Do you know why the PCB produces tin beads? 2021-09-30

There are many reasons for the production of tin beads , Among them, the common cause is caused by steel mesh . The steel mesh is convenient for the solder paste to leak into PCB On the pad , The opening has an inclination angle ( such as 45 degree ), Ideally, when demoulding , The shape and thickness of the solder paste left on the pad are good .

PCB There are four reasons for producing tin beads , Let's take a look 4 Kind of ：

1. The production of tin beads is related to the flux
The flux will remain under the components or PCB Plates and carriers ( Pallets for selective welding ) Between . If the flux is not preheated sufficiently and is in the PCB The plate burns out before touching the tin wave , There will be tin splashing and the formation of tin beads . therefore , The preheating parameters recommended by the flux supplier should be strictly followed .

Whether the tin beads will adhere to PCB On board depends on substrate material . If tin beads and PCB The adhesion of the plate is less than the gravity of the tin bead , The tin beads will bounce off the board and fall back into the tin VAT .

under these circumstances , Solder mask is a very important factor . A rougher solder mask will have a smaller contact surface with the solder beads , Tin beads do not stick easily PCB On the circuit board . During lead-free welding , High temperature will make the solder mask smoother , It is easier to cause tin beads to stick to the board .

 

2.PCB Tin beads produced by the gas release of volatile substances in the plate and solder mask

If PCB If there is a crack in the metal layer of the plate through hole , When these substances are heated, volatile gases will escape from the cracks , stay PCB Tin beads are formed on the component surface of the board .

3. stay PCB The tin beads formed when the circuit board leaves the liquid solder

When PCB When the plate is separated from the tin wave , The circuit board will pull out the tin post , When the tin column breaks and falls back into the tin cylinder , The splashed solder will fall on the board to form tin beads .

therefore , When designing tin wave generator and tin cylinder , Attention should be paid to reducing the landing height of tin . Small landing height helps to reduce tin slag and tin splashing .

The use of nitrogen will aggravate the formation of tin beads , Nitrogen atmosphere can prevent the formation of oxide layer on the surface of solder , Increased the probability of tin bead formation , meanwhile , Nitrogen also affects the surface tension of the solder .

 

4. Causes of steel mesh

The steel mesh is convenient for the solder paste to leak into PCB On the pad , The opening has an inclination angle ( such as 45 degree ), Ideally, when demoulding , The shape and thickness of the solder paste left on the pad are good .

PCB There are many reasons for producing tin beads , Through analysis, the common cause is caused by steel mesh .

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