Buried Strain Gauge with Resin Film Cushioning for PCB Impact Testing
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Solution Overview
Problem
Conventional multilayer printed circuit boards face challenges in accurately measuring strain within solder joints due to breakage of resistive elements during impact resistance tests and inaccurate strain measurement caused by cracks in the insulative resin layer and through-holes.
Innovation Solution
A multilayer printed circuit board design featuring a strain gauge with a resistive element held tight between polyimide or thermoplastic resin films, buried in the substrate and electrically connected via a viahole filled with conductive material, allowing for accurate strain measurement without breaking the resistive element, even under impact.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a strain gauge with a resistive element is buried in the circuit board to measure strain, then strain measurement capability is improved, but the resistive element is broken due to cracks in the insulative resin layer during impact resistance tests
Solution Approach 1:
The patent applies beforehand cushioning by introducing a cushioning layer made of elastic resin between the resistive element and the insulative resin layer. This elastic resin layer acts as a buffer that absorbs impact energy and prevents crack propagation to the resistive element during impact resistance tests, thereby protecting the strain gauge's measurement capability while improving reliability under impact conditions.
2Ease of operation
If a through-hole is formed to electrically connect the resistive element to measuring electrodes, then electrical connectivity is improved, but strain measurement accuracy is reduced due to strain occurrence at the through-hole
Solution Approach 1:
The patent applies the taking out principle by extracting the electrical connection function from the through-hole structure and relocating it to the surface of the circuit board. Measuring electrodes are provided on the surface layer, and conductive paste is used to electrically connect the resistive element to these surface electrodes, eliminating the need for through-holes that would introduce strain and compromise measurement accuracy.
3Reliability
If the resistive element is held tight between resin films to prevent breakage, then reliability is improved, but the structure becomes more complex
Solution Approach 1:
The patent applies merging by integrating the cushioning function directly into the existing resin film structure. The elastic resin cushioning layer is combined with the insulative resin layer, allowing the same resin material to serve dual purposes: providing electrical insulation and protecting the resistive element from impact. This reduces structural complexity while maintaining reliability.
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
This design prevents crack propagation in the resin layer, maintains the integrity of the resistive element, and enables precise strain measurement, including near solder bumps, enhancing the accuracy of stress distribution analysis.
Implementation Method 1
a strain gauge having a resistive element held tight between resin films formed from polyimide or thermoplastic resin is buried in the substrate
Data Source
AI summary
There are provided a multilayer printed circuit board and a testing piece for the printed circuit board, including a substrate having an inner-layer conductor circuit and one or more outer-layer conductor circuits formed on the substrate with an insulating layer laid between the substrate and outer-layer conductor circuit, wherein a strain gauge having a resistive element held tight between resin films formed from polyimide or thermoplastic resin is buried in the substrate, and electrodes electrically connected to the resistive element are exposed to outside from the resin film and are electrically connected at exposed portions thereof to a viahole. Even if a crack is caused by an impact test to take place in the insulative resin layer, the resin film layers prevent the crack from spreading and thus the resistive element forming the strain gauge will not be ruptured. Also, not only the information on strain in the surface layer of the circuit board but also information on strain in a desired position can be measured accurately, so that an actual stress given to the circuit board can accurately be determined.