Browse Prior Art Database

Method for SMT passive components

IP.com Disclosure Number: IPCOM000019050D
Publication Date: 2003-Aug-27
Document File: 5 page(s) / 319K

Publishing Venue

The IP.com Prior Art Database

Abstract

Disclosed is a method for surface-mount technology (SMT) passive components. Benefits include improved yield and improved process robustness.

This text was extracted from a Microsoft Word document.
At least one non-text object (such as an image or picture) has been suppressed.
This is the abbreviated version, containing approximately 56% of the total text.

Method for SMT passive components

Disclosed is a method for surface-mount technology (SMT) passive components. Benefits include improved yield and improved process robustness.

Background

         Sporadic assembly yield loss ranges from 7.0% to 0.5% of 0402 passive components (such as resistors and capacitors) due to shift skew and tombstoning on motherboards. Defects are only observed on passive pad designs with a solder-mask defined (SMD)/metal define (MD) combination (see Figures 1 and 2). The red dash line indicates the position of the 0402 component.

The SMD pad is connected to a ground plane that is larger than the MD pad, which is a signal pad (see Figure 3). As a result, heat is released faster on the SMD pad, causing a pulling force towards the MD pad. Analysis of the SMD/MD pad area differences indicates that a small area difference (1:1) causes tombstoning and shift skew problems as compared to large SMD/MD pad area differences.

         Components designated as 0402s have a dimension of 4 mm x 2 mm. The requirement to reduce the dimension and weight of electronic components continues as SMT advances. Smaller passive components enable more compact printed circuit board assemblies (PCBAs). This trend is expected to continue. However, the size reduction presents several challenges, including:

•         Assembly-process robustness

•         Sufficient yield

•         Manufacturing cost per board

•         Difficulty in reworking passive components

•         Visual inspection difficulty

•         Insufficient test coverage

         Conventional containment actions include:

•         Prioritize PCB suppliers according to their capability to control the SMD/MD pad opening and minimize yield loss

•         Reduce the stencil aperture opening from 21 mils x 21 mils (square shape) to 18 mils x 18 mils (square shape), which reduces the solder paste volume printed onto 0402 PCB pads and reduces the uneven pulling effect that causes tombstoning or shift skew

         These containment actions do not provide robust solutions in high volume manufacturing. Furthermore, the conventional 0402 design with SMD/MD pads contribute 3-13% fallout (see Figure 4). The conventional PCB SMD/MD pad combination with a small SMD pad opening has a ratio of area delta between SMD/MD of 1:1. The conventional design with the small SMD pad opening causes an uneven heat dissipation rate on one side and results in an unbalanced pulling effect. When the solder joint forms on the MD side, the opposite component terminal from the SMD pad lifts up and causes the tombstone effect, which is a lump of solder that adheres to the passive component terminal.

Description

         The disclosed method is an enlarged SMD pad opening which increases...