Cohu Test Fixtures

Cohu Test Fixtures

Background:

Cohu is an industry leader in back-end computer chip testing. The chip, referred to as a device under testing (DUT), is picked up, transported, and plunged into a test socket by Cohu's pick & place handler, specifically by the socket loading kit (SLK). 

During testing, the DUT must be thermally controlled to a desired temperature set-point, and the DUT must be inserted into the socket with an accurate force to ensure proper electrical contact for the tests. Each DUT typically has thousands of individual contact points that contact individual spring loaded pogo pins that complete the circuit to the tester. Insufficient force leads to contact failures; however, too much force leads to socket damage.

Figure (1): Our project will specifically be compatible with Cohu's Eclipse XT Machine, which is shown above.

Objectives:

Currently, the force applied to a DUT is achieved by inputting a pressure through the system, and the force is calculated through theoretical analysis from the input pressure, using F = PA. The objective of this project is two-fold:

Figure (2): The SLK's motion is shown above.

By the end of this project:

The Universal Force Test Fixture must:

• • Fit all current (and future) SLK modules

  • Be confined to the testing area of the Eclipse XT

  • Be limited to the z-axis height of SLK heads

  • Be accurate to ± 0.1%

  • Not allow for bending or deflection

  • Be user-friendly for the client

The Mock DUT Fixture must:

• • Fit all current (and future) SLK modules

  • Have a 25 mm x 25 mm contact surface to interface with thermal heads

  • Mimic thermal loads the handler will experience during testing

  • Optimize time constant to desired temperature

By the end of this project, we will have delivered:

Before the COVID-19 outbreak, the deliverables were as follows:

• • Fabricated force test fixture and mock DUT fixture

  • Completed CAD models/drawings with little/no alterations needed

  • Completed Bill of Materials (BOM)

  • Instruction manual on how to operate each device

After the COVID-19 outbreak, the project deliverables were redefined to be:

• • Increased theoretical analysis in lieu of experimental testing

  • Completed CAD models/drawings with little/no alterations needed

  • Completed Bill of Materials (BOM)

  • Instruction manual on how to assemble and operate each device

Design:

Universal Force Measurement Fixture:

Our design solution consists of an attachment of a force measuring sensor to an individual SLK head. The SLK with its force sensor attached to it would then push against a plate. This design allows the user to measure the force applied to each individual head. Each SLK head has threaded holes that allows the user to attach a sensor to each respected SLK head. Ease of use for the user was highly prioritized due to the user possibly having to attach and detach the adapter multiple times (up to 16 times). This design includes the use of captive screws and alignment holes to allow the user to easily attach the adapter to each SLK head. The sensor chosen for this design is the S-beam load cell due to it's high accuracy (0.005% error).

Figure (3): Picture shown above is an individual SLK head and its threaded holes

Figure (4): Picture shown above is the SLK adapter with an S-beam load cell

Figure (5): Picture shown above is the X2 SLK with the adapter attached to it and the plate the S-beam load cell will push against

Mock DUT Fixture: 

Our design consists of a metal block that will contact the thermal head of the SLK head. Inside this metal block, there will be a heater that will heat the metal block, a temperature sensor to determine the temperature of the block, and a switch to prevent the device from overheating. Our secondary objective's design is similar to the first objective's design in that it will be attached to an individual SLK head using an adapter. The adapter will be made of an insulating material to prevent the heat from the metal reaching the metal casing of the SLK head as heat may damage that part of the SLK. The metal that was chosen for our block is copper. Aluminum was considered due to its low density, but after further consideration, copper was chosen due to its high thermal conductivity and heat capacity. A cartridge heater was chosen as the heater due to its high output wattage and its common application in heating plates. 

Figure (6): Picture shown above is the device under testing (DUT) with the X16 adapter attached

Summary of Performance Results:

Force Test Fixture:

Several analysis were done in place of experimental results due to the COVID-19 situation.

Figure (7): Displacement of X1 Force Fixture Adapter from maximum load of 500 pounds

Figure (7) shows the total deformation received on the aluminum X1 force fixture adapter from a 500 pound load. The displacement is minimal, roughly .003 mm. This displacement was more than satisfactory as the maximum displacement requirement is 0.1mm.

Figure (8): FEA simulation of worst case loading on the Force Receiving Plate

Using a base plate thickness of 19mm and 6 standoffs, a maximum displacement of 0.053 mm and a factor of safety of 3.31 was obtained.

Mock DUT Fixture:

The Mock DUT Fixture was also analyzed using FEA in ANSYS. The two main concerns with the mock DUT was that 1) it should heat up as fast as possible, and 2) holes must be properly sized to allow for thermal expansion.

Figure (9): Transient Thermal animation 

These results show that after 10 seconds, the mock DUT will be heated from 22℃ to nearly 150℃, resulting in a uniform temperature distribution. Initially, in early designs, calculations suggested that it would take nearly one minute to heat the copper mock DUT by this much. However, that was with a previous design iteration, where the cartridge heater was normal to the contact surface, and the mock DUT had much more mass. This design has made efficient use of space, and the results satisfy the functional requirements of the mock DUT. 

Figure (10): Equivalent stresses induced in the mock DUT due to thermal expansion

These results show that the max stress received on the mock DUT from thermal expansion is 93 MPa. With a tensile strength of 210 MPa, we have a factor of safety of 2.25 on the mock DUT from stresses resulting from thermal expansion. This is acceptable, and will significantly improve the number of cycles to failure! 

Link to Executive Summary

Link to Final Presentation delivered on MAE Senior Project Day (6/10/2020)