- Pre-Heating from ambient temperature to 80 degrees C
- Vision alignment of the die to be underfilled
- Locate surface to dispense (z axis)
- Dispense the fill pass – Multiple passes may be required
- Dispense the fillet pass – May not be required depending on chip size or underfill material selection
- Post-Heating – Product dependent
Capillary Underfill Process
Overview
http://www.gpd-global.com/text/apps/underfill.html
The Capillary Underfill process is used to encapsulate the bottom side of a silicon die. Encapsulation is typically thought to cover the top surface, typically where fragile interconnects are. But in this case, the fragile interconnects are on the bottom side of the die. The die to be underfilled are termed flip chip. The reason for underfilling is to compensate for differences in thermal expansion rates of the two joining materials: solder bumps and FR4 or ceramic. Without this underfill or bonding material, the life expectancy of a flip chip would not be very long as cracking would be inherent. The underfill also adds rigidity to the product; that is, giving it the ability to survive high impact shock without fracturing or separating completely from the substrate.
A successful underfill process results in complete encapsulation of the bottom side of the die with an even meniscus on all 4 sides, void of air entrapment. If air is trapped it could cause a defect during expansion due to high operating temperatures.
Recommended Equipment^
MAX II Series
GPD Global recommends the MAX II Series or DS Micro Series for heated underfill applications. The systems are equipped with either 1, 2, or 3 heated zones for the substrate depending on the machine configuration and application. Heating of the substrate, up to 100 degrees C, is done via custom contact vacuum fixturing or non-contact via forced air. Other available system features are Substrate Temperature Monitoring in the Pre-Heat, Closed-Loop Process Calibration, Full View Illumination, fully automatic nozzle calibration and true nozzle cleaning.
DS Micro II
For optimal control of underfill, an LX Auger Valve equipped with WebFlow yields high through puts, excellent material control and dispense heights of up to 2 mm (0.080″). LX Auger Valves equipped with WebFlow yield +/-3% repeatability or better. WebFlow is easily maintained and replaced through a tool-less design. No special calibration, alignment, or set up is required.
Process
The basic steps to an underfill process are:
Inline systems are recommended for underfill due to the ability to use pre- and post-heaters. Pre-heating is used to quickly ramp a product from ambient temperature to process temperature. Different methods of pre-heating are available. The most common method is a custom contact vacuum fixture. This offers the quickest and most uniform heat transfer. A universal heating method utilizes forced air. Forced air allows any substrate to be heated no matter the geometry or size, but time to heat is longer vs. contact heating. To ensure that the substrates are not overheated, a temperature monitoring system has been employed. If the substrate reaches the operating temperature, heat is removed and the substrate is transported to the workarea. Post-heating is used after the dispense process to continue the flowing process. Depending on the application, the post-heater may be used to gel an underfill to hold it in place before the next step. …more (pdf)
