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Information about Cohu and Our Sponsor:
Information about Cohu and Our Sponsor:
Cohu is a global leader in semiconductor manufacturing. They do test and handle equipment, thermal subsystems, test contacting, vision inspection, MEMS test solution, and print circuit board. They mainly focus on increase the efficiency and reduce the cost of the products.
Sponsor: Ernest Blanco.
The Original Design and Project Background:
The Original Design and Project Background:
Figure 1: Top-Down View of DUT and Bottom-Up View of Heater Assembly
Figure 2: Cross-Sectional View of Pick and Place Fixture and DUT
The existing suction cup solution limits the contact area of the thermal control unit. Cohu’s existing solution utilizes four vacuum cups (see Figure 1) that contact the corners of the chip in order to create suction for the pickup. The DUT (device under test) then comes into contact with the thermal head (see Figure 2), which brings the DUT to the required temperature for testing. Because the vacuum cups in this case take up a significant portion of the testable area of the DUT, complete thermal dissipation through the chip requires more time. Because these test handlers must do millions of test cycles as quickly as possible, minimizing the time to test each chip is crucial. We are working with Cohu on their pick and place mechanism. Right now their picker uses vacuum cups that restrict the inside area of their heating mechanism, they want us to come up with a pickup mechanism that extends the contact area of their heating mechanism.
Objective:
Objective:
Pick up the device under test using vacuum, transports and plunge it into testing socket.
Heat or cool semiconductor chip from -55 degrees C to 175 degrees C.
Test multiple sealing mechanisms that move the seal to the periphery of the semiconductor chip.
Test mechanism that simulates the pick up and heating of the system.
Automation
Operate for millions of cycles.
Figure 3: Design of Test Fixture
Figure 4: Section View of Fixture
Design Principle:
Design Principle:
To improve Cohu's existing 4 cups design with the peripheral seal design.
Figure 3 and Figure 4 are our preliminary test fixture design. It has a chip bed and uses a vacuum pump to create negative pressure for pickup. The Figure 3 and Figure 4 are only showing CAD version of main body of the fixture.
Results:
Results:
We were able to complete the fabrication and assembly of our fixture with all systems functioning. The actuating system successfully maintains its desired positions over many cycles using end-stop switches for homing and a stepper motor for precise position control. The pressure system switches between positive and negative pressure as desired when attempting to pick up or replace the DUT. The heating system brings the heat sink to the desired temperature and uses P&ID control to maintain a precise temperature.
While the system works as designed, some changes were necessary during the assembly process to bypass manufacturing challenges:
The soldering between the bellow and the seal adapter successfully mated the two parts together, but were not able to hold a vacuum, so the connection between the parts was also superglued. This will likely limit the working temperature of the fixture, so we do not recommend running the fixture at high temperatures.
The screws mating the shell and the heat sink were designed to use sealing washers in order to maintain an airtight seal, but we ended up using spare gasket material from the adapter gasket.
The vacuum inlet was designed to be J-B welded in order to ensure a seal, but we also used spare gasket material for this junction.
Due to manufacturing missteps in both the shell and the heatsink, we were unable to fit a gasket as originally intended to penetrate the heating cartridge wires through the shell. Instead, a combination of electrical tape and superglue was used to create the seal. This seal is imperfect and at times will prevent the fixture from successfully picking up the DUT.
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