PCB Tech - What is the process technology of PCBA lead forming

The main purpose of PCBA 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 release. After the PCBA component is soldered and debugged, it will undergo vibration and high and low temperature shocks. Stress test. Under such environmental stress conditions, the device body and PCB solder joint strength will be tested to a certain extent. Through the formation of integrated circuit leads, part of the stress formed in the environmental stress test process will be eliminated, and the stress relief mainly reflects On all leads or wires between the root of the lead of the formed component and the welding point, to ensure that the lead or wire between the two restriction points has free expansion and contraction, and prevent the harmfulness of the components and the welding point due to mechanical vibration or temperature changes. The stress plays a key role in improving product reliability. Therefore, the lead forming of integrated circuits has been paid more and more attention by the product production department.

What are the technological requirements for PCBA integrated circuit lead forming

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 editor of Pater Technology will explain and analyze 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 PCB component body 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 empirical value is 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. The calculation method is as follows:

Calculation method formula

in:

R-The radius of the inner corner of the lead bend

r-The corner radius of the forming die

σs-material yield limit, MPa

E-material elastic modulus

t-lead thickness (or lead diameter), mm

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.