inSMTIn the production process, it is often encountered that the boards produced after reflow soldering have solder beads. The formation of solder beads makes the product quality unguaranteed and the appearance not smooth. So, what causes reflow soldering solder beads and how can we solve it? Next, Shenzhen JJY Solder Paste Factory will explain it to you:

What causes reflow solder beads

The reflow soldering curve can be divided into4The sections are for preheating, holding, reflux and cooling. The purposes of preheating and holding are respectively.

1The temperature curve is incorrect

In order to makePCBThe surface is60~90sRise inward150℃And keep it warm for about a while90sThis can not only reducePCBIn addition to the thermal shock of components, it is more important to ensure that the solvent in the solder paste can partially evaporate to prevent excessive solvent from splashing during re-soldering due to the rapid increase in temperature, which would cause the solder paste to come out of the solder pad and form solder beads.

2The quality of solder paste

The metal content in solder paste is usually in (90±0.5)%If the metal content is too low, it will lead to excessive flux components. Therefore, excessive flux will cause flying beads due to the difficulty in volatilization during the preheating stage.

Water vapor in solder paste/The oxygen content increases. Since solder paste is usually refrigerated, if the recovery time is not ensured when it is taken out of the refrigerator, it will lead to the entry of water vapor. In addition, the cap of the solder paste bottle should be tightly closed after each use. If it is not tightly closed in time, it will also cause the entry of water vapor.

3Printing and surface mount technology

In the printing process of solder paste, due to the misalignment between the template and the solder pad, if the misalignment is too large, it will cause the solder paste to flow out of the solder pad. After heating, it is easy to form solder beads.

Therefore, the clamping of the template should be carefully adjusted to ensure there is no loosening. Additionally, a poor printing working environment can also lead to the formation of solder beads. The ideal ambient temperature is (25±3)℃The relative humidity is50%~65%.

4The thickness and opening size of the template

If the thickness of the template and the opening size are too large, it will lead to an increase in the amount of solder paste used and also cause the solder paste to overflow outside the solder pad, especially for templates manufactured by chemical etching methods.

Reflow soldering solution for generating solder beads

1Therefore, it is usually necessary to pay attention to the heating rate, adopt moderate preheating, and have a good platform to allow most of the solvent to evaporate, thereby inhibiting the formation of tin beads.

2After the solder paste printed on the template is completed, the remaining part should be dealt with separately. If it is put back into the original bottle, it will cause the solder paste in the bottle to deteriorate and also produce solder beads.

3The surface mount technology processZDirectional pressure is an important cause of tin beads and often does not attract people's attention. Some surface mount technology (SMT) machines are due toZThe shaft head is positioned based on the thickness of the components, which may cause the components to stick togetherPCBThe phenomenon of squeezing the solder paste out of the solder pad in the previous moment will cause obvious solder beads in this part of the solder paste. In this case, the size of the tin beads produced is slightly larger and usually only needs to be readjustedZThe height of the shaft can prevent the formation of tin beads.

4The solution is to select a template of appropriate thickness and the design of the opening size. Generally, the opening area of the template is the size of the solder pad90%.

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