The main purpose of lead forming is to ensure that the device leads can be soldered to the corresponding pads of the PCBA. Additionally, it addresses the issue of stress relief. After the PCBA components are soldered and tested, they undergo environmental stresses such as vibration, and impacts from high and low temperatures. Testing under such conditions assesses the strength of both the device body and PCB solder joints. By forming the integrated circuit leads, some of the stress encountered during environmental tests can be alleviated. Stress relief primarily involves shaping all leads or wires between the component lead base and the soldering point to ensure free expansion and contraction, thereby safeguarding against mechanical vibrations or temperature-induced damage to critical components and solder joints. Stress management is crucial for enhancing product reliability, making lead forming a focal point in modern production practices.

Except under special circumstances, there are three methods for wiring out integrated circuit leads: top wiring, middle wiring, and bottom wiring. However, regardless of the wiring method used, the forming mechanism does not vary significantly, differing mainly in process control. Based on practical experience and compliance with relevant standards, the following analyzes several key technical parameters of integrated circuit lead forming:

1. Shoulder width (A)

This refers to the distance from the lead root to the first bending point, as depicted in Figure 1. During the forming process, the shoulder width on both sides of the device should ideally be symmetrical. The lead should not be bent at the device body root. The minimum size should be 2 times the lead diameter or 0.5mm. This condition requires comprehensive consideration of the corresponding PCBA pad size, with adjustments made as needed.

2. Welding surface length (B)

This is the distance from the lead cut point to the second bending point, illustrated in Figure 1. To ensure welding reliability, for round leads, the overlap length on the pad should be at least 3.5 times the lead diameter, up to a maximum of 5.5 times, but not less than 1.25mm. For flat leads, the overlap should be at least 3 times the lead width, up to 5 times, with the lead end surface at least 0.25mm from the pad edge. When the flat lead width is less than 0.5mm, the overlap length should not be less than 1.25mm.

3. Standoff height (D)

This is the distance between the component body and the mounting surface after forming, shown in Figure 1. The standoff height should range from a minimum of 0.5mm to a maximum of 1mm. Maintaining an adequate standoff height during lead forming is crucial for stress relief, preventing hard contact between the component body and the PCB surface, which could otherwise damage the device. Additionally, this space facilitates effective penetration of conformal coating and potting compound beneath the chip body, enhancing adhesion to the PCB and improving vibration resistance.

4. Lead bending radius (R)

To ensure good coplanarity of the integrated circuit’s lead welding surface (not exceeding 0.1mm), the lead bending radius must be carefully controlled during forming. Different materials and lead thicknesses (diameters) exhibit varying degrees of rebound during forming, necessitating specific bending radius controls. According to IPC610D, for lead thicknesses less than 0.8mm, the minimum bending radius should equal the lead thickness; for thicknesses greater than 0.8mm, the radius should be between 1.5 to 2.0 times the lead thickness. Empirical values and theoretical calculations guide the determination of the fillet radius of the forming die and the inner fillet radius of the lead.

5. Lead forming coplanarity

Coplanarity refers to the vertical distance between the lowest and highest pins of the device. It is a critical parameter in integrated circuit lead forming. Poor coplanarity can lead to uneven force distribution on the device body, affecting product reliability. JEDEC specifies a coplanarity tolerance of 0.1016mm for device leads. Factors influencing coplanarity include the design of the forming mold rails and the stability of operational procedures, as well as lead warpage during handling. Coplanarity is typically qualitatively assessed by visual inspection on a flat surface, though quantitative measurements can be made using a profiler or optical pin scanner.

6. Pin skew

Lead skew refers to the deviation of the formed lead from its theoretical position relative to the package centerline. Judged qualitatively by appearance, it can be assessed by placing the formed integrated circuit on the PCB processing pad for observation. The maximum lateral deviation should not exceed 25% of the lead width. Accurate measurement can be achieved using a profile projector or optical pin scanning system, with a requirement that pin skew be less than 0.038mm. Factors contributing to lead skew include forming, cutting, and the inherent structure of the lead itself.

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This revision aims to enhance clarity and readability while maintaining technical accuracy.