BY KAI-FUNG LAU AND CHI-HUNG LEUNG
The integrated circuit (IC) packaging industry currently seems to be concentrating on the development of non-leaded packages (such as chip scale ball grid arrays [CSBGAs] and QFNs), but leaded products, especially the gull-wing type surface-mount packages, are still playing an important role in the IC market.
 Figure 1. a) Straight lead, b) J-lead, and c) gull-wing lead. |
Leaded IC packages can be divided into three broad categories: straight lead (Figure 1a), J-lead (Figure 1b) and gull-wing lead (Figure 1c). A plastic dual in-line package (PDIP) is a typical example of a straight lead package, which is mainly for through-hole printed circuit board (PCB) assembly. J-lead formed profiles can be found in plastic leaded chip carrier (PLCC) or SOJ type packages. Gull-wing formed leads can be found in the quad flat pack (QFP) and thin small-outline (TSOP) style packages. Since gull-wing leaded packages are still broadly used in surface mount products, this article will focus primarily on key aspects of the gull-wing lead forming process.
Lead Profile Quality Requirements
Although users typically have their own stringent dimensional and visual quality requirements, their package outlines usually conform to JEDEC or EIAJ specifications. (See Figure 2 for a typical lead profile.) Most often, the critical dimensions are those directly affecting the quality of PCB assembly. These are typically defined as: 1) coplanarity; 2) lead position, which can be sub-divided into lead skew and lead offset; 3) lead spread; and 4) stand-off height. For lead visual quality, the major concerns are lead tip burrs, solder scratches and solder cracks.
Coplanarity
Coplanarity is the perpendicular distance between the lowest resting plane and the highest lead (Figure 3), and is usually measured by profile projectors or optical lead scanners. Generally, the maximum coplanarity allowance based on tooling outgoing requirements will not exceed 0.050 mm.
 Figure 2. Typical gull-wing lead profile. |
Contributors to the most significant coplanarity problems are trimmed dam bar conditions and package warpage. Dam bar trimming design may affect the coplanarity if burrs from cutting are excessive. If the dam bars are cut alternately, the width of the lead at the dam bar area may vary. Also, the burrs generated may be in an alternating pattern. This causes the lead cross section to vary with position, and therefore different degrees of spring-back can result after forming.
 Figure 3. Definition of coplanarity. |
For QFP packages, there is a linear relationship between coplanarity and package warpage. For TSOP packages, warpage has a greater relative impact on the stand-off and total package height, which are generally important in applications where a TSOP package is used. A critical part of a sound design of lead forming tooling is the ability to tolerate package warpage.
Lead Skew
Lead skew is the deviation of a formed lead from its theoretical position as measured with respect to the center line of the package. It is usually measured by using profile projectors or optical lead scanning systems. It will affect the lead footprint location of the package when attached to the PCB. Usually, lead skew should be less than 0.038 mm, depending on the package type. Figure 4 shows a typical lead skew configuration. The causes of lead skew can be related to many factors, including molding, dam bar cutting, forming and the lead configuration itself. Different types of lead skew and their causes are shown in Table 1.
 Figure 4. Typical lead skew configuration. |
One of the main contributors to lead skew is the dam bar trimming method. For fine pitch products, the dam bar may be trimmed in an alternating fashion (i.e., trimming all of the even leads first and then the odd leads), or they may be trimmed in one shot. An alternating trimming configuration gives stronger punch and die designs, but can induce severe lead skew problems at the forming process. Figures 5a and 5b show sample outcomes of leads formed with different trimming methods. One-shot trimming (Figure 5b) gives controllable lead skew, while alternating trimming (Figure 5a) gives uncontrollable skewing of paired leads.
The Forming Mechanism
The various approaches to forming involve a range of both basic solid forming mechanisms and complex roller forming systems. The latter have been developed to accommodate different package types and process requirements.
 Table 1. Different types of lead skew and their related causes. |
Each forming method has its own advantages and disadvantages. The choice of a particular mechanism for a certain product type depends primarily on the packaging and process requirements. For example, cam and swing cam solid forming approaches are preferred for TSOP forming. The inclined solid forming mechanism will have drawbacks, such as solder build-up and scratching, but it does have the advantage of a simple tool design for low-cost applications. A swing cam roller forming mechanism will give better preformance in preventing solder build-up, but there is typically a higher tool cost with this approach.
 Figure 5. Induced lead skewing from alternating dam bar trimming and one shot dam bar trimming. |
The results of detailed evaluations of different forming methods show that the roller forming process induces much lower stress on the leads during forming compared to solid forming processes. The greater stress induced by solid forming processes can exaggerate the factors that result in lead skew or lead shift. In addition, a roller forming mechanism is able to achieve a smaller bending radius compared to those formed by solid forming mechanisms. A comparison of inclined solid forming and swing cam roller forming mechanisms is shown in Figure 6.
 Figure 6. Comparison of inclined solid forming and swing cam roller forming mechanisms. |
AP
Kai-Fung Lau is engineering manager and Chi-Hung Leung is tooling design manager at ASM Assembly Automation, Post Encapsulation Product Group. For more information, contact Kai-Fung Lau at ASM, 6th Floor, Watson Center, Kung Yip Street, Kwai Chung, Hong Kong; 852-2619-2302; Fax: 852-2619-2129; E-mail: [email protected].
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