Establishing Flip Chip Coplanarity by Pressure Measurement
Successful flip chip assembly of large die with many bumps for applications such as image sensors depends upon the substrate and the die that is being placed upon it having parallel, coplanar surfaces when they are brought in contact. Any deviation from coplanarity can cause open or poor electrical connections and may even misalign the die by imparting a sideways sliding motion during placement pressure.
Two conventional approaches for establishing and checking coplanarity rely on optical or laser methods. An optical autocollimator projects an image onto the mirrored surface of the substrate, measuring the relative angle by the deviation of the reflected image from the projected one. Mechanical controls adjust the angle of the substrate until the images are superimposed at coplanarity.
Laser-based coplanarity adjustment depends upon a comparison of laser beam reflections from shiny targets at several locations on the substrate. Again, the substrate angle may be mechanically adjusted to achieve coplanarity.
Many flip chip bonders do not have either of these expensive add-ons. Establishing or verifying coplanarity then becomes a cumbersome trial-and-error procedure of making sample assemblies that can be removed and microscopically checked for coplanarity. This eliminates the expensive optical or laser equipment, but at the cost of considerable time consumed in repeated samples and adjustments. Verification that coplanarity is maintained through a production run means more samples
Sensor Products Inc. offers a pressure-sensitive film as a simpler, quicker, less costly method to establish the coplanarity of the two surfaces. The sensors are thin-film elements that measure contact pressure and distribution between any two mating surfaces. Placing pressure-sensitive film between the surfaces and bringing them into contact at the normal assembly pressure provides a permanent color-coded “footprint” of the pressure applied at every point.
The colors show the magnitude of the pressure. The uniformity of pressure across the contact area verifies coplanarity. The location and magnitude of any non-coplanarity is readily seen, for easy readjustment. Re-verifying during a production run simply means inserting another piece of film.
About George A. Riley
George A. Riley has more than 20 years experience in microelectronics packaging, including 10 years developing and manufacturing flip chip assemblies for more than 50 customers. He is an industry consultant, educator, and web entrepreneur who has authored more than 90 papers and articles. He can be reached at (508) 753-3572, by emailing firstname.lastname@example.org or by visiting his website www.flipchips.com