Lead Frame Bottom Tie Bar Geometry for Crack-Resistant Packaging
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Solution Overview
Problem
Optoelectronic semiconductor devices face a high risk of cracks in the package body due to thermal stress during the soldering process, particularly at the corners of rectangularly shaped lead frame parts.
Innovation Solution
The design incorporates trapezoidal-shaped bottom tie bars that narrow towards the top side, reducing the risk of cracks by minimizing stress concentrations and requiring only minor adaptations to existing manufacturing processes, allowing for thinner package walls and improved handling.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If rectangularly shaped lead frame parts are used, then manufacturing is simple and handling is easy, but the risk of cracks in the package body increases due to thermal stress during soldering
Solution Approach 1:
The patent applies asymmetry by changing the lead frame part shape from rectangular to trapezoidal. The trapezoidal shape with inclined lateral sides eliminates the sharp corners present in rectangular designs, creating an asymmetric geometry that redistributes thermal stress more evenly during soldering processes. This structural asymmetry directly addresses the crack risk while maintaining manufacturability through standard fabrication processes.
Solution Approach 2:
The patent implements parameter changes by modifying the geometric parameters of the lead frame parts. Specifically, the lateral sides are designed with a defined inclination angle rather than being perpendicular to the base, and the top width is reduced relative to the base width. These parameter changes transform the stress distribution characteristics, reducing concentration at corners while preserving ease of manufacture through controlled geometric adjustments.
2Ease of operation
If the lead frame sheet is singulated between adjacent device units, then individual devices can be separated for assembly, but the bottom tie bars at border lines are affected and may increase crack risk
Solution Approach 1:
The trapezoidal shape of bottom tie bars creates an asymmetric stress distribution pattern that avoids the sharp corner configurations present in rectangular designs. When singulation occurs at border lines, the inclined sides of adjacent trapezoidal units meet in a manner that distributes stress continuously, preventing the stress concentration that would occur at the corners of rectangular tie bars subjected to the same singulation process.
Data Source
AI summary
In an embodiment a lead frame sheet includes a plurality of device units, each device unit configured for an optoelectronic semiconductor device and having at least two metallic lead frame parts configured for being applied to an electric component at a top side and configured for being soldered onto an external carrier at a bottom side opposite the top side, wherein each one of the lead frame parts includes at least one bottom tie bar connecting adjacent ones of the device units, wherein, at a border line between adjacent ones of the device units, the bottom tie bars form part of the bottom side, and wherein, towards the top side, the bottom tie bars narrow such that each bottom tie bar has a local minimum or a global minimum of a width at the top side and along the border line.