Understanding Microelectronics Packaging Design and Manufacturing Processes
Introduction
Microelectronics packaging are meticulously designed to serve specific functions at the subsystem and customer levels. These packages undergo various processes, such as die attach, wire bonding, sealing, and multiple tests, including environmental, electrical, and mechanical tests, to become functional device housings. This article delves into these processes, discussing their intricacies, impacts on package design, and manufacturing.
Die Attach
Die attach is a critical process that involves attaching dice to the package. Two primary methods are prevalent:
Eutectic Die Attach Process
In this method, dice are attached to a gold surface within the package, termed the die attach area. The attachment typically involves ultrasonic vibration of the die, creating a scrubbing motion for optimal wetting of surfaces. This wetting is crucial for eliminating bonding voids and ensuring a thermal path between the package and die contact. Eutectic die attach can occur through a metal preform or in situ eutectic formation. The process necessitates a forming atmosphere of hydrogen to achieve a flawless fillet and braze flow. Critical parameters include scrubbing energy, time, and temperature control (420-430°C). Clean surfaces devoid of organics are essential to prevent dewetting. Gold thickness should be a minimum of 75 microinches, depending on braze conditions. The reliability of this process hinges on void-free attachments to avoid hot spots in the die, which can degrade chip performance.
Wire Bonding Process
The wire bonding process connects the device to the I/O terminals of the package using gold or aluminum as connecting media. Wire diameters range from 0.5-5 mils for gold and 0.7-20 mils for aluminum, while ribbon bonding is utilized for microwave subsystems with specific thicknesses and widths. Various bonding mechanisms are employed:
- Ultrasonic Bonding: Utilizes sonic vibrations for scrubbing the wire onto the bond site.
- Thermosonic Bonding: Similar to ultrasonic bonding but involves heat application to increase bond strength.
- Thermocompression Bonding: Employs a one-time force to compress the wire into the bond site.
In all metallurgical bonding processes, intimate contact between the surfaces being bonded is crucial for reliable bonding. Contaminant-free surfaces are essential to ensure bond integrity.
Sealing
After mounting and connecting devices, packages require sealing to protect the enclosed components. Two primary sealing types exist:
- Non-Hermetic Seal: Involves lid/package adhesive sealing with typical hermeticity values greater than 10^-6 cc/sec atm.
- Hermetic Seal: Achieves hermeticity values of 10^-3 cc/sec atm or better through processes like solder seal, seam seal, or laser sealing. Leak tests, either gross or fine, validate hermeticity:
-Helium Detection: Pressurizes a lidded package with helium and detects leakage rates proportional to hermeticity.
-Radioactive Krypton Gas Detection: Similar to helium detection but uses radioactive krypton gas, with a counter measuring leak rates and hermeticity.
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