The assembly quality of SMT is directly related to the PCB pad design, and the size ratio of the pad is very important.
If the PCB pad design is correct, a small amount of skew during mounting can be corrected by reflow soldering again. On the contrary, if the PCB pad design is incorrect, even if the mounting position is very accurate, component position shifts, suspension bridges, etc. will appear after reflow soldering. Welding defects.
The basic principle
- According to the analysis of the solder joint structure of various components, in order to meet the reliability requirements of solder joints, the PCB pad design should master the following key elements:
- Symmetry: The pads at both ends must be symmetrical to ensure the balance of the surface tension of the molten solder.
- Pad spacing: Ensure that the component ends or pins and the pads are properly overlapped. If the pad spacing is too large or too small, it will cause welding defects.
- The remaining size of the pad: the remaining size after the component end or lead overlaps with the pad must ensure that the solder joint can form a meniscus.
- Pad width: It should be basically the same as the width of the component end or pin.
Pad Size Solderability Defects
- Pad size varies
The size of the pad design must be consistent, the length must be suitable for the range, and the extension length of the pad must have a suitable range. If it is too short or too long, tombstone phenomenon will easily occur. Inconsistent pad size and uneven pulling force can also cause device tombstoning.
- Pad width wider than device pins
The design of the pad should not be too wide than the component, and the width of the pad is only 2mil wider than the component. Too wide a pad width can lead to problems such as component displacement, empty soldering, and insufficient tin on the pad.
- Pad width narrower than device leads
The width of the pad design is narrower than the width of the component, and the pad area that the component contacts is small during SMT placement, which can easily cause the component to stand sideways or flip.
- Pad length is longer than device pins
The designed pad should not be too long than the component pins. Excessive flow of flux in the SMT reflow process beyond a certain range will cause the components to shift to one side.
- The inter-pad pitch is shorter than the device
The problem of pad spacing short circuit generally occurs in the IC pad spacing, but the inner spacing design of other pads cannot be much shorter than the component pin spacing, and a short circuit will be caused if the value exceeds a certain range.
- Pad lead width is too small
The same component is in the SMT patch, and the defect of the pad will cause the component to be biased. For example, if a pad is too small or part of the pads are too small, there will be no tin or less tin, resulting in different tensions at both ends and deviation.
A real case due to small pad material deviation
The material pad size does not match the PCB package size
- Description of the problem: When a product is in SMT production, the inductance is found to be offset during the visual inspection after reflow soldering. After inspection, it is found that the inductance material does not match the pad. The pad size of the material soldering end is 3.31mm, and the pad size of the PCB package is 2.51. 6MM, the material will be twisted after welding.
- Impact of the problem: The electrical connectivity of the material will deteriorate, which will affect the performance of the product, and seriously cause the product to fail to start normally;
- Problem extension: If you cannot purchase materials with the same size as the PCB pad, and the inductance and current resistance can meet the needs of the circuit, then you still face the risk of changing the board.
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Chip standard package pad inspection
The soldering reliability inspection of the Chip standard package can be divided into three points: “pad length”, “pad width”, and “internal distance between pads”.
Chip parts produced by different brands may have different sizes, and DFM will create a component library (component geometric model library) for each model corresponding to each brand for analysis and inspection.
In addition to the production process, welding accessories, and materials, the soldering defects produced by PCBA during the production process have a lot to do with the pad design of PCBA.
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