Organic Coated Lead Frame for Wire Bond Reliability
Solution Overview
Problem
The attachment of a semiconductor die to a lead frame using solder paste often results in flux and tin deposition on lead fingers, compromising the integrity of subsequent wire bonds due to tin cloud formation, leading to unreliable or low-peel strength bonds.
Innovation Solution
A method involving a coating of organic compounds on lead fingers, which prevents flux and tin from seeping through and allows for their removal during a de-coating process, ensuring clean surfaces for wire bonding.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If solder paste is used to attach the die to the die pad, then reliability and thermal conductivity are enhanced, but flux and tin are deposited on the lead fingers causing tin cloud formation that compromises wire bond integrity
Solution Approach 1:
A coating layer is applied to the lead fingers as an intermediary barrier between the solder paste/flux and the lead finger surface. This coating prevents direct contact and deposition of harmful substances while allowing the soldering process to proceed normally. The coating acts as a protective mediator that can be subsequently removed to reveal clean lead finger surfaces for wire bonding.
Solution Approach 2:
The coating is applied to the lead fingers before the die attach process. This preliminary action prepares the lead fingers by creating a protective barrier that will prevent contamination during the subsequent soldering operation. The coating is applied in advance, before the harmful deposition occurs, allowing for easy removal afterward.
2Reliability
If the lead fingers are coated to prevent flux and tin deposition, then wire bond integrity is improved, but an additional coating and de-coating process step is required
Solution Approach 1:
The coating material used on the lead fingers is designed to be inexpensive and easily removable. This disposable-like coating serves its protective function during the soldering process and then can be quickly and economically removed without significant cost or complexity. The coating is sacrificed to protect the lead finger surface, similar to a disposable protective layer.
Solution Approach 2:
The coating material undergoes a parameter change from a protective state during soldering to a removable state afterward. The coating is designed with specific chemical or physical properties that allow it to be easily altered or removed after serving its protective function. This parameter change enables the coating to transition from protecting the surface to being easily eliminated.
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 enhances the reliability of wire bonds by removing tin clouds and flux, thereby improving the overall package reliability and ensuring strong connections between wires and lead fingers.
Implementation Method 1
A method involves coating lead fingers with an organic compound coating that prevents flux and tin from seeping through and contacting the lead fingers
Implementation Method 2
removing the coating from the lead fingers, wherein removing said coating removes the tin and flux from the lead fingers
Data Source
AI summary
A lead frame having a coating of organic compounds on its lead fingers prevents tin and flux from contaminating the lead fingers after die attach. The coating is removed prior to wire bonding. The coating allows for reliable second bonds (bond between wires and lead fingers) to be formed, decreasing the likelihood of non-stick and improving wire peel strength.