Lead Frame Edge Plating for Optical Inspection of Solder Joints
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Solution Overview
Problem
The formation of ill-formed toe fillets during the solder reflow process in no-leads semiconductor packages can lead to failed optical inspections, despite adequate bonding, due to the lesser affinity of solder on the exposed copper surfaces along the package sides, which are not plated and may be oxidized and rough from the singulation process.
Innovation Solution
A lead frame with pre-plated solderable surfaces and features on its edges, combined with film-assisted molding, prevents mold compound from filling these features and ensures a consistent, solderable surface for forming well-defined optically-inspectable solder features during reflow, enhancing the formation of toe fillets.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the contacts are plated with solderable metal on the active surface only, then the affinity for solder is enhanced on the bottom surface, but the exposed copper surfaces along the sides have lesser affinity for solder due to oxidation and roughness from singulation
Solution Approach 1:
The patent applies local quality by extending solderable plating specifically to the side surfaces of the contacts where toe fillets form, while maintaining other surface properties. This localized plating ensures consistent solder affinity exactly where needed for optical inspection, without affecting other contact surfaces.
Solution Approach 2:
The patent implements preliminary action by pre-plating the side surfaces of contacts with solderable metal before the singulation process. This pre-application of solderable coating protects the copper surfaces from oxidation and ensures consistent solder affinity is present before the contacts are exposed during package fabrication.
2Area of stationary object
If the package uses no leads extending from the perimeter, then space on the PCB is reduced and mounting density is increased, but automatic optical inspection becomes difficult due to lack of visible solder features
Solution Approach 1:
The patent uses color changes (or rather, surface property changes) by creating toe fillets with plated surfaces that have different optical characteristics. The solderable plating on the side contacts creates visible, optically-detectable toe fillet features that stand out during automatic optical inspection, solving the visibility problem of leadless packages.
Solution Approach 2:
The patent introduces an intermediary element - the toe fillet formed by solder on the plated side contact surfaces - that serves as a visible indicator of proper solder bonding. This intermediary feature allows optical inspection systems to detect adequate bonding without requiring traditional lead extensions.
3Device complexity
If the exposed copper surfaces along the package sides are left unplated, then manufacturing complexity is reduced, but the toe fillet formation is inconsistent leading to failed optical inspections
Solution Approach 1:
The patent applies local quality by selectively plating only the specific regions of contact surfaces that are exposed during singulation and will form toe fillets. This targeted approach adds minimal complexity compared to full contact plating, while ensuring consistent optical inspection results where needed.
Solution Approach 2:
The patent changes the surface parameter (solderability) of the exposed contact regions by applying solderable plating. This parameter change transforms the oxidized, low-solderability copper surfaces into high-solderability plated surfaces, ensuring consistent toe fillet formation and optical inspection pass rates.
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 approach ensures consistent formation of well-formed toe fillets, improving the reliability of automatic optical inspection and reducing the likelihood of misinterpretation of adequate solder bonds as failed, even when the package sides have irregular or oxidized surfaces.
Implementation Method 1
Surface tension of the solder to portions of the contacts that have an affinity for solder results in some of the solder extending beyond the edge of the mounted component along the PCB contact, and rising along the edge of the contact of the mounted component away from the PCB.
Data Source
AI summary
A mechanism is provided by which optically-inspectable features formed during surface mount bonding of no-leads packages are enhanced. Embodiments of the present invention use a lead frame having features that will lie upon the edges of the finished semiconductor device package, where molding material is prevented from lying in those features through the use of a preplaced film on the lead frame or film-assisted molding in conjunction with a mold chase that conforms to the features provided on the lead frame. Embodiments use a lead frame that has a pre-plated solderable surface, such that the exposed features enhance formation of the optically-inspectable features during solder reflow operations of PCB mounting.