Solder Paste Layer for PoP Bonding Oxidation
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Solution Overview
Problem
The semiconductor industry faces challenges in manufacturing package on package (PoP) structures due to issues with surface oxidation of metal balls used for bonding, which degrades the bonding quality between die packages and substrates, leading to reduced reliability and yield.
Innovation Solution
A solder paste layer is applied over metal balls to prevent oxidation, and a reflow process is used to bond the solder paste with contact pads, forming intermetallic compounds that enhance bonding strength, while a molding compound is used to support the structure and manage thermal expansion mismatches.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If metal balls are used for bonding in PoP structures, then bonding is enabled between die packages and substrates, but surface oxidation occurs which degrades bonding quality and reliability
Solution Approach 1:
A solder paste layer is introduced as an intermediary substance between the metal ball and the environment. This solder paste layer acts as a protective barrier that prevents direct exposure of the metal ball surface to oxidizing conditions, thereby eliminating surface oxidation without interfering with the bonding function. The solder paste is applied to cover the metal ball surface, and during reflow processing, it forms a protective coating that maintains metal ball integrity.
Solution Approach 2:
The solder paste is applied to the metal ball surface before the bonding process occurs. This preliminary application of solder paste creates a protective coating in advance, preventing oxidation from occurring during subsequent processing steps. The solder paste is applied prior to reflow processing, ensuring the metal ball surface is protected before any potential oxidation can occur.
2Ease of manufacture
If traditional bonding methods are used without solder paste, then processing steps are simpler, but bonding strength and reliability are reduced due to oxidation
Solution Approach 1:
The solder paste serves as a mediator that enhances bonding strength while integrating seamlessly into the existing manufacturing process. The solder paste layer, when applied and reflowed, creates strong intermetallic compounds at the bonding interface, significantly improving bond strength without adding complex processing steps. The application process uses existing printing or dispensing equipment, maintaining manufacturing simplicity.
3Strength
If underfill formation is used to support PoP structures, then structural support is provided, but manufacturing cost and process complexity increase
Solution Approach 1:
The need for underfill formation is extracted and eliminated from the manufacturing process. By applying solder paste to the metal balls before bonding, the solder paste itself provides the structural support and stress relief functions that would otherwise require underfill material. This removes the underfill formation step from the process, reducing both complexity and cost while maintaining structural integrity.
Solution Approach 2:
The solder paste layer performs multiple functions simultaneously: it prevents oxidation of the metal ball surface, provides bonding material for strong attachment, and serves as a substitute for underfill by providing structural support and stress management. This multi-functionality eliminates the need for separate underfill material and 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
The solution improves the bonding strength between die packages and substrates, increases the reliability of PoP structures, and reduces processing costs by eliminating the need for underfill formation, thereby enhancing integration density and manufacturing efficiency.
Implementation Method 1
A solder paste layer is applied over metal balls to prevent oxidation
Implementation Method 2
a reflow process is used to bond the solder paste with contact pads, forming intermetallic compounds that enhance bonding strength
Implementation Method 3
forming intermetallic compounds that enhance bonding strength
Implementation Method 4
a molding compound is used to support the structure and manage thermal expansion mismatches
Data Source
AI summary
The described embodiments of mechanisms of forming a die package and package on package (PoP) structure involve forming a solder paste layer over metal balls of external connectors of a die package. The solder paste layer protects the metal balls from oxidation. In addition, the solder paste layer enables solder to solder bonding with another die package. Further, the solder paste layer moves an intermetallic compound (IMC) layer formed between the solder paste layer and the metal balls below a surface of a molding compound of the die package. Having the IMC layer below the surface strengthens the bonding structure between the two die packages.