Printed Wiring Board Pad Thickness Control for Flip-Chip Reliability
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Solution Overview
Problem
Existing printed wiring boards face challenges in achieving reliable flip-chip connections due to the thickness difference between pads and conductive circuits, leading to issues with solder agglomeration and connection reliability.
Innovation Solution
A method for manufacturing printed wiring boards involves forming pads thicker than conductive circuits, with a solder bump profile that is thicker on the pads than on the conductive circuits, using a multi-step process involving plating resist layers and etching to create a robust solder connection.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the pad thickness is made greater than the conductive circuit thickness, then connection reliability is improved and solder agglomeration is prevented, but the manufacturing process complexity increases due to multiple plating resist layers and etching steps
Solution Approach 1:
The manufacturing process is segmented into distinct stages: first forming a base metal film, then applying first and second plating resist layers sequentially, performing multiple plating operations, and finally etching to create the thickness differential. This segmentation allows precise control over pad thickness while maintaining manufacturing feasibility through systematic process steps.
Solution Approach 2:
The first plating resist layer is formed and removed before the second plating resist layer is applied. This preliminary action creates a structured approach where each plating operation builds upon the previous one, enabling precise thickness control of the pad relative to the conductive circuit while organizing the complex manufacturing process into manageable sequential steps.
2Reliability
If the solder bump on the pad is made thicker than on the conductive circuit, then heavier pressure can be applied during assembly improving connection reliability, but the risk of solder bridges increases
Solution Approach 1:
The solder bump thickness is made non-uniform, with thicker solder specifically on the pad area and thinner solder on the conductive circuit areas. This local quality differentiation allows the pad to withstand heavier assembly pressure for improved connection reliability while the thinner solder on conductive circuits minimizes the risk of solder bridges between adjacent traces.
3Manufacturing precision
If multiple plating resist layers are used to form the pad structure, then precise thickness control is achieved, but the manufacturing time and cost increase
Solution Approach 1:
Multiple plating operations are merged into a single manufacturing flow, where the first plated-metal film and second plated-metal film are formed in sequence on the same substrate. This merging approach achieves precise thickness control through cumulative plating while consolidating operations to reduce overall manufacturing time compared to separate processing steps.
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 approach enhances connection reliability by allowing heavier pressure on the pads during assembly, preventing solder bridges and maintaining insulation distance, thus improving the reliability of electronic component mounting.
Implementation Method 1
forming a plated-metal film on the metal film exposed from the plating resist
Implementation Method 2
etching to reduce the thickness of the plated-metal film exposed from the etching resist
Implementation Method 3
forming a solder film on the pad and portion of the conductive circuit exposed through the opening
Implementation Method 4
forming a solder bump on the pad by agglomerating solder on the pad through a reflow
Data Source
AI summary
A method for manufacturing a printed wiring board includes forming a metal film on a surface of an insulative board, a plating resist on the metal film, and a plated-metal film on the metal film exposed from the plating resist, covering a portion of the plated-metal film with an etching resist, etching to reduce thickness of the plated-metal film exposed from the etching resist, removing the resists, and forming a wiring having a pad and a conductive circuit thinner than the pad by removing the metal film exposed through the removing of the plating resist, a solder-resist layer on the surface of the board and wiring, in the layer an opening exposing the pad and a portion of the circuit contiguous to the pad, a solder film on the pad and portion of the circuit exposed through the opening, and a solder bump on the pad by solder reflow.