Large Inner Solder Pads for BGA Thermal Stress Relief
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Solution Overview
Problem
The coefficient of thermal expansion mismatch between semiconductor dies and package substrates in Ball Grid Array (BGA) packaging leads to high stress at solder balls under the die, causing mechanical failures due to thermally induced stress.
Innovation Solution
Forming large solder pads under the die area with at least 5 times greater average area than standard pads, which combine multiple power or ground sites, to increase the reliability and thermal path of connections, thereby reducing stress-induced failures.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If standard-sized solder pads are used under the die, then the package structure is simple and manufacturing is easy, but high stress concentration occurs at the solder balls leading to mechanical failure
Solution Approach 1:
The patent applies local quality by creating inner solder pads with significantly larger area (at least 5 times greater than outer solder pads) specifically in the high-stress region under the die. This localized modification addresses the stress concentration problem only where needed, without complicating the entire solder pad structure. The inner pads are positioned within the inner portion defined by the die perimeter, providing enhanced reliability precisely where thermal expansion mismatch causes maximum stress.
2Reliability
If larger solder pads are formed under the die, then crack propagation is delayed and connection reliability is enhanced, but manufacturing precision requirements increase
Solution Approach 1:
The patent segments the solder pad structure into two distinct types: inner solder pads located within the inner portion (under the die) and outer solder pads located in the outer portion. The inner pads have at least 5 times greater area than outer pads, creating a clear segmentation that simplifies manufacturing guidelines. This segmentation allows different fabrication approaches for different regions, reducing overall manufacturing precision requirements compared to a uniform pad design.
3Temperature
If multiple power or ground sites are combined into single large pads, then thermal path is improved and stress distribution is enhanced, but the number of solder interconnects is reduced
Solution Approach 1:
The patent merges multiple power or ground sites into single large inner solder pads. Each inner pad can combine multiple power or ground BGA sites under the die, improving thermal path efficiency by providing larger thermal mass and better heat dissipation. This merging also simplifies the interconnect architecture by reducing the total number of solder joints while maintaining or improving electrical and thermal performance.
Data Source
AI summary
A package substrate (12) has a die (14) mounted on a first side. One or more inner solder pads (20-23) are on an inner portion of a second side. A perimeter of the inner portion is aligned with a perimeter of the die. The one or more inner solder pads are the only solder pads on the inner portion. The one or more inner solder pads number no more than five. A plurality of outer solder pads (16) are on an outer portion of the second side. An average of areas of the one or more inner solder pads is at least five times an average of areas of the one or more inner solder pads. The plurality of outer solder ball pads are for receiving solder ball balls. The outer portion is spaced from the perimeter of the inner portion. The outer portion and the inner portion are coplanar.