Lead Frame Terminal Solder Thickness Ensuring Portion
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Solution Overview
Problem
Semiconductor devices with mold resins of low linear expansion coefficients experience stress and solder cracks due to thermal expansion mismatches, and existing solutions either increase package size or require precise alignment, leading to cost and reliability issues.
Innovation Solution
A lead frame design with a solder thickness ensuring portion on the terminal portion, formed by stamping, which provides a thinner outer region facing the conductive pattern, reducing stress concentration and preventing solder cracks while maintaining alignment accuracy and device size.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If a mold resin with a low coefficient of linear expansion is used to reduce package warping, then the degree of package warping due to temperature change is reduced, but the difference between the coefficients of linear expansion of the mold resin and the leads is increased, resulting in significant stress to the soldering joint
Solution Approach 1:
The patent applies local quality by creating a solder thickness ensuring portion with a depressed facing surface in the outer region of the terminal portion, while maintaining a flat back surface. This localized structural modification allows the solder joint to accommodate thermal expansion stress without compromising overall package stability, resolving the contradiction between low warping and stress reduction.
2Reliability
If the outer region of the terminal portion is made thinner to reduce stress concentration, then solder crack occurrence is reduced, but the structural strength of the lead may be compromised
Solution Approach 1:
The invention implements local quality by thinning only the outer region of the terminal portion's facing surface while keeping the back surface flat and maintaining adequate thickness in the center region. This localized thinning reduces stress concentration at the solder joint interface without compromising the overall structural strength of the lead, thereby improving reliability without sacrificing strength.
3Reliability
If a projection is added to the lead terminal portion to improve soldering joint reliability, then the reliability of soldering joint is improved, but the terminal portion area is increased, resulting in larger package size
Solution Approach 1:
Instead of adding a projection to improve soldering joint reliability as in conventional approaches, this patent inverts the approach by creating a depressed facing surface in the outer region of the terminal portion. This inverted structure reduces stress concentration and improves reliability without increasing the terminal portion area, thereby avoiding package size increase.
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 lead frame's solder thickness ensuring portion effectively reduces solder crack occurrence due to thermal expansion, maintaining device reliability and size without increasing costs or alignment complexity.
Implementation Method 1
the occurrence of crack in a soldering joint between a terminal portion of a lead and a wiring board due to a temperature change can be reduced
Data Source
Figure 1
Figure 2(a)~2(c)
Figure 3A~3A(2)
AI summary
The occurrence of crack in a soldering joint between a terminal portion of a lead and a wiring board due to a temperature change is reduced even with the use of a mold resin with a low coefficient of linear expansion. A lead frame 1 according to one embodiment includes a lead part 2 including an inner lead 3 and an outer lead 4 connected to the inner lead 3, and a frame unit 5 supporting the lead part 2. The inner lead 3 has a terminal portion 3a having a facing surface 3b and a back surface 3c in the opposite side from the facing surface 3b. The facing surface 3b faces a conductive pattern of a wiring board. An outer region of the terminal portion 3a is provided with a solder thickness ensuring portion 6 where the facing surface 3b is depressed toward the back surface 3c. The solder thickness ensuring portion 6 is thinner than a center region of the facing surface 3b. A center region of the back surface 3c is flat without a depression.