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Design of Rectangular Sheet Type Component Solder Pad

by: Nov 23,2018 107 Views 0 Comments Posted in PCB Design & Layout

Abstract:

Thought the design software has been very advanced and convenient, and has been improving fast, it can’t satisfy all the designer, especially no solder pad library responding to all kinds of SMC or SMD. So analyze the influences of PCB solder pad on the reliability of PCA, and according to the relative quality requirements, give some simple design methods.

Solder Pad Design:

1.    Rectangular Sheet Type Component Solder Pad:

The inspection of welding quality of rectangular components, whether in the national army standard or IPC standard, is to check the welding material wetting state on the end face of the welding end, and there is no requirement on both sides of the welding end. Therefore, the design of such component pad must ensure that the welding material climbing height on the end face of the welding end reaches above 1/3 of component thickness.

2.    Solder Pad Spacing:

The test was carried out with the example of the welding pad of 0805 components. The size of the component is shown in figure 1. The spacing between the pads is 2.0mm, 0.6mm and 0.4mm, respectively. The solder end materials of the component are tin lead and silver palladium coating.

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It is shown from the experimental results that the mounting accuracy is high when the diameter is 1.8mm, and the components are unstable and easy to move before soldering. The component of this design will form a qualified solder joint if it is exactly overlapped on the pad during soldering. However, in the actual soldering process, the solder paste is melted in liquid state in the backflow area, and the components float on it. If the distance between the pads is too large, the contact area of the soldering end will be reduced. At the same time, coupled with the vibration of the transmission mechanism and other factors, it is easy to cause uneven wetting force at both ends of the components, which leads to the phenomenon of tombstoning. The result is exactly the same, which means that a diameter of 1.8 mm is too big. While when the diameter is 0.6 mm, the component end is completely placed on the pad. Moreover, the solder paste has strong adhesive force on the components, which is not easy to move, and the wetting force on both ends of the components is equal during soldering, and finally a good solder joint is formed. The specific results of the experiment are shown in the following sheet 1.

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In addition, the cleaning process after soldering will also affect the spacing of the pads. If choosing the no-clean solder paste to use when soldering and it is no need to clean after soldering, the diameter can be the reliability of soldering, the solder pad width is larger than the component body will not result in a wide on the mechanical strength or solder joint shape defects. However, in the actual production, the width of the rectangular sheet type component’s solder pad is greater than the component body’s width, once the component placement accuracy is not high, or if there is deviation during soldering process, the components of displacement deviation cannot receive correction because two sides of the components are effecting by solder wetting force at the same time. Instead as the component solder pad width is less than or equal to the component body width, or if there is deviation or there is pasted on the welding beyond solder pad component in the process of lateral force and one of the solder pad sides will not effect by the wetting force, such components at both ends by torque force components can be back to the solder pad, which have the effect of error correcting component pasted on. Therefore, it is recommended that the width of the pad should not exceed the width of the component body.

4.    Pad Length:

For rectangular sheet type components, the solder flux of the solder ends to form the solder joints depends on the extent of the length of the solder pad, /span> given the thickness of the template, the longer the pads provide the more solder flux. Gtb3243-98 stipulates that "the height of rectangular sheet type component soldering shall be greater than 1/3 the thickness of the component body", so the length of the protruding component end of the pad shall provide the minimum amount of solder flux required to meet this requirement.

5.    Requirements of Pad Design:

Based on the above points, the rectangular sheet type components can be designed by using the following method. The solder pad and its formula of the component is shown in figure 2 and figure 3.

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Note:

(1) The above unit of measurement is mm.

(2) When there are welding pads on PCB with pin center distance less than 0.5mm, when the length of capacitive welding pads is designed, the K value should be as large as possible.

(3) The smaller the component size is, the smaller the K value should be when designing the spacing between welding pads.

(4) K values in the above three formulas are not necessarily equal.


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