Electronic Design - Introduction to SMT assembly quality and PCB pad design

PCBA processing includes SMT assembly and PCB board production, and the assembly quality of SMT has a direct and very important relationship with PCB pad design. If the PCB pad design is correct, a small amount of skew during mounting can be corrected due to the surface tension of the molten solder during reflow soldering (called self-positioning or self-correction effect); on the contrary, if the PCB pad design is incorrect, Even if the placement position is very accurate, welding defects such as component position deviation and suspension bridge will occur after reflow soldering.

1. PCB pad design

According to the analysis of the solder joint structure of various components, in order to meet the reliability requirements of the solder joint, the PCB pad design should master the following key elements:

1. Symmetry-the pads at both ends must be symmetrical to ensure the surface tension of the molten solder is balanced.

2. Pad spacing-ensure proper overlap size between component ends or pins and pads. Too large or small pad spacing can cause soldering defects.

3. The remaining size of the pad-the remaining size of the component end or pin and the pad after the overlap must ensure that the solder joint can form a meniscus.

4. The width of the pad-should be basically the same as the width of the component tip or pin.

2. The quality of solder paste for SMT chip processing and the correct use of solder paste

There are certain requirements for the metal powder content of the solder paste, the oxygen content of the metal powder, the viscosity, and the thixotropy in the chip processing.

If the solder paste has a high metal powder content, the metal powder will splash as the solvent and gas evaporate when the reflow soldering temperature rises. If the oxygen content of the metal powder is high, the splash will be aggravated and the tin bead will be formed. In addition, if the viscosity of the solder paste is too low or the shape retention (thixotropy) of the solder paste is not good, the solder paste pattern will collapse after printing, and even cause adhesion. Soldering defects such as solder balls and bridges will also be formed during reflow soldering.

Improper use of solder paste, for example, the solder paste is taken out of the low temperature cabinet and used directly. Because the temperature of the solder paste is lower than room temperature, water vapor condensation occurs, that is, the solder paste absorbs moisture in the air, and after stirring, the water vapor is mixed in the solder paste, and then the solder is heated. When the water vapor evaporates to bring out the metal powder, the water vapor will oxidize the metal powder at high temperature, splash and form tin beads, and cause problems such as poor wetting.

Three, SMT chip processing solder paste printing quality

According to statistics, on the premise that the PCB design is correct and the quality of the components and the printed board are guaranteed, 70% of the quality problems of the surface assembly are caused by the printing process. Whether the printing position is correct (printing accuracy), the amount of solder paste, whether the amount of solder is uniform, whether the solder paste pattern is clear, whether there is adhesion, whether the surface of the printed board is contaminated by the solder paste, etc., directly affect the soldering quality of the surface assembly board. .

Fourth, the solder ends and pins of the components, and the quality of the pads of the printed circuit board

When the solder ends and pins of the components, the pads of the printed circuit board are oxidized or contaminated, or the printed board is damp, the reflow soldering will produce welding defects such as poor wetting, false soldering, tin beads, and voids.

5. SMT chip processing and mounting components

The three elements of placement quality: correct components, accurate positions, and proper pressure (patch height).

1. The components are correct-it is required that the type, model, nominal value and cabinet polarity of each assembly tag component must meet the requirements of the product assembly drawing and schedule, and cannot be pasted in the wrong position.

2. Accurate position-the ends or pins of the components and the land pattern should be aligned and centered as much as possible.

3. Pressure (SMD height)--SMD pressure (height) should be appropriate, and the solder ends or pins of the components should be immersed in solder paste not less than 1/2 thickness. For general components, the amount of solder paste extrusion (length) should be less than 0.2mm, and for narrow-pitch components, the amount of solder paste extrusion (length) should be less than 0.1mm. If the mounting pressure is too small, the solder ends or pins of SMT components float on the surface of the solder paste, and the solder paste cannot stick to the components, and it is prone to position shifts during transfer and reflow soldering. Excessive placement pressure and excessive solder paste squeeze out will easily cause solder paste adhesion, and bridging will easily occur during reflow soldering, and components will be damaged in severe cases.