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Method for a staggered T-shaped pad to eliminate bridging defects for ultra-fine-pitch leaded packages

IP.com Disclosure Number: IPCOM000029166D
Publication Date: 2004-Jun-16
Document File: 4 page(s) / 106K

Publishing Venue

The IP.com Prior Art Database

Abstract

Disclosed is a method for a staggered T-shaped pad to eliminate bridging defects for ultra-fine-pitch leaded packages. Benefits include improved functionality and improved performance.

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Method for a staggered T-shaped pad to eliminate bridging defects for ultra-fine-pitch leaded packages

Disclosed is a method for a staggered T-shaped pad to eliminate bridging defects for ultra-fine-pitch leaded packages. Benefits include improved functionality and improved performance.

Background

              Finer pitch leaded packages are required in high-density products, such as portable devices that are lighter and smaller, faster, and integrated with more functions. Conventionally, the most commonly used leaded packages are 0.5-mm pitch. Board assembly is well established without major yield loss. However, the trend towards finer pitches, such as 0.4 mm and 0.3 mm, creates challenges in board assembly, especially the risk of connecting adjacent mounting pads (bridging, see Figure 1). For example, recent builds with multiple 0.4-mm pitch leaded devices in their products and reference design resulted in a high number of solder bridging defects of ~20,000 dpm (~80% assembly yield loss). Multiple engineering solutions, such as stencil aperture size/design, solder mask between pads, and solder paste change, were assessed and evaluated but none of the options resolved the problem (see Figure 2).

      Conventionally, no solution exists to eliminate this assembly defect. Process optimization in solder print processing minimizes the problem but is not sustainable due to a narrow process margin. The failure is detectable visually and is reworkable using a conventional rework process, such as touch-up with a soldering iron. As a result, this defect is contained through rework at most surface mount technology (SMT) manufacturing houses.

              Typical pad design for leaded packages is rectangular with the same dimensions for all the pads. Stencil aperture is designed with similar pad dimensions but with a narrower width to reduce the opportunity for bridging to occur. However, this solution is not effective as the solder volume varies from the printing process or stencil-to-stencil variation.

General description

      The disclosed method uses a staggered T-shaped pad design for ultra-fine-pitch devices....