often encounter substitution requirements for devices of different package sizes, resulting in mismatch between the device and the PAD, and solder balls are prone to be produced. Therefore, substitution should be avoided as much as possible.

2. PCB is damp

There is too much moisture in the PCB, and when it passes through the reflow oven after mounting, gas is generated due to the rapid expansion of moisture and tin beads are generated. It is required that the PCB must be dry and vacuum packaged before being put into SMT production. If it is damp, it needs to be baked in an oven before use. For organic solder protection film (OSP) boards, baking is not allowed. Calculated according to the production cycle, OSP boards can be put on line for production within 3 months, and replacement of materials is required for more than 3 months.

3. Selection of solder paste

The solder paste significantly affects the quality of soldering. The metal content, oxide content, metal powder particle size, and solder paste activity in the solder paste all affect the formation of tin beads to varying degrees. Metal content and viscosity. Normally, the volume ratio of the metal content in the solder paste is about 50%, the mass ratio is about 89% to 91%, and the rest is flux (Flux), rheology modifier, viscosity control agent, solvent, etc. If the flux ratio is too large, the viscosity of the solder paste will decrease. In the preheating zone, the force generated by the flux vaporization is too large and it is easy to produce tin beads.

The viscosity of the solder paste is an important factor affecting the printing performance. It is usually between 0.5 and 1.2 KPa·s. During stencil printing, the viscosity of the solder paste is about 0.8 KPa·s. When the metal content increases, the viscosity of the solder paste increases, which can more effectively resist the force generated by the vaporization in the preheating zone, and can also reduce the tendency of the solder paste to collapse after printing, which can reduce the tin bead.

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 bonding process after the metal powder is melted. During the reflow soldering stage, the oxide content on the surface of the metal powder will increase, which is not conducive to the "wetting" of the pad and the formation of tin beads. Therefore, vacuum operation must be required in the metal powder (Powder) process to prevent the powder from oxidizing.

The particle size and uniformity of the metal powder. Metal powder is a very fine spherical particle, its shape, diameter and uniformity all affect its printing performance. The finer particles have a higher oxide content. If the proportion of fine particles is large, the printing resolution will be better, but it is easy to produce sags, which will increase the number of tin beads; the larger the proportion of particles will increase the amount of tin. The uniformity varies greatly, which will lead to an increase in tin beads.

Solder paste activity. The activity of the solder paste is not good, and it dries too fast. If an excessive amount of diluent is added, in the preheating zone, the force generated by the gasification of the diluent will easily produce tin beads. If you encounter solder paste with poor activity, it is best to stop using it immediately and replace it with a good activity.

Board problem causes tin beads and its control method

If the stencil is too thick, the solder paste printing will be thick, and solder balls are likely to be produced after reflow soldering. Selection principle of template thickness:

Template thickness principle

If the template is too thick and there are too many tin beads, make the template again as soon as possible. The opening of the template is not treated with anti-tin bead, which is easy to produce tin bead. Regardless of lead-free or lead-free, the chip openings of the tin-printing template must be anti-ball openings. Improper, excessively large, or offset template openings will cause tin beads to be produced. The size of the PAD determines the size of the template opening. The most critical element of the template opening design is the size and shape. In order to avoid excessive solder paste printing, the opening size is designed to be less than 10% of the corresponding pad contact area; the lead-free template opening design should be larger than that with lead, so that the solder paste covers the pad as much as possible.

Improper adjustment of parameter setting of flashing machine leads to tin bead and its control method

The solder paste has sag after printing, and tin beads may be formed after passing through the reflow oven. The solder paste has sag, which is related to the pressure, speed, and demolding speed of the printing machine squeegee. If there is solder paste collapse, the pressure, speed or demolding speed of the squeegee must be re-adjusted to reduce the collapse and reduce the occurrence of tin bead. The printing starts when the position is not aligned properly, and the printing is offset, so that part of the solder paste is stained on the PCB, and solder beads may be formed. The solder paste printed 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 main parameter in the production, usually equal to the thickness of the template (1+10%±15%). Too thick leads to the collapse of the edges and the formation of tin beads. The printing thickness is determined by the thickness of the template, and has a certain relationship with the settings of the machine and the characteristics of the solder paste. The micro adjustment of the printing thickness is often achieved by adjusting the squeegee pressure and printing speed.

Mounting pressure problem causes tin beads and its control method

If the pressure setting of the middle part of the patch is too large, when the component is pressed on the solder paste, a part of the solder paste may be squeezed under the component. During the reflow soldering stage, this part of the solder paste will melt and easily form tin beads, so you should choose Appropriate placement pressure.

Furnace temperature problem causes tin beads and its control method

The reflow curve can be divided into four stages: preheating, heat preservation, reflow and cooling. In the preheating stage, the temperature is increased to between 120 and 150 degrees, which can remove the volatile solvent in the solder paste and reduce the thermal vibration to the components; at the same time, vaporization will occur inside the solder paste. If the metal powder in the solder paste is bonded If the force is less than the force generated by vaporization, a small amount of solder paste will overflow from the PAD and leave, and some will hide under the chip resistor and form a solder ball after reflow. It can be seen that the higher the preheating temperature and the rapid heating of the preheating zone, the atmospheric phenomenon will increase, and the easier it is to form tin beads. Therefore, adjust the temperature of the reflow oven, reduce the speed of the conveyor belt, and adopt a moderate preheating temperature and preheating speed to control the tin bead.

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