PCB Solder Ball Land Structure for Thermal Cycling Reliability
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Solution Overview
Problem
Current PCB manufacturing methods face challenges in achieving improved thermal cycling reliability and economical surface processing of conductive patterns, particularly in the context of decreasing semiconductor package thickness and increasing memory capacity, where flip chip bonding is used instead of wire bonding.
Innovation Solution
A method involving a substrate with an insulating layer and a protected layer, where a nickel first metal layer and a copper second metal layer are formed, followed by an organic solderability preservative (OSP) film, to create a solder ball land and mount a semiconductor chip, with specific thickness ranges for each layer to enhance solderability and prevent oxidation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a conventional single-layer copper surface processing is used, then manufacturing cost is reduced, but thermal cycling reliability deteriorates
Solution Approach 1:
The surface processing structure is segmented into multiple functional layers: a nickel layer (first metal layer) for oxidation prevention and a copper layer (second metal layer) for solderability enhancement. This segmentation allows each layer to perform its specific function optimally, improving thermal cycling reliability while maintaining manufacturing efficiency.
Solution Approach 2:
The invention uses a composite metal structure combining nickel and copper layers. The nickel layer provides oxidation resistance and structural stability, while the copper layer ensures excellent solderability. This composite approach resolves the contradiction by achieving both reliability and functional performance without excessive complexity.
2Reliability
If copper layer thickness is increased to improve solderability, then solderability improves, but copper dissolution into solder ball increases
Solution Approach 1:
The nickel layer is applied locally on the protected layer to create a controlled interface. This localized nickel barrier prevents excessive copper dissolution into the solder ball while maintaining adequate copper layer thickness for solderability. The nickel layer acts as a selective barrier that allows beneficial copper dissolution (for solder joint formation) while preventing harmful excessive dissolution.
Solution Approach 2:
The nickel layer serves as an intermediary between the protected layer and the copper layer. It mediates the interaction between copper and solder, controlling the dissolution process. The nickel layer enables solderability through the copper layer while limiting copper loss to the solder ball, thus resolving the contradiction between solderability and copper content control.
3Reliability
If nickel layer thickness is increased to prevent oxidation, then oxidation prevention improves, but manufacturing cost increases
Solution Approach 1:
The nickel layer thickness is optimized to a specific range (1 μm to 20 μm) to achieve the minimum effective oxidation prevention. This parameter optimization ensures adequate protection against oxidation while minimizing material consumption and manufacturing cost. The copper layer thickness is similarly optimized (0.05 μm to 2 μm) to balance solderability with copper dissolution control.
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 improves thermal cycling reliability and economic efficiency by forming an intermetallic compound between the nickel and copper layers, ensuring strong bonding and reliable soldering while maintaining low copper content in solder balls, thus addressing the need for smaller, lighter electronic components.
Implementation Method 1
forming a first metal layer on the protected layer, forming a second metal layer on the first metal layer, the second metal layer, including copper, and forming an organic solderability preservative (OSP) film on the second metal layer
Implementation Method 2
an intermetallic compound including an alloy of nickel, copper, and tin may be formed between the nickel layer and each of the solder ball
Data Source
AI summary
A method of manufacturing a semiconductor package according to the present inventive concepts comprises preparing a printed circuit board (PCB) including a protected layer, exposing a portion of the protected layer from the insulating layer, forming a solder ball land by processing the exposed surface of the protected layer, forming a solder ball on the solder ball land, and mounting a semiconductor chip on the solder ball formed on the PCB. The solder balls include copper of about 0.01 wt % to about 0.5 wt %.