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Method for PCB pad design for passive component assembly

IP.com Disclosure Number: IPCOM000010743D
Publication Date: 2003-Jan-15
Document File: 4 page(s) / 299K

Publishing Venue

The IP.com Prior Art Database

Abstract

Disclosed is a method for printed circuit board (PCB) pad design for passive component assembly. Benefits include improved throughput, improved reliability, and improved assembly process robustness.

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Method for PCB pad design for passive component assembly

Disclosed is a method for printed circuit board (PCB) pad design for passive component assembly. Benefits include improved throughput, improved reliability, and improved assembly process robustness.

Background

        � � � � � Sporadic assembly yield loss ranges from 0.5% to 7.0% of passive components (resistors and capacitors) due to shift skew and buildup (tombstoning) on motherboards. Defects are observed on pad designs with solder mask defined (SMD):metal define (MD) combinations but not on MD:MD pad combinations.

        � � � � � The requirement to reduce the dimension and weight of electronic components continues as surface mount technology (SMT) advances. Smaller components enable more compact PCB assembly (PCBA) and will continue to be used in future motherboard design. However, this situation posts a challenge to assembly process robustness and high yield. Reworking components is a time consuming process. The small form factor of the components increases the risk of manufacturing escapees due to constraints in visual inspection and to test coverage limitations.

        � � � � � Two conventional PCB pad designs exist:

•        � � � � SMD:MD (see Figure 2)

•        � � � � MD:MD (see Figure 3)

        � � � � � The SMD pad design is preferred conventionally due to an easier PCB fabrication process for the ground plane. The SMD pads opening’s tolerance is +/-20% of the target. Controlling the target dimension of CAD dimension of 20 mils x 20 mils on the final SMD pad is difficult. Its reproducibility is poor and varies from supplier to supplier. The MD pad opening is much more reproducible.

        � � � � � Typically, the SMD pad dissipates heat at a slower rate due to connecting to a large piece of ground plane. The uneven heat dissipation can cause an unbalanced pulling effect. A solder joint occurs on the MD side and lifts up the opposite component terminal from the SMD pad. This tombstoning effect appears as one lump of solder at a passive component terminal (see Figure 4).

        � � � � � Conventional conta...