Package-on-Package Alignment via Pre-formed Substrate Marks
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Solution Overview
Problem
Conventional Package-on-Package (PoP) alignment processes face challenges due to process variations in sawing bottom packages, leading to inaccuracies in solder ball alignment, and surface oxidation of solder balls makes identification difficult.
Innovation Solution
The use of pre-formed alignment marks on the device die and/or in the molding material, which are exposed and used to accurately determine the positions of electrical connectors, allowing for precise alignment of top and bottom packages during the reflow process.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If alignment marks are formed in the reflow boat to locate solder balls in bottom packages, then alignment of solder balls can be performed, but process variation in sawing causes variation in alignment accuracy
Solution Approach 1:
Alignment marks are formed on the bottom package substrate before the sawing process. This preliminary positioning ensures that the alignment reference is established prior to any cutting operations, so that when packages are later separated by sawing, the alignment marks remain on the substrate and provide a consistent reference frame for locating solder balls, eliminating alignment variation caused by sawing process variations
Solution Approach 2:
The bottom package substrate serves as an intermediary carrier that holds both the alignment marks and the solder balls in fixed relative positions. This substrate acts as a stable reference frame that is not affected by the sawing process, allowing accurate location of solder balls through the alignment marks even after packages are separated
2Ease of operation
If solder balls are used as alignment marks, then alignment can be performed, but surface oxidation makes identification difficult
Solution Approach 1:
A dedicated alignment mark structure is introduced as an intermediary element that provides a clear, oxidation-resistant visual reference for alignment. This alignment mark is formed on the substrate and can be designed with high-contrast features or protective coatings that prevent oxidation, making it easily identifiable during the alignment operation without the identification difficulties associated with oxidized solder ball surfaces
Solution Approach 2:
The alignment mark can be formed with distinct visual characteristics such as contrasting colors or reflective properties that make it easily distinguishable from surrounding features. This visual differentiation ensures reliable identification during alignment operations, overcoming the difficulty of identifying oxidized solder balls which have altered surface properties
3Object-affected harmful factors
If molding compound covers solder balls to protect them, then protection is provided, but laser ablation or drilling is required to expose them for bonding
Solution Approach 1:
The protective covering is segmented into different functional zones: the molding compound continues to protect the main body of solder balls, while a specific region (the alignment mark area) is left exposed or differently treated. This segmentation allows the solder balls to remain protected during storage and handling while providing access to alignment features and enabling bonding without requiring aggressive laser ablation of the entire molding compound
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 method ensures accurate alignment and bonding of solder balls, reducing the impact of process variations and surface oxidation issues, resulting in improved reliability and consistency of the PoP structure.
Implementation Method 1
The solder balls in the bottom packages are located through locating the alignment marks in the reflow boat
Implementation Method 2
The solder balls in the top package and the bottom package may then be bonded through the solder balls in the bottom package
Data Source
AI summary
A method includes aligning a top package to a bottom package using an alignment mark in the bottom package, and placing the top package over the bottom package, wherein the top package is aligned to the bottom package after the placing the top package over the bottom package. A reflow is then performed to bond the top package to the bottom package.