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ELECTRICAL SHIELD DESIGN FOR IMPROVED MECHANICAL RELIABILITY

IP.com Disclosure Number: IPCOM000004635D
Original Publication Date: 2001-Mar-07
Included in the Prior Art Database: 2001-Mar-07
Document File: 1 page(s) / 6K

Publishing Venue

Motorola

Related People

Krishna Jonnalagadda: AUTHOR [+3]

Abstract

Combining multiple small electrical shields into a single larger shield with multiple interior compartments achieves the same electrical isolation, while greatly improving the collateral benefit of the shield in reducing the mechanical strain near sensitive area-array components.

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ELECTRICAL SHIELD DESIGN FOR IMPROVED MECHANICAL RELIABILITY

by Krishna Jonnalagadda, David Read and Andrew Skipor

SUMMARY

Combining multiple small electrical shields into a single larger shield with multiple interior compartments achieves the same electrical isolation, while greatly improving the collateral benefit of the shield in reducing the mechanical strain near sensitive area-array components.

PROBLEM

Area array components, such as ball grid arrays (BGA) and their successors, allow significant reduction of the required area, or footprint, of electronic subsystems on printed circuit boards (PCB), even while the functionality of these subsystems is ever-improving. Such components present multiple challenges to the mechanical reliability of the solder joints connecting such components to their supporting PCB. The surface mount design of these components eliminates the compliance of the electrical leads used in previous through-hole designs, and the components are becoming larger and stiffer as the chip size increases.

The unavoidable result of such changes is that some traditionally electrical components are being required to perform mechanical functions. The classic example of this is the solder joint, which formerly was considered to have only an electrical function, but now has mechanical functionality as well, especially in surface mount technology.

Another such multi-functional component is the radio frequency (RF) shield used to isolate sensitive components on the printed circuit board PCB assembly. Traditional shield design has focused on a single large shield or a number of smaller shields usually depending on the electrical requirements and manufacturing costs. Large shields can be effective for reducing solder joint stresses at an area array component. However, a small shield may be electrically necessary.

SOLUTION

Here we propose a new design, which purposefully incorporates the mechanical and electrical advantages of both large and small shields. The prior art was either a large shield or a number of smaller shields (Figure 1), which involved electrical and mechanical tradeoffs. The invention is a new shield design that leverages the positive aspects of both the single shield and multiple shield design.

Finite element analysis was applied to demonstrate that a single large shield is a better mechanical design choice compared to the multiple shield configuration for reducing the stresses at the solder joints i.e. increasing solder joint fatigue life. Strain leakage was observed beyond the outer edges of the shield. In the singe shield case, the strains at the inner edges of the BGAs were much lower than either the multiple shield or no-shield case. Experimental data with strain gages confirmed the conclusions of the analysis.

The new design calls for a single large shield with chambers or compartments simulating several small shields see Figure 2. The new design with chambers provides at least as much stiffness as the single shield while acco...