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Method for a groove-type component terminal and pad design

IP.com Disclosure Number: IPCOM000022664D
Publication Date: 2004-Mar-24
Document File: 8 page(s) / 282K

Publishing Venue

The IP.com Prior Art Database

Abstract

Disclosed is a method for a groove-type component terminal and pad design. Benefits include improved functionality and improved performance.

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Method for a groove-type component terminal and pad design

Disclosed is a method for a groove-type component terminal and pad design. Benefits include improved functionality and improved performance.

Background

Tombstoning is a common failure mode in surface mount technology. The problem is exacerbated as component size becomes smaller (see Figure 1).

Tombstoning can occur with a difference in the melting time between two pads and with a difference in the wetting forces between two pads. Before reflow, solder paste tackiness holds the component during the chip mounting and board transfer (see Figure 2).

The vectors F1 and F2 indicate the direction of the tackiness from the solder paste. The vector F3 indicates the direction of the weight of chip component. The vector X indicates the distance from the component center of gravity

During the reflow, the wetting force from the molten solder plays an important role. The difference in temperature on the two pads at any time during the reflow leads to the imbalance (see Figure 3).

In the case of difference in temperature, tombstoning occurs if:

F2 (X) < F5 cos α (X) + F4 sin θ (X)

Simplified, tombstoning occurs if:

F2 < F5 cos α + F4 sin θ

An example of the occurrence with a difference in the melting time between two pads is when both pads have different designs. One has a metal defined (MD) pad and the other has a solder mask defined (SMD) pad (a larger copper pad compared to the metal defined pad). The lack of solder mask opening control causes pad area variation, due to differences in solder mask materials and individual supplier compensation methods. A result is pad heating variation, which leads to a difference in the solder paste melting rate from one side of the component to the other. The difference in the heat rate leads to a difference in surface tension and creates shifting or tipping off the pad in the reflow zone during ramp up or down (see Figure 4).

An example of the occurrence with a difference in the wetting forces between two pads is when components are mounted and shift/skew.

These two examples are inherent design and process issues, which is not eliminated or cost effective, if control is imposed.

         The issue is conventionally solved by reducing the stencil aperture opening on a solder masked pad. For example, changing the 1:1 stencil aperture to an oval shape reduces the amount of paste deposited on the pads. The resulting lower pulling force reduces the difference in the pulling force between the two terminals (see Figure 5).

         No constructive solution available for all implementations due to tombstoning.

If component is placed at a 180-degree angle, the orientation is called billboard.

General description

The disclosed method is the design of a component terminal and pad with a groove to direct the solder wetting force. The groove may be designed in one or two directions. The one-direction design creates a gap within the terminal to reduce the wetting force by limiting the ris...