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Method for a pad design with a variety of shapes

IP.com Disclosure Number: IPCOM000130560D
Publication Date: 2005-Oct-26
Document File: 3 page(s) / 72K

Publishing Venue

The IP.com Prior Art Database

Abstract

Disclosed is a method for a pad design with a variety of shapes. Benefits include improved performance and improved reliability.

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Method for a pad design with a variety of shapes

Disclosed is a method for a pad design with a variety of shapes. Benefits include improved performance and improved reliability.

Background

      The size of chip components is scaling down and input/output (I/O) density is increasing. This chip scaling leads an increased incidence of tombstoning and bridging. Tombstoning occurs when the melt times of two pads differ. It also occurs when the wetting forces of two pads differ.

      Before reflow, solder paste tackiness holds the component during the chip mounting and board transferring (see Figure 1). The figure illustrates the following values:

•             F1, F2 = Tackiness from solder paste

•             F3 = Weight of chip component

•             X = Distance from component center of gravity

      During the reflow, wetting force from the molten solder plays an important role. The difference in temperature on the two pads at any time during the reflow will lead to the imbalance moment. In the case of difference in temperature, tombstone occurs if the following equation is true (see Figure 2):
F3 (X) < F5 cos α (X) + F4 sin θ  (X)

which simplifies to:

F3 < F5 cos α + F4 sin θ

      The equation above contains the following values:

•             F2 = Tacky force of solder paste

•             F3 = Weight of chip component

•             F4, F5 = Wetting force of molten solder at two different edges of the terminal

      Bridging occurs when the pastes of two adjacent pads are in close proximity. Typically, paste is splashed away from its original location and goes near another pad when placed by a placement machine with force. The problem is becoming increasingly critical because smaller part sizes require closer placement.

              The conventional solution is to reduce the stencil aperture opening on a solder-masked pad to reduce the amount of paste deposited on the pads. For example, a 1:1 stencil aperture may be changed to an oval shape. As a result, the difference in pulling force between two terminals is reduced.

      No conventional solution exists to completely prevent tombstoning or bridging. Attempted solutions add to the manufacturing cost.

      Conventional pads are square or rectangular shaped.

General description

      The disclosed method is an on-pad design that increase assembly-process robustnes...