Stencil Aperture Geometry for Concave PCB Solder Joints
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Solution Overview
Problem
The reliability of solder joints in surface-mount technology, particularly in power management integrated circuits, is compromised due to miniaturization, leading to crack formation and reduced performance caused by thermal expansion and contraction, as well as oxide formations and the production of brittle intermetallic compounds in convex solder joints.
Innovation Solution
A solder paste stencil with apertures designed to create a non-convex solder joint by controlling the curvature of the solder fillet through specific stencil aperture dimensions and solder paste application, ensuring the solder paste contacts only the surface of the terminal facing the PCB pad, thereby minimizing stress concentration and promoting a concave solder joint.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If a convex solder joint is formed using conventional stencil aperture design, then the solder joint provides initial mechanical strength and electrical connection, but it creates excessive stress concentration at the interface during thermal cycles leading to crack formation and propagation
Solution Approach 1:
The patent inverts the conventional convex solder joint geometry to create a concave solder joint. Instead of the solder forming a protruding convex shape that concentrates stress at the interface, the stencil aperture is designed to produce a concave geometry where the solder recedes from the terminal interface. This inversion fundamentally changes the stress distribution, moving the maximum stress away from the vulnerable terminal-solder interface and into the bulk solder material, thereby preventing crack initiation and propagation during thermal cycling.
Solution Approach 2:
The patent applies parameter changes by modifying the stencil aperture dimensions and geometry to control the solder joint shape. Specific aperture parameters (such as opening size, shape, and positioning relative to the terminal and pad) are optimized to produce the desired concave solder joint profile. This parameter optimization ensures that the solder fillet forms with the correct curvature to achieve low-stress concentration at the interface while maintaining adequate mechanical strength and electrical connection.
2Reliability
If the stencil aperture is designed to create a concave solder joint, then stress concentration is minimized and crack propagation is reduced, but the manufacturing precision of the stencil aperture dimensions must be tightly controlled
Solution Approach 1:
The patent establishes specific parameter ranges for the stencil aperture dimensions that balance reliability improvement with manufacturing feasibility. By defining optimal aperture sizes, shapes, and tolerances, the patent enables consistent production of concave solder joints without requiring extreme manufacturing precision. The parameter optimization includes specifying aperture opening dimensions, aspect ratios, and positional relationships to the terminal and pad, allowing standard manufacturing processes to achieve the desired results.
Solution Approach 2:
The patent employs partial action by designing the stencil aperture to deposit solder paste with specific volume and distribution characteristics that are sufficient to create the concave joint geometry without requiring excessive precision. The aperture is designed to deliver just the right amount of solder material to form the desired profile, accommodating normal manufacturing variations while still achieving the reliability benefits of the concave geometry.
3Shape
If solder paste is applied to control the curvature of the solder fillet, then the concave joint geometry is achieved, but the solder paste application process must be precisely controlled
Solution Approach 1:
The patent implements preliminary action by pre-configuring the stencil aperture geometry before solder paste application. The aperture is designed in advance with specific dimensions and shapes that will produce the desired concave solder joint curvature when solder paste is deposited through it. This preliminary design of the aperture ensures that the solder paste application process does not require high precision control, as the aperture itself guides the formation of the correct joint geometry.
Solution Approach 2:
The patent optimizes solder paste application parameters (such as paste viscosity, deposit thickness, and screen pressure) in conjunction with the stencil aperture design to achieve consistent concave joint formation. By adjusting these parameters within defined ranges, the patent enables reliable production of the desired solder fillet curvature without requiring extremely precise control of the application process.
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 solution significantly reduces stress on the solder joint interface, minimizing crack propagation and enhancing the performance and reliability of QFN packages by forming a lower-stress concave solder joint that withstands thermal cycles without disrupting electrical functionality.
Implementation Method 1
During a reflow process, the solder is heated to form a bond between the QFN terminal 104 and the PCB pad 106
Implementation Method 2
During repeated thermal cycles, the crack 101 of the crack formation image 100 of FIG. 1 is formed due to thermal expansion and contraction
Data Source
AI summary
Micro solder joint and stencil design. In one embodiment, a stencil for depositing solder on a printed circuit board (PCB) includes a plurality of stencil apertures, a first stencil aperture of the plurality of apertures having an aperture wall defining an aperture perimeter. The aperture wall is configured to not extend beyond an outer edge of a PCB pad provided on the printed circuit board, the aperture wall is also configured to not extend beyond an outer edge of a terminal of a surface mount component, and the first stencil aperture is configured to receive solder paste to form a non-convex solder joint between the PCB pad and the terminal.