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Method for staggered leads for fine-pitch bridging prevention

IP.com Disclosure Number: IPCOM000022660D
Publication Date: 2004-Mar-24
Document File: 6 page(s) / 96K

Publishing Venue

The IP.com Prior Art Database

Abstract

Disclosed is a method for staggered leads for fine-pitch bridging prevention. Benefits include improved performance and improved reliability.

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Method for staggered leads for fine-pitch bridging prevention

Disclosed is a method for staggered leads for fine-pitch bridging prevention. Benefits include improved performance and improved reliability.

Background

Process complexity increases with the usage of fine-pitch technology. With a pitch size of close to 0.4 mm and less, the surface mount technology (SMT) process faces an increased challenge to reduce defects. Conventionally, fallout for 0.4-mm pitch SMT component (see Figure 1) yield is bad at ~80-90% yield loss during early builds for the components. Yield loss fluctuates due to solder bridging through process development and the difficulty of process control in obtaining consistent paste printing.

No absolute solution is available. The conventional containment action is to reduce the stencil aperture opening. The previously 8x9-mils opening is reduced to a 7x8-mils opening for a pitch size of 0.5 mm. This solution has its advantages and disadvantages. A reduced amount of paste printed on the pad enables less opportunity for bridging to occur. As a result, fallout is reduced from a 0.5% yield loss to a 0.1% yield loss. However, as the pitch size gets smaller, the margin for error is reduced. If an inadequate amount of paste is deposited, insufficient solder or complete opens can potentially occur. Insufficient solder at the pins poses a reliability concern for the early propagation of fatigue cracks.

         Technologies that are conventionally available in the marketplace includes fine-pitch gull wing/extended lead technology and leadless chip carrier (LCC) technology. Fine pitch gull wing/extended lead (FP) technology has a fine pitch of 0.5 mm (see Figures 2 and 3). LCC technology has a fine pitch of 0.5 mm (see Figure 3 and 4). Conventional technologies are process sensitive to bridging due to the small distance between adjacent leads (see Figure 3).

Note: In the figures, the value X signifies the pitch. The value Y is one of the following distances:

•         From the LCC lead to the LCC lead in a conventional package

•         From the gull wing/extended lead to the gull wing/extended lead package

•         From the LCC lead to the extended lead in staggered leads package

         The value Y” is the distance from the adjacent lead (LCC to LCC or extended lead to extended lead).

General description

The disclosed method is a hybrid of LCC fine-pitch technology and gull wing/extended lead fine-pitch technology in alternate or staggered patterns to increase the distance between adjacent leads, preventing the process bridging.

         The key element of the method is a package with leads underneath the package body similar to LCC and leads extending out from the package body in a staggered pattern.

The purpose of the disclosed method is to solve the high yield loss contributed by bridging of fine pitch on 0.4-mm pitch and smaller pitch components.

Advantages

         The disclosed method...