PCBA Tech - Common defects and causes of SMT reflow soldering

In SMT chip processing,reflow soldering is a very important process. It is a soldering process in which the chip components are combined with the pads on the circuit board through high temperature and then cooled together, which has great stability in the use of the circuit board. Influence. Some process defects are also prone to occur in reflow soldering, and the reasons need to be analyzed and resolved in a targeted manner to ensure product quality. 

Defect Pad refers to the surface of the pad in the surface mounting technology (SMT) process, the surface of the pad is bad or can not meet the design requirements of the state.The following is mainly for you to organize and introduce the common defects and cause analysis of SMT reflow soldering.

1.Tin beads

Reasons:

1.The silk screen hole and the pad are not aligned, and the printing is not accurate, which makes the solder paste dirty the PCB.

2.The solder paste is exposed to too much in an oxidizing environment, and too much water in the air is sucked.

3.The heating is not precise, too slow and uneven.

4.The heating rate is too fast and the preheating interval is too long.

5.The solder paste dries too fast.

6.The flux activity is not enough.

7.Too many tin powders with small particles.

8.The flux volatility is inappropriate during the reflow process.

The process approval standard for tin beads is:when the distance between the pads or printed wires is 0.13mm, the diameter of the tin beads cannot exceed 0.13mm, or there can be no more than five tin beads within a 600mm square range.

Two.open the way

Reasons: 

1.The amount of solder paste is not enough.

2.The coplanarity of the component pins is not enough.

3.The tin is not wet enough (not enough to melt, and the fluidity is not good), and the tin paste is too thin to cause tin loss.

4.The pin sucks tin (like rush grass) or there is a connection hole nearby. The coplanarity of the pins is particularly important for fine-pitch and ultra-fine-pitch pin components. One solution is to apply tin on the pads in advance. The pin sucking can be prevented by slowing down the heating speed and heating the bottom surface more and less heating on the top surface.

Three, solder cracks

Reasons: 1. The peak temperature is too high, causing the solder joints to suddenly cool down, and the solder cracks are caused by excessive thermal stress caused by chilling;

2.The quality of the solder itself;

Four.Hollow

Reasons: 1. The influence of materials. The solder paste is damp, the metal powder in the solder paste has high oxygen content, the use of recycled solder paste, the component pins or the pads of the printed circuit board substrate are oxidized or contaminated, and the printed circuit board is damp.

2.The influence of soldering process: the preheating temperature is too low and the preheating time is too short, so that the solvent in the solder paste cannot escape in time before hardening and enter the reflow area to generate bubbles.

Five,Tin Bridge

Generally speaking, the cause of solder bridges is that the solder paste is too thin, including low metal or solid content in the solder paste,low thixotropy, easy squeezing of the solder paste, too large solder paste particles, and too small flux surface tension. Too much solder paste on the pad, too high peak reflow temperature, etc.

SMT reflow soldering is a relatively complicated process, which is susceptible to various factors, and various defects appear, which are generally difficult to eliminate. Understand the common defects of SMT reflow soldering and their causes, and pay more attention to the operation process to avoid defects., And once there is a problem, it can be analyzed and solved in time.

Currently, the main soldering process in SMT (Surface Mount Technology) still follows the steps of printing the solder paste, placing the components accurately in an automated placement machine, and finally soldering them in a reflow oven. For BTCs (which may refer to a specific type of large thermal pad), traditional soldering methods often result in large voids in the pad due to their generally large area.

In order to deal with this problem, the industry has taken a series of measures, such as pre-baking printed boards and components, optimising stencil design, etc., in which the stencil's dot-matrix openings are designed to provide escape routes for volatile substances, such as water vapour and solvents, inside the solder paste during reflow soldering to reduce the rate of voids. However, the practical results of these measures are not always satisfactory.

It has been shown that extending the reflow's constant temperature zone time can reduce voiding to a certain extent, but this also introduces a new problem - an increased risk of pillow effect (HoP) in BGA (ball grid array package) devices. In particular, BCT (which may refer to another type of pad) ground pads have more volatiles due to their large area and the amount of solder paste used, as well as a longer path from the centre of the pad, making ground pads more susceptible to the formation of large voids.

The key to solving the voiding problem more effectively is to ensure that the volatiles can be discharged smoothly. One innovative approach is to place a pre-made solder pad on the ground pad after printing the solder paste and before mounting. The melting point of this prefabricated solder sheet should match that of the solder paste, and its thickness should be slightly less than the thickness of the stencil (0.2 mm less than the stencil thickness is recommended). In the preheating and constant temperature zones, the prefabricated solder sheet is able to support the component and provide more time and space for the volatile substances in the solder paste to evaporate.

It is worth noting that the solder used for SMT is usually packaged in tape and reel packaging for easy automated placement using a placement machine, a feature that makes the method easier and faster to apply in practice. By introducing prefabricated solder sheets, the voiding rate of the BTC thermal pads has been significantly improved, which in turn has dramatically increased the qualification rate and quality reliability of the product. This method is not only simple and easy to implement, but also has a mature process and significant effect.