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Method for BGA attachment utilizing a solder fillet for large, high stiffness, low CTE packages

IP.com Disclosure Number: IPCOM000006851D
Publication Date: 2002-Feb-06
Document File: 5 page(s) / 89K

Publishing Venue

The IP.com Prior Art Database

Abstract

Disclosed is a method for ball grid array (BGA) attachment using a solder fillet for large, high stiffness, low CTE packages. Benefits include reduced complexity and decreased cost.

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Method for BGA attachment utilizing a solder fillet for large, high stiffness, low CTE packages

Disclosed is a method for ball grid array (BGA) attachment using a solder fillet for large, high stiffness, low CTE packages. Benefits include reduced complexity and decreased cost.

Background

              A conventional method of decreasing package/substrate-induced reliability issues on the die is to decrease the CTE mismatch between the die and the package. That is, reduce the CTE of the package to approximate 3 ppm/C. Ceramic substrates achieve higher levels of first level (L1) interconnection and die level reliability this way. However, decreasing the CTE of the package increases the CTE mismatch between the package and the system motherboard, inducing large strains on the solder joints connecting them.

              A design rule of thumb in the industry recommends restricting the use of BGA as second level (L2) interconnects in ceramic packages to package sizes less than 32.5 mm square (or <500 I/O connections at 1.27-mm pitch). For larger packages, CGA (column grid array) joints are recommended (see Figure 1). In this type of interconnection, solder columns (commonly 1.3-mm to 2.25-mm high) interconnect the package and the motherboard. Because of the larger standoff height of the solder joints, shear strain in the joints are decreased, increasing the reliability of the joints. The fatigue life of CGA attachment is conventionally 5-10X that of BGA attachment.

              The use of CGA increases the package cost significantly. The increased standoff that CGA requires may also restrict its use.

              The same arguments and limitations that apply to ceramic substrates apply to similar (other low-CTE high-stiffness) substrates, like alloy42 core substrates.

              Conventionally, solutions like column grid array (CGA) are used for large ceramic packages. Similar solutions are expected for use with other low CTE packages.

General description

              The disclosed method extends the use of BGA as the L2 interconnect scheme for large size, low CTE, high stiffness (such as ceramic or metal core) packages. The key element is the method is that in addition to the BGA solder balls, a solder fillet is also required at the edges of the package, connecting the package to the motherboard (see Figure 1).

              The disclosed method is only effective for high-stiffness substrates, such as ceramic or metal core substrates. This solution is less effective for low-stiffness low-CTE substrates, such as low-CTE organic substrates.

Advantages

              The advantages of the disclosed method are reduced complexity and decreased cost.

Detailed description

              The disclosed method supplements BGA attachment with a BGA fillet (see Figure 2).  The edges of the package and some part of the sides will have to be metallized. In the case of the second variant as shown in the figure, only the edges need to be metallized. The corresponding region in the motherboard also has to be metallized. To form...