Cause and prevention of welding on SMT surface mount

With the wide application of SMT surface mount technology, the requirement of SMT technology is higher and higher. SMT welding is closely related to the whole process of assembly. Once welding problems occur, it will affect the quality of products and cause losses.


Bridging


A bridge is a solder path that connects two or more adjacent pads and contacts the pads to form a conductive path. The bridging causes of the most serious collapse of solder or excess solder printing edge, or size of weld substrate tolerance, SMD mount offset caused by SOP, QFP, in circuit miniaturization trend stage, bridging may cause short circuit, using of the products.

Preventive measures:

1, the size of the substrate welding area to meet the design requirements, SMD mounting position should be within the specified range.

2, to prevent the solder paste printing poor edge, the substrate wiring gap, solder coating coating accuracy must meet the requirements.

3, formulate appropriate welding process parameters to prevent the mechanical vibration of the welding belt.


Two solder ball


Solder ball refers to the welding process, because of the spatter and other reasons, in the circuit board in the unnecessary position of the formation of scattered ball. Heating the solder ball have occurred in the welding process and the rapid dispersion caused by solder and solder printing, the dislocation, is also related to the collapse of edge, pollution etc..

Preventive measures:

1, according to the type of welding to implement the corresponding preheating process.

2, according to the setting of the heating process of welding, welding heating in too bad to avoid.

3, the use of solder paste to meet the requirements, no moisture absorption and other problems.


Three crack


Welding PCB in the near welding zone, because of the difference between the solder and the thermal expansion of the joint, it will cause the microcrack of SMD because of the influence of solidification stress or contraction stress when it is hot or hot. After welding, the impact stress and bending stress of SMD must also be reduced during punching, cutting and transportation of PCB.

Preventive measures:

1. The design of surface mount products needs to take into account the gap between thermal expansion and correct heating conditions and cooling conditions.

2, choose good ductility solder.


Four, pull the tip


Pull point means the tip or burr of the solder joint. The reason is that the solder is too much, the flux is little, the heating time is too long, the welding time is too long, and the welding angle of the iron is not proper.

Preventive measures:

1 、 choose proper soldering flux to control the quantity of solder.

2, according to PCB size, whether multilayer plate, the number of components, whether or not installed components, such as setting preheat temperature.


Five, the film problem (Manhattan phenomenon)

The Manhattan phenomenon refers to the phenomenon that one end of the rectangular chip element is welded on the pad and the other end is raised. The main reason for this phenomenon is uneven heating at both ends of the component, uneven heating direction, melting of solder paste and welding area size, SMD shape and wettability.

Preventive measures:

1, adopt reasonable preheating way to realize uniform heating when welding.

2, reduce the surface tension of the SMD end when the solder is melted.

3, the length of the substrate welding area should be set appropriately, the printing thickness of the solder should be set correctly.


Six, poor wetting


Wettability is defined as the welding of solder and substrate in the welding process, and the reaction between the metal and the metal is not generated after infiltration, resulting in leakage welding or less welding failure. The main reason is that the surface of the welding zone is contaminated or stained with solder flux or caused by the metal compound layer on the surface of the joint.


Preventive measures:

1. Perform proper welding procedures, and make good anti fouling measures on the surface of the substrate and on the surface of the component.

2. Select suitable solder and set reasonable welding temperature and time.


The above is about the SMT surface mount welding bad reasons and preventive measures, I believe we have some understanding. Because of the SMT welding process is relatively cumbersome, can not avoid some problems in welding work, we should learn to analyze the causes of every problem, and seek solutions and preventive measures, to reduce welding defects.


 

(Source: internal information)

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