24 October 2001Manufacturing / Production Technology, Hardware & Services
Information from Zetech
Flip chip technology is providing PCB designers with a myriad of options for increasing processing speed while reducing substrate area. Whether assembled into BGA packages or directly onto the substrate as a DAC (direct chip attach), flip chip technology is increasing the demands on electronics manufacturers for better control of material deposition in the flip chip, underfill process.
Figure 1. A variety of dispense fill and fillet patterns can be used to facilitate material flow under the chip
Shock, moisture and connectivity are key considerations in creating a reliable flip chip assembly. A proper underfill stabilises the chip's connection to the substrate and reduces the number of boards and packages requiring rework.
First, material is dispensed along one or more sides of the chip. The material typically is pre-heated to 40-60°C via a syringe heater to reduce material viscosity. To dispense the material, the needle is positioned 2-5 mils from the chip and 6-8 mils from the substrate. This allows the material to flow under the chip to prevent ploughing material onto the top of the chip or build up around the sides of the needle.
Figure 2. The needle is positioned 2–5 mils from the chip and 6–8 mils from the substrate to prevent dispense material from ploughing onto the top of the chip or build up around the sides of the needle
During the underfill phase, complete and precise coverage under the chip is crucial. If too much material is dispensed, the operator will flood the die in a BGA package or run the underfill material into other chips on a high-density assembly. Too little material results in uneven coverage and a less stable package. In this phase, the substrate is heated to 70-100°C to enhance capillary action under the chip and to assist material in releasing from the tip of the needle. Heat is applied through a heated vacuum fixture.
Finally, a fillet can be dispensed along one or more sides of the chip, sealing the underfill material and further protecting the chip. Again, the needle must be positioned precisely on the X, Y and Z-height coordinates to ensure accurate deposition. The package or assembly is then sent through final convection heating at 70-100°C to stabilise the material prior to batch curing.
GPD manufactures the DS series of liquid dispensing equipment for micro volume flip chip underfill applications. The systems offer less than 1 mil location accuracy in the dispense area using a traceable glass plate procedure. Pre-dispense, dispense and post-dispense heating is controlled within ±3°C.
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