The Capillary Underfill process consists of dispensing a void-free fluid to encapsulate the bottom side of a silicon die or BGA device. Encapsulation covers the top surface, typically where fragile interconnects are located, but in the case of Capillary Underfill, the fragile interconnects are located on the underside of the component. Underfilling enhances the connection strength of electrical contacts and compensates for differences in thermal expansion rates of the two joining materials that could lead to product failure. Underfill is typically used for applications in high shock environments or when consistent reliability is required.
The Underfill process requires substrates to be at an elevated temperature for proper processing. These elevated temperatures reduce fluid viscosity. Good dispensing is done with pumps with a positive shutoff mechanism for controlled dispensing. Our NCM5000 Jetting Pump and our innovative Progress Cavity Displacement (PCD) are two of these pumps. A jetting pump has the advantage of dispensing small droplets from up to 4 mm above the surface while maintaining a small wetout area. This benefits small devices and devices with passives in close proximity. The PCD is a continuously volumetric pump that does not vary in volume even when viscosity and temperature change. It dispenses Underfill for larger devices quickly and with excellent precision and without the need for continued calibration.
The basic steps of an underfill process are:
- Pre-Heating from ambient temperature to 80 degrees C.
- Vision alignment of the device.
- 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 device size or underfill material selection.
- Post-Heating - product dependent.
Inline systems are recommended for Underfill due to their ability to use pre- and post-heaters. Pre-heating is used to quickly heat 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 fixture 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. The pre-heat area has an optional sensor to monitor product temperature. If the substrate reaches the operating temperature, heat is removed and the substrate is transported to the work area. The work area also has an optional TempView Imaging System to monitor all areas to be underfilled. Post-heating is used after the dispense process to continue the flowing process. Depending on the application, the post-heater may be used to continue flow for larger devices before going to final cure.
Choosing an appropriate Precision Dispensing System and pump for underfill process is vital. We recommend the MAX II or DS Series for heated underfill applications. The systems are equipped with 1, 2, or 3 heated zones for the substrate depending on machine configuration and application. Heating the substrate, up to 100 degrees C, is done via custom contact vacuum fixturing or via non-contact 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.
The recommended pumps for underfill applications are the Volumetric PCD Pump Series and NCM5000 Jetting Pump. Progressive Cavity Displacement (PCD) dispense technology excels at dispensing underfill on components 10 mm and larger because of its ability to dispense fluids at a controlled, high flow rate. Coupled with GPD Global's standard S Type taper tip nozzles, controlled flow rates are unmatched. For devices smaller than 10 mm or have very tight keep-out areas, we recommend NCM5000 Jetting Pump. NCM5000 Jetting pump is fast for small droplets, it minimizes wet out area and optimizes dispense rate for small volume.