Must-see! Classification of SMT Reflow Soldering Problems, Helping to Avoid Detours in Production



Author: Jack Wang

In the surface mount technology (SMT) process of FR4 printed circuit boards, reflow soldering is a crucial step. This step is prone to various defects, which can be classified into printing defects and mounting defects. Common defects include insufficient solder, short circuit, tombstoning, offset, missing component, extra component, wrong component, reverse installation, cracking, solder ball, cold solder joint, void, abnormal gloss, etc. Among them, tombstoning, cracking, solder ball, cold solder joint, void, and abnormal gloss are defects specific to the post-soldering stage. The following is a detailed description of each defect:

Tombstoning: One end of the component lifts off the pad and tilts upward or stands upright.

Short circuit: Solder connects between two or more solder joints that should not be connected, or the solder of the solder joint fails to connect properly with the adjacent conductors.

Offset: The component deviates from the predetermined position in the horizontal, vertical, or rotational direction on the pad.

Cold solder joint: The solderable end of the component fails to make an effective connection with the pad.

Reverse installation: The installation direction of the polar component is incorrect.

Wrong component: The model and specification of the component installed in the designated position do not meet the requirements.

Missing component: No component is pasted in the position where a component is required to be installed.

Exposed copper: The green solder mask on the surface of the PCB peels off or is damaged, exposing the copper foil.

Blistering: The surface of the PCB/FR-4 PCB undergoes local expansion and deformation.

Solder hole: Air holes and pinholes appear in the component solder joints after passing through the reflow oven.

Solder crack: Cracks occur on the surface of the solder.

Hole blocking: Solder paste remains in the vias such as through holes, screw holes, etc.

Pin warping: The pins of the multi-pin component are warped and deformed.

Side standing: The side of the component's soldering end is directly soldered.

Cold solder joint (poor contact): The component is not soldered firmly and is likely to disconnect under the influence of internal and external stresses.

Silk screen reversal and whiting: The component list is silk-screened on the FR4 printed circuit board, but the silk-screened font of the product name and specification cannot be recognized.

Cold solder (unmelted solder): The surface of the solder joint is dull, the solder crystals are not completely melted, and a reliable soldering effect cannot be achieved.

The stencil which is also known as the surface mount template, is a special mold in the surface mount technology process. Its main function is to assist in the deposition of solder paste and accurately transfer an appropriate amount of solder paste to the corresponding positions on the FR4 printed circuit board. With the development of surface mount technology, the stencil is also widely used in rubber processing (such as the red adhesive process). There are mainly two types of stencils:

Chemical etching template: The template openings are formed through chemical etching, which is suitable for making brass and stainless steel templates.

characteristics: 

lThe opening is bowl-shaped, which is not conducive to the release of solder paste. 

lIt is only suitable for printed components with a PITCH value greater than 20 mils (such as 25 - 50 mils). 

lThe template thickness is 0.1 - 0.5mm. 

lThe opening size error is 1mil (position error). 

lThe price is lower than that of laser-cut and electroformed templates.

Laser-cut template: The final opening is formed by laser cutting.

characteristics: 

lThe upper and lower openings are naturally trapezoidal, and the upper opening is usually 1 - 5mil larger than the lower opening, which is conducive to the release of solder paste. The aperture error is 0.3 - 0.5mil, and the positioning accuracy is less than 0.12mil. 

lThe price is between that of chemical etching and electroformed templates. 

lThe smoothness of the hole wall is not as good as that of the electroformed template. 

lThe template thickness is 0.12 - 0.3mm. 

lIt is usually used for the printing of FR4 printed circuit boards with a component PITCH value less than or equal to 20 mils.

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Author: Jack Wang