Lead Frame Exposed Pad Trenches for Solder Void Reduction
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Solution Overview
Problem
Semiconductor packages with exposed pads experience high solder voiding levels during PCB attachment, affecting thermal and electrical performance, and existing solutions like U.S. Pat. No. 6,872,661 B1 are inefficient and unsuitable for power applications due to low thermal mass and complex production processes.
Innovation Solution
A lead frame based semiconductor package with a cross hatch trench or grid pattern on the exposed pad, where channels are machined into the lead frame to allow mold compound to fill and create non-wettable paths for solder, reducing voiding by enabling air outgassing and enhancing mechanical interlocking.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If patterns are put in solder paste deposit or die attached flag to combat solder voiding, then solder voiding is reduced, but the thermal mass in die attach area is low and the process becomes complicated
Solution Approach 1:
The exposed pad surface is segmented into multiple channels or trenches that divide the solder joint area into smaller regions. This segmentation allows trapped gases to escape through the channels during soldering, reducing void formation while maintaining a simple single-step molding process without requiring multiple etch back steps or separate die attached flags.
Solution Approach 2:
The invention adds a vertical dimension to the exposed pad by creating channels or trenches that extend through the pad thickness. This dimensional change provides pathways for gas escape during soldering, addressing the voiding problem without requiring complex planar patterns or multiple processing steps.
2Reliability
If etch back steps are performed after molding to create patterns, then solder voiding is reduced, but production efficiency decreases and special processes are required
Solution Approach 1:
The channels or trenches are formed in the exposed pad during the lead frame manufacturing process, before the molding step. This preliminary action eliminates the need for post-molding etch back steps, maintaining high production efficiency while providing the necessary pathways for solder void reduction.
Solution Approach 2:
The channel formation process is merged with the existing lead frame manufacturing process, combining multiple functions into a single integrated process. This eliminates the need for separate post-molding processing steps, maintaining productivity while achieving solder void reduction.
3Reliability
If epoxy material is used in die attached process to create patterns, then solder voiding is reduced, but the package cannot be used in power applications
Solution Approach 1:
The invention replaces the chemical approach (epoxy material) with a mechanical approach (physical channels or trenches in the pad structure). This substitution allows the solution to work with various die attach materials including those suitable for power applications, while still providing effective pathways for gas escape during soldering.
4Reliability
If multiple separate die attached pads are used, then solder voiding is reduced, but thermal mass in die attach area is low
Solution Approach 1:
Instead of using multiple separate pads, the invention segments the single exposed pad surface by creating channels or trenches within it. This maintains the continuous metal structure and thermal mass of a single pad while providing segmentation benefits for gas escape pathways during soldering.
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 solution minimizes solder voiding, ensuring optimized thermal and electrical performance, suitable for power applications, and simplifies the manufacturing process by integrating patterning with existing lead frame production, resulting in higher quality packages with reduced voiding and enhanced mechanical stability.
Implementation Method 1
The channels allow for continuous out gassing of entrapped air during the reflow process
Implementation Method 2
The mold compound enters into the channels during the process of application thereof and provides adequate mechanical interlocking between the lead frame and the molding compound
Implementation Method 3
The solder is unable to adhere to such epoxy materials and as a result a channel or path is formed in the solder joint
Data Source
AI summary
A method of manufacturing a semiconductor package, where the package includes a surface for attachment of the package to a device by a joint formed of a connective material in a joint area of the surface. The method is characterised in that it comprises the step of patterning one or more channels on the surface which channels extend away from the joint area towards an edge of the surface. Also the method has the step of applying a compound to one or more channels which compound interacts with the connective material, such that when the semiconductor package is attached to the device the interaction defines one or more paths in the connective material. These correspond to the one or more channels on the surface and allow the passage of waste material away from the joint area to the outer edge of the surface.