Flip Chip Lead Frame Wettability Control
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Solution Overview
Problem
In flip chip packaging, solder balls tend to uncontrollably collapse during reflow when using a lead frame with lead fingers, leading to structural and functional damages due to short circuits, and forming a solder mask on these small lead fingers is difficult, time-consuming, and costly.
Innovation Solution
The process involves forming attachment and non-attachment areas on the lead frame by depositing a wetting material and surface treating to create a wettability differential, allowing controlled collapse of solder balls during reflow without a solder mask, using techniques like photolithography, laser ablation, and oxidation to define these areas.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a solder mask is formed on the lead frame to prevent solder ball collapse, then solder ball stability is improved, but manufacturing complexity and cost increase significantly
Solution Approach 1:
The patent applies local quality by creating distinct wetting and non-wetting regions on the lead frame surface. The wetting region promotes solder ball attachment and controlled collapse, while the non-wetting region prevents unwanted solder migration. This localized property differentiation eliminates the need for a complete solder mask while achieving the same reliability benefit.
Solution Approach 2:
The patent changes the surface energy parameters of the lead frame through chemical treatments (such as oxidation or plasma treatment) to create areas with different wettability. By modifying the surface chemistry rather than adding a physical mask layer, the process simplifies manufacturing while maintaining solder ball stability during reflow.
2Reliability
If a solder mask is formed on the lead frame to prevent short circuits, then electrical reliability is improved, but manufacturing time and cost increase
Solution Approach 1:
The patent performs preliminary surface treatment on the lead frame before solder ball placement to pre-establish the wetting patterns. This preliminary action creates the necessary surface conditions in advance, eliminating the need for subsequent solder mask formation and associated alignment and curing steps, thereby reducing overall manufacturing cycle time.
3Reliability
If solder balls are allowed to collapse during reflow, then better electrical contact is achieved, but uncontrollable collapse causes short circuits and structural damage
Solution Approach 1:
The patent introduces the wetting material as an intermediary between the solder ball and the lead frame. This intermediary layer controls the interaction during reflow, allowing beneficial collapse for electrical contact while preventing harmful uncontrolled migration that would cause short circuits. The wetting material mediates the collapse process to achieve desired outcomes without adverse effects.
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 reduces the risk of structural and electrical damages by confining reflowed solder balls to the attachment area, preventing unwanted bonding and migration, thus ensuring a more reliable microelectronic package assembly.
Implementation Method 1
depositing a wetting material and surface treating to create a wettability differential
Implementation Method 2
using techniques like photolithography, laser ablation, and oxidation to define these areas
Implementation Method 3
using techniques like photolithography, laser ablation, and oxidation to define these areas
Implementation Method 4
using techniques like photolithography, laser ablation, and oxidation to define these areas
Implementation Method 5
allowing controlled collapse of solder balls during reflow without a solder mask
Data Source
AI summary
Processes of assembling microelectronic packages with lead frames and/or other suitable substrates are described herein. In one embodiment, a method for fabricating a semiconductor assembly includes forming an attachment area and a non-attachment area on a lead finger of a lead frame. The attachment area is more wettable to the solder ball than the non-attachment area during reflow. The method also includes contacting a solder ball carried by a semiconductor die with the attachment area of the lead finger, reflowing the solder ball while the solder ball is in contact with the attachment area of the lead finger, and controllably collapsing the solder ball to establish an electrical connection between the semiconductor die and the lead finger of the lead frame.