QFN Leadframe Solder Printing for Wettable Sidewalls
Solution Overview
Problem
Quad-flat no-leads (QFN) packages face reliability issues due to non-wetting solderability caused by oxidation of exposed copper surfaces after singulation, leading to poor Board Level Reliability (BLR) performance.
Innovation Solution
Implementing low-cost and fast metallic printing processes to selectively print solderable material on risk-prone areas, ensuring all exposed copper surfaces are coated with solder, creating a wettable surface on both upper and side surfaces of the leadframe mounting pads.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If traditional leadframe encapsulation is used, then manufacturing simplicity is maintained, but exposed copper surfaces oxidize causing poor solderability and reduced reliability
Solution Approach 1:
Solder material is deposited on the leadframe outer ends before the encapsulation molding process. This preliminary action ensures that the copper surfaces are protected with a solder coating prior to encapsulation, preventing oxidation during subsequent handling and assembly operations while maintaining simple leadframe structures
Solution Approach 2:
The patent changes the material parameter of the leadframe outer ends by depositing solder material (tin, lead, or alloys) onto the copper surfaces. This material transformation prevents oxidation and improves solderability without fundamentally changing the leadframe structure or requiring complex additional processes
2Reliability
If post-encapsulation plating processes are implemented, then solderability is improved, but manufacturing cost and time increase
Solution Approach 1:
Instead of performing plating after encapsulation, the patent performs solder material deposition before encapsulation. This preliminary action eliminates the need for post-encapsulation plating processes, reducing manufacturing steps, cost, and time while ensuring the solder-coated surfaces are protected during molding
Solution Approach 2:
The patent extracts and eliminates the post-encapsulation plating process from the manufacturing flow by performing the solder deposition earlier in the process sequence, before encapsulation occurs. This removal of unnecessary post-processing steps improves productivity while maintaining BLR performance
3Ease of manufacture
If copper surfaces are left exposed after singulation, then manufacturing simplicity is maintained, but oxidation occurs reducing connection integrity
Solution Approach 1:
Solder material is deposited on the copper surfaces before singulation occurs. This preliminary coating protects the copper from oxidation during and after singulation while maintaining simple manufacturing processes without requiring additional post-singulation treatment steps
Solution Approach 2:
The solder material coating creates a protective barrier that effectively creates an inert environment around the copper surfaces, preventing oxygen access and oxidation. This protective layer maintains copper integrity without requiring actual inert atmosphere processing during manufacturing
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 enhances the integrity of solder joints by preventing copper oxidation, thereby improving BLR performance and eliminating the need for costly post-plating processes, ensuring reliable connections between QFN packages and PCBs.
Implementation Method 1
the exposed leadframes, which are the exposed edges of the leadframes used to connect singulated QFN packages to PCBs, typically have a non-wetting solderability issue due to oxidation of the exposed copper surfaces of the leadframe edges after package singulation
Data Source
AI summary
The disclosed principles provide for implementing low-cost and fast metallic printing processes into the QFN and other no-leads package assembly flow to selectively print solderable material in areas that would otherwise be susceptible to corrosion and thus pose reliability risks. The problem of copper corrosion and poor BLR performance in no-leads packages because of remaining exposed copper areas after package singulation is solved by employing selective metallic printing processes in the assembly flow to coat all risk-prone areas with solder material. For example, for no-leads packages that are formed using printed leadframes, solder can be deposited through inkjet, screen, stencil, or photonic printing into the grooves which are formed after passivating the packages at the strip level. The singulating occurs through the grooves having solder printed therein, and results in wettable upper and sidewall surfaces of the outer ends of the leadframe for each package.