Stand-off blocks prevent BGA corner bridging

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Solution Overview

Problem

Ball grid array (BGA) components experience warpage due to Coefficient of Thermal Expansion (CTE) mismatch during the reflow process, leading to corner bridging defects and potential electrical shorts as solder balls touch each other, which is difficult to detect and can cause reliability issues.

Innovation Solution

Incorporating stand-off blocks at the corners of the BGA assembly to maintain a minimum stand-off height and prevent substrate warpage from causing solder balls to touch, thereby preventing corner bridging by using cylindrical blocks placed on designed-in pads before the reflow process.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If BGA components are subjected to reflow process, then solder joints are formed, but substrate warpage occurs due to CTE mismatch causing corner bridging defects

Engineering Contradiction:

Improvesolder joint reliabilityVSAvoidsubstrate flatness

Core Design Contradiction:

ReliabilityVSManufacturing precision

Solution Approach 1:

Stand-off blocks are placed on the substrate before the reflow process to pre-establish a minimum stand-off height at corner regions. This preliminary positioning prevents the substrate from warping into contact with the PCB at corners during heating, thereby preventing corner bridging defects before they can occur.

Inventive Principle:

Principle #10Preliminary action

Solution Approach 2:

The stand-off blocks act as intermediary elements between the substrate and the PCB. These blocks physically separate the substrate from the PCB at corner regions, mediating the thermal expansion forces and preventing direct contact between solder balls and PCB pads that would cause bridging defects.

Inventive Principle:

Principle #24Intermediary (Mediator)

2Reliability

If stand-off blocks are added to prevent corner bridging, then assembly yield improves, but device complexity increases

Engineering Contradiction:

Improveassembly yieldVSAvoidassembly structure

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

Stand-off blocks are placed only at specific corner regions of the substrate where bridging defects are most likely to occur, rather than uniformly across the entire substrate. This localized approach provides precise protection against corner bridging while minimizing the addition of components and overall assembly complexity.

Inventive Principle:

Principle #3Local quality

Applied Scientific Principles

This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.

Function Achieved in This Case

The stand-off blocks effectively prevent corner bridging by maintaining sufficient space between solder balls during the reflow process, ensuring no electrical shorts and improving assembly yield and long-term reliability by mitigating substrate warpage effects.

Implementation Method 1

Ball grid array (BGA) components experience warpage due to Coefficient of Thermal Expansion (CTE) mismatch during the reflow process

Methodology Applied

Scientific EffectCoefficient of Thermal Expansion (CTE) mismatch: Thermal Expansion

Data Source

PatentUS10638597B2Stand-off block

Publication Date: 2020.04.28 CISCO TECHNOLOGY INC

AI summary

An apparatus may be provided. The apparatus may comprise a substrate and a circuit board. A ball grid array structure may be disposed between the substrate and the circuit board. In addition, a stand-off structure may be disposed between the substrate and the circuit board. The stand-off structure may be adjacent to the ball grid array structure.