PCB Underfill Blocking Area Prevents Solder Loss
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Solution Overview
Problem
Conventional printed circuit boards experience solder loss during reflow due to weak adhesive forces between the metal layer and solder resist layer, caused by increased pressure from humidity in the underfill, leading to product defects.
Innovation Solution
A printed circuit board structure is developed with a blocking area formed in the underfill between the solder resist layer and the semiconductor chip, which surrounds the semiconductor chip and exposes the metal line, preventing solder escape by blocking the path of solder movement during reflow.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a conventional solder resist layer is applied over the metal layer, then the wiring circuit is covered to prevent unintentional connection, but the adhesive force between the metal layer and solder resist layer becomes weak due to pressure increase from humidity in the underfill during reflow, causing solder resist layer separation and solder loss
Solution Approach 1:
The solder resist layer is segmented by forming openings at specific locations where the underfill will contact the metal layer. This segmentation allows the underfill to directly contact the metal layer in critical areas, maintaining strong adhesion while preserving the solder resist coverage in other areas to prevent unintentional connections.
Solution Approach 2:
The solder resist layer is selectively removed to create openings only in areas where underfill contact is needed. This local modification allows the underfill to exert pressure directly on the metal layer in specific regions, maintaining adhesive strength where needed while preserving the solder resist's protective function in other regions.
2Reliability
If the solder resist layer is applied as the outermost layer, then insulation and prevention of unintentional connection are achieved, but the solder resist layer separates and lifts during reflow due to pressure from humidity in the underfill, causing solder to be pressed out and pass through the space
Solution Approach 1:
The solder resist layer is segmented by forming openings that allow the underfill to contact the metal layer directly. This prevents the buildup of pressure that would otherwise cause the solder resist layer to separate and lift, thereby preventing solder from being pressed out through the separation space.
Solution Approach 2:
The opening acts as an intermediary structure that allows controlled interaction between the underfill and metal layer. This controlled interaction prevents the harmful pressure buildup that would cause solder resist separation and subsequent solder loss, while maintaining the overall integrity of the solder resist layer.
3Reliability
If the underfill is applied to electrically connect the semiconductor chip to the pad, then electrical connection is achieved, but the humidity in the underfill causes pressure increase during reflow, leading to solder resist layer separation and increased solder loss
Solution Approach 1:
The solder resist layer is segmented by forming openings that allow the underfill to contact the metal layer directly. This segmentation relieves the pressure buildup caused by humidity expansion in the underfill, preventing solder resist layer separation while maintaining the electrical connection function of the underfill.
Solution Approach 2:
The openings are formed in the solder resist layer before the underfill is applied. This preliminary action prepares the structure to accommodate the pressure increase from humidity in the underfill during reflow, preventing subsequent solder resist layer separation and solder loss.
Data Source
AI summary
Disclosed is a printed circuit board according to an embodiment. The printed circuit board comprises: a base board; a metal layer, including a pad and a metal line formed in the base board; a solder resist layer that is formed on the base board on which the metal layer is formed and has an opening through which the surface of the metal line is exposed; and an underfill that is formed between the solder resist layer and a semiconductor chip electrically connected to the pad and includes a blocking area formed in the opening.