PCBA Tech - PCB chip processing-spatter analysis

PCB chip processing-spatter analysis
In the stage of solder paste printing for SMT patch processing on PCBA circuit boards, there are many production links due to the fact that the control measures of the engineering process are not in place, which will cause some minor quality problems. For example, the production of splashes. Maybe those who work in SMT processing plants may have a certain understanding.

Solder spatter actually includes solder spatter and flux flying objects, which are caused by the boiling of the flux or the contamination of the solder paste during reflow soldering. These spatters can fly from the solder joint to a few millimeters or even tens of millimeters away from the solder joint.

Solder spatter often causes problems. If the solder spatters on the solder mask, tin beads will be formed; if it falls on the surface of the button or gold finger, it will form slight "bumps" that affect the contact. Flux spatter usually does not cause The problem, but if it falls on the surface of the button or the finger, it will form a watermark-like stain. Due to the insulation of the flux, there will also be a risk of contact.

1. Reasons

1. Spatters are mostly caused by the moisture absorption of the solder paste. Due to the large amount of hydrogen bonding, the water molecules accumulate considerable heat energy before it finally breaks and evaporates. Excessive heat energy combined with water molecules directly exploded vaporization activity. That is, splashes are produced. Solder paste exposed to a humid environment or solder paste using hygroscopic additives will increase the absorption of moisture. For example, once the water-soluble (also known as water-washing type) solder paste is exposed to 90% RH for 20 minutes, more spatters will be generated.

2. In the process of solder paste reflow. The process of solvent volatilization, water vapor volatilization produced by reduction, and solder coalescence causes the flux droplets to be squeezed out. It is not only a normal physical process of solder paste reflow soldering, but also a common cause of flux and solder splash. in. Solder aggregation is a major cause. When reflowing, the inside of the solder powder melts. Once the surface oxide of the solder powder is eliminated by the flux reaction, countless tiny solder droplets will fuse and form a whole solder. The faster the flux reaction rate, the stronger the cohesion driving force, which can be expected to produce more serious spatter.

The effect of flux reaction rate or wetting rate on flux spatter has been studied. The wetting time is the most important factor in determining the flux spatter, and the slower wetting rate is not prone to flying.

3. Dirty wiping of the screen may also cause contamination of the tin ball at the bottom of the template, which will eventually remain on the surface of the PCB, resulting in a phenomenon similar to tin splashing. If the improvement measures are not effective, this may be the reason.

2. Suggestions for improvement

Spatter can be improved or eliminated by increasing the preheating temperature or prolonging the preheating time. The reasons are as follows:

1. The absorbed water is dried out;

2. More oxides are produced during preheating, so the condensation process is slowed down;

3. Due to the loss of volatile substances, the flux gains greater viscosity, making its reaction rate with solder oxides slower;

4. Since the flux medium is more viscous, the solidification of the solder powder is slower.

However, it should be noted that too high or too long preheating may cause poor wetting and voids.

Generally speaking, the ways to reduce spatter are:

1. Process

1. Guan avoids solder paste printing in a humid environment.

2. Use a long preheating time and or a high preheating temperature curve.

3. Use air atmosphere for reflow welding.

2. Materials

1. Use solder paste (flux) with low moisture absorption.

2. Use a solder paste (flux) with a slow wetting rate.
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