The reason and prevention strategy of surface mount welding

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**Poor Wetting**

Poor wetting occurs in the soldering process when the solder does not adequately react with the substrate in the welding area, leading to incomplete or ineffective solder joints. The primary causes of poor wetting include contamination on the surface of the welding area, residues from flux, or the formation of a metal compound layer on the joint's surface, such as sulfides on silver or oxides on tin. Additionally, if residual elements like aluminum, zinc, or cadmium in the solder exceed 0.005%, the effectiveness of the flux is reduced due to moisture absorption, which can also lead to poor wetting. In wave soldering, the presence of gas on the surface of the substrate can further contribute to this issue. To address poor wetting, it is essential to adopt appropriate soldering techniques, implement anti-contamination measures on the surfaces of both the substrate and components, select suitable solder, and establish reasonable welding temperatures and times.

**Bridging**

Bridging is primarily caused by excessive solder, significant edge collapse after solder printing, misalignment of the substrate welding area, or an offset in SMD mounting. In SOC (System on Chip) and QFP (Quad Flat Package) circuits that are becoming increasingly fine, bridging can lead to electrical short circuits, negatively affecting product performance.

Corrective measures to prevent bridging include:

1. Ensure proper solder paste printing to avoid poor edge conditions.

2. Set the size of the substrate welding area to meet design specifications.

3. Ensure the SMD mounting position falls within the designated range.

4. Maintain appropriate wiring gaps and coating accuracy for solder resistors.

5. Establish appropriate welding process parameters to prevent mechanical vibrations from the welding machine's conveyor belt.

The reason and prevention strategy of surface mount welding

**Cracks**

When a welded PCB exits the welding zone, micro-cracks may form in the SMD due to differences in thermal expansion between the solder and the joined components. This can happen due to quenching or heating, leading to solidification or contraction stress. Additionally, care must be taken to minimize impact stress on the SMD during handling and transportation.

In designing surface mount products, it is important to consider the reduction of thermal expansion gaps and correctly set the heating and cooling conditions. Selecting a solder with good ductility is also essential.

**Solder Balls**

Solder balls often form during the soldering process, especially with rapid heating, which can cause solder to be propelled away. Other contributing factors include improper printing alignment or edge collapse, as well as contamination.

Preventive measures include:

1. Avoid excessive heating during soldering and adhere to the specified temperature profile.

2. Remove defective solder printing, including bad edge conditions or misalignment.

3. Use solder paste that meets the requirements for moisture absorption.

4. Implement a suitable preheating process based on the welding type.

**Suspension Bridge (Manhattan)**

A "bad suspension bridge" refers to a situation where one end of a component lifts out of the soldering area, leaning upward or erect. This issue can arise from rapid heating, unbalanced heating direction, poor solder paste selection, inadequate preheating, and incorrect sizes of the welding area. The shape of the SMD itself and its wettability can also influence this phenomenon.

To prevent a bad suspension bridge, take the following measures:

1. Ensure proper storage conditions for SMDs.

2. Correctly size the substrate welding area.

3. Reduce the surface tension at the end of the SMD when the solder melts.

4. Set the appropriate solder printing thickness.

5. Adopt reasonable preheating methods to achieve uniform heating during soldering.

Why Choose China PCBA Supplier KSPCBA as Your surface mount welding Manufacturer?

– Experienced and skilled team

– State-of-the-art equipment

– Strict quality control

– Excellent customer service

– Competitive price