In the wave soldering process of PCB Assembly, although the residual tin bead on the circuit board is not the most serious disadvantage, it is a time bomb with the most unstable quality. The generation of tin beads should be generated when the printed circuit board leaves the liquid solder surface. Although there are many reasons for the formation of tin beads, there are two main reasons:
When the PCB is separated from the tin wave, a connected tin wire will be pulled out between the component pins and pads/pads on the PCB and the liquid level of the tin pool, just like cheese drawing. When the tin wire continues to be pulled The short circuit between adjacent solder joints is prone to occur when the long and eventually fractured rebound pins or pad ends, and the solder falling back to the tin pool will splash tin liquid and form tin beads on the PCB board.
The second is that some tin liquid will drop directly from the pins to the tin pool to produce splashes. If the adhesion between the tin bead and the surface of the PCB is less than the gravity of the tin bead itself, then the tin bead will bounce off the PCB and fall back into the tin pool, otherwise the tin bead will remain on the PCB.
When the PCB leaves the liquid tin, some gases that were originally surrounded and restricted by a large area of liquid tin and cannot escape (from ambient air, flux volatiles, moisture generated after high heat, and the gas generated by the solder itself) will be in the solder. The splatter escaped from the weakest point of the enclosure and brought out the residual tin before curing was attempted. If the gravity of the solder ball is less than its adhesion to the PCB, it will remain on the surface of the PCB.
Most of the above reasons can be effectively solved or improved as long as a smoother surface solder mask green paint is used.
Second, It is not recommended to use NSMD (non-solder mask limited) pad design on the solder side of the circuit board in contact with the tin wave. Because the area without the solder mask green paint will become rougher, and it is easier for the solder beads to adhere to the surface of the PCB.
Third, Properly changing the tin removal angle (between the plane wave and the track) during wave soldering can also improve the situation of wire drawing rebound. In theory, reducing the tin removal angle can reduce the distance of tin liquid dripping from the pin and reduce the strength of splashing, but If the angle is too small, it will be unfavorable for the air-encapsulated gas to be discharged. Therefore, an angle with the best quality balance can only be obtained through an experimental.
Of course, there are other factors in the formation of tin beads, such as PCB moisture absorption, incomplete flux spraying, insufficient preheating, etc., which may cause tin bead problems. In addition, the improper design of the wave soldering carrier may cause tin infiltration at the edge of the mask opening.
The reasons for the occurrence of wave solder beads and possible solutions are listed below.
When the pcb leaves the liquid tin, some gases that are originally surrounded by a large area of liquid tin and cannot escape (from ambient air, flux volatiles, and water vapor generated by high heat) will splash from the surrounding weak position before the solder solidifies out, and brought out the miserable tin. If the gravity of the solder ball is less than its adhesion to the PCB, it will remain on the surface of the PCB.
- Choice smoother green painting of solder mask will be solved or improved mainly issue.
- Try not to use NSMD (non-solder mask limited) pad design on the solder side of the circuit board. Because the area without solder mask green paint will become rough, it is easier for the solder beads to adhere to the surface of the pcb.
- Properly changing the tin removal angle (between the plane wave and the track) during wave soldering can also improve the situation of wire drawing rebound. In theory, reducing the tin removal angle can reduce the distance of tin liquid dripping from the pin and reduce the strength of splashing, but If the angle is too small, it will be unfavorable for the air-encapsulated gas to be discharged. Therefore, an angle with the best quality balance can only be obtained through an experimental.
- The preheating temperature of wave soldering is insufficient, and the flux is not fully exerted.
- The detining angle is too small, and the gas packet cannot be discharged smoothly.
- Improper design or maintenance of the wave solder mask carrier, resulting in tin seepage or flux residue on the edge of the mask mouth.
- Turning on nitrogen (N2) will make it easier for the gas of the solder species to break through the surface tension of the tin pool and splash, because nitrogen will reduce the oxidation rate of the tin wavefront and make the surface tension smaller.
- Properly increase the preheating temperature or prolong the preheating time, so that the solvent and water vapor of the flux can completely evaporate.
- Increase the detinning angle.
- Confirm the quality of the wave solder mask carrier.
- You can try turning off the nitrogen for wave soldering to see how it works.
- Flux remains under the part body. During the preheating stage, if the flux is not completely volatilized, it will quickly generate gas when it enters the tin wave, and sputtering will bring out the tin. Once it stays on the surface of the pcb, tin beads will be formed.
- If the flux is sprayed with water on the PCB, splashing will also occur.
- Be sure to stay on the preheat temperature and time requirements of the flux manufacturer.
- Confirm that the flux is properly stored and free from moisture contamination.
- Confirm whether the compressed air source for spraying flux is dry, and there is oil filter and water filter.
Solder mask quality related reasons:
- A rougher solder mask green paint will provide the opportunity for solder beads to adhere to the surface of the PCB.
- The tin melting temperature of the lead-free process is relatively high. If the temperature resistance quality of some solder mask layers is not improved, the solder mask layer will be softened and made sticky at higher temperatures, and the solder balls will also be easily adhered to the solder mask layer.
- Choose a smooth surface solder mask green paint.
- Moderately adjust down the wave soldering temperature or require the PCB board factory to choose a solder resist green paint with better temperature resistance.
The reasons for the oxidation of PCB or parts pins, moisture absorption and dampness:
The plated through hole (PTH) of the PCB or the oxidation or dip of the component pins will form air holes and cause residual tin splashing.
Confrom incoming quality of pcb and component, proper control storage temperature and humidity, strictly control first in first out, don’t use expire materials.
Tin pool reason:
The tin pool contains insufficient tin, the tin liquid is oxidized, and there are too many impurities.
If it is a leaded process, when the tin content in the tin pool is less than 61.4%, some pure tin must be appropriately added to increase the amount of solder.
There are too few through-holes on the PCB, resulting in poor exhaust during wave soldering, resulting in gas explosion and splashing of tin beads.
Properly design some vent holes on the PCB.