PCB Technical - PCBA integrated circuit lead forming process

The main purpose of the lead forming is to ensure that the device leads can be soldered to the corresponding pads of the PCBA; on the other hand, it mainly solves the problem of stress relief. After the PCBA components are welded and debugged, environmental stresses such as vibration and high and low temperature impact will be carried out. The test, under such environmental stress conditions, will form a certain test of the strength of the device body and PCB solder joints. Through the formation of the integrated circuit leads, part of the stress formed during the environmental stress test will be eliminated. The stress relief is mainly reflected in Forming all the leads or wires between the root of the component lead and the soldering point to ensure that the lead or wire between the two restriction points has free expansion and contraction, and to prevent harmful components and soldering joints from being caused by mechanical vibration or temperature changes. Stress plays a key role in improving product reliability. Therefore, integrated circuit lead forming has attracted more and more attention from product production departments.

Except for special circumstances, there are three ways to wire out the integrated circuit leads, which are top wire way, middle wire way and bottom wire way. However, no matter which wire way, the forming mechanism will not be much different, only in process control. It's different. According to the actual use experience and in accordance with the relevant requirements of the standard, the following analyzes several key technical parameters of integrated circuit lead forming:

1. Shoulder width (A)

That is, the distance from the root of the lead to the first bending point. As shown in Figure 1, the shoulder width on both sides of the device should be basically the same during the forming process. The lead should not be bent at the root of the device body., The minimum size is 2 times the lead diameter or 0.5mm. Under this condition, the size of the corresponding PCBA pad should also be considered comprehensively, and then appropriate adjustments should be made according to actual needs.

2. Length of welding surface (B)

That is, the distance from the cutting point of the lead to the second bending point of the lead, as shown in Figure 1. In order to ensure the reliability of the welding, for the round lead, the length of the lead lapped on the pad should be at least 3.5 times Lead diameter, the maximum is 5.5 times the lead diameter, but should not be less than 1.25mm; for flat leads, the length of the lead lapped on the pad should be at least 3 times the lead width, and the maximum is 5 times the lead width. The end surface after cutting the foot is at least 0.25mm from the edge of the pad. When the width of the flat lead is less than 0.5mm, the overlap length is not less than 1.25mm;

3. Station height (D)

That is, the distance between the body of the component and the mounting surface after forming, as shown in Figure 1. The minimum distance is 0.5mm and the maximum distance is 1mm. In the process of component lead forming, it is very necessary to provide a certain size of station height. The main reason is to consider the problem of stress release, to avoid the formation of hard contact between the component body and the PCB surface, resulting in no stress release space, and then Damage the device. On the other hand, in the three-proof and potting process, the three-proof paint and potting glue can be effectively immersed into the bottom of the chip body. After curing, it will effectively improve the adhesion strength of the chip to the PCB and enhance the anti-vibration effect.

4. Lead bending radius (R)

In order to ensure that the lead welding surface of the integrated circuit has good coplanarity (not greater than 0.1mm) after the lead of the integrated circuit is formed, due to the rebound of the device lead during the forming process, the rebound coefficient of different materials and different lead thickness (diameter) has a certain degree Therefore, the lead bending radius should be controlled during the lead forming process to ensure good coplanarity of the lead welding surface after forming, and the warpage does not exceed 0.25mm. IPC610D stipulates that when the lead thickness is less than 0.8mm, the minimum lead bending radius is the lead 1 times the thickness; when the lead thickness (or diameter) is greater than 0.8mm, the minimum lead bending radius is 1.5 to 2.0 times the lead thickness. In the actual forming process, on the one hand, the above-mentioned empirical values are used for reference, and on the other hand, it is determined by theoretical calculation. The main parameters to be determined are the fillet radius of the forming die and the inner fillet radius of the lead.

5. Coplanarity of lead forming

Coplanarity is the vertical distance between the lowest landing plane and the highest pin. Coplanarity is one of the important parameters of integrated circuit lead forming. If the coplanarity of the device is not good and exceeds the specified allowable range, it will cause uneven force on the device body and affect the reliability of the product. JEDEC stipulates the device The lead forming coplanarity is 0.1016mm. The main factors that cause poor coplanarity are the following aspects: First, the design of the forming mold's rails is unreasonable, and the coplanarity is poor, so it needs to be adjusted appropriately in the design; on the other hand, it is also related to the operation stability of the operator. It also has a lot to do with the warpage of the device leads during the turnaround process. The evaluation of the coplanarity of the formed integrated circuit leads is usually qualitatively judged by appearance. The method is to place the formed integrated circuit on a flat surface with good flatness, and observe the pins on the flat surface with a 10x magnifying glass. For the location, qualified units can purchase a profiler or an optical pin scanner for quantitative measurement.

6. The pin is skewed

Lead skew refers to the deviation of the formed lead from its theoretical position measured relative to the centerline of the package. Under normal circumstances, qualitative judgment can be made by appearance. The main method is to place the formed integrated circuit on the PCB processing pad to be soldered, observe the relative position of the pin and the PCB pad, and ensure that the maximum lateral deviation does not exceed 25% of the lead width. This is the minimum requirement. On the other hand, it can be accurately measured by a profile projector and an optical pin scanning system. The pin skew should be less than 0.038mm. The cause of lead skew may be related to many factors, including forming, lead cutting, forming, and the lead structure itself.