Flip-chip devices underfilling

MIL-STD-883 Method 5011 coordinated and released Thermoplastic film adhesives Silver-glass die-attach adhesives introduced First underfills for flip-chip devices... 

Lastly, adhesives are used to dissipate stresses that may be generated from thermal excursions, mechanical shock, vibration, or moisture. Specially formulated adhesives are effectively used as underfills for flip-chip devices and ball-grid-array packages to compensate for mismatches of expansion coefficients among the solder, the silicon chip, and the ceramic or plastic-laminate substrate. Low-stress adhesives are also used to attach fragile devices such glass diodes and to dampen stresses due to vibration. 

Adhesives used to fill spaces underneath a flip-chip device or BGA package must flow readily and rapidly when dispensed at the edges of the chip in order to completely fill the gap and free space surrounding the solder bumps. To be effective, the adhesive must be drawn into the narrow space by capillary action. The theory behind underfilling is based on capillary-flow behavior between closely spaced parallel plates (Fig. 2.5) by considering the drop in pressure (p) across a liquid-vapor interface ... 

Rework of underfill flip-chip devices and ball-grid-array packages 212 References 212... 

In a similar process, known as polymer-film interconnect (PFI), an insulative thermoplastic film is laminated over the devices at the wafer stage, and vias are opened over the bonding pads using a laser. At that point, either the normal solder bumps can be formed or a silver-filled conductive adhesive can be stencil printed into the vias to form polymer bumps. After printing, the epoxy is B-staged and the flip-chip devices are diced. In assembly, the devices are heated to a temperature that completes the cure of the B-staged bumps and simultaneously reflows the thermoplastic underfill material. 

Figure 6.3 Reliability of flip-chip devices with and without underfill.

The first specifications for adhesives were generated by the staff of NASA and the DoD who were prompted by the high reliability that was required of microcircuits used in aerospace programs. These specifications covered primarily die and substrate attachments for hermetically sealed integrated circuits, hybrid microcircuits, and multichip modules. Subsequently, with the increased use of surface-mount adhesives in the assembly of commercial printed-wiring boards and underfills for flip-chip devices, industry associations took the lead in generating the requirements and test methods. 

Underfill for flip-chip-attached devices. With the availability and increased use of flip-chip devices, a need has arisen to reinforce the connections and dissipate stresses resulting from mismatches in expansion coefficients among the solder, the substrate, and the device. This need has been met by underfilling the space between the solder connections with specially formulated underfill adhesives (Fig. 1.5). Underfill adhesives are... 

Rework of Underfill Flip-Chip Devices and Ball-Grid Array Packages... 

Reworking underfilled flip-chip devices or BGA parts also presents new challenges since both solder and adhesive are present and both must be removed. New underfill formulations have had to be developed that soften, but do not char at the temperatures required to liquefy the solder. Some equipment manufacturers have modified their flip-chip bonders to include a rework station, In general, the chip undergoing rework is spot heated to... 

Figure 5.22 shows a specimen process flow for flip-chip gluing with ICA. Conductive adhesive is stenciled on to the substrates. The devices are then placed on the chip, and the conductive adhesive is thermally cured. As in flip-chip soldering, underfill Is applied as mechanical protection and cured. 

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