What are the advantages for this flip chip technology?
Polymer flip chip assembly has several unique advantages: • Low Cost — Materials & Stencil Printing Technology Used for Polymer Flip Chip Assembly Are Inherently Low In Cost – Very Little Material Is Consumed or Lost In the Process & Most Of the Unused Silver Is Reclaimable • Low Temperature — Process Temperatures Are Typically Held Between 70-150°C – This Enables Heat-Sensitive Devices & Substrates To Be • Flip Chip Bonded That Would Otherwise be Damaged Using Solder-Reflow • Clean Environment — The Technology Is Clean From Both a Process & Environmental Standpoint – There Is No Flux or Post Cleaning Solvents Required, Therefore, This Assembly Process Is Suitable for a Clean Room Environment – Since the Epoxy Contains No Lead, the Materials Are Safe for the Environment • Fine Pitch — The Small Particle Size & the Thixotropic Rheology Of the Conductive Epoxies Enables Very Small Features To Be Stencil Printed – Epoxy Bumps Can Be Printed As Small As 50µm In Pitch • Flexible Connection — Conductive Polymers Used for This Technology Have a Low-To-Moderate Modulus Of Elasticity, Which Enables the Material To Respond Well To Thermally Induced Stress – the Lower Processing Temperature Also Reduces the Effects Of CTE (Coefficient Of Thermal Expansion) Mismatch Between the Device & Substrate
How does this technology work?
At its simplest, conductive polymer bumps are stencil printed onto individual chips or substrates before the parts are aligned and flip chip bonded. The polymer bumps are subsequently oven cured at low temperature. In some applications, the assembled components are under-filled with a non-conductive epoxy for added mechanical integrity and moisture resistance. Since the epoxy may develop electrical shorts between fine-pitch contacts if too much pressure is applied during placement, rigid stand-off bumps may be used to fix the correct spacing between the surfaces being joined.
Can the parts be disassembled and repaired after curing?
Generally, no. Once the under-fill epoxy has cured, the parts are firmly attached and cannot be easily separated without damaging either the chip or substrate. If no under-fill epoxy is present (depending on the number of bumps present), it may be feasible to remove the chip with a combination of spot heating and shear force.
Which metal finishes are the conductive epoxies compatible with?
Conductive epoxies work best when applied on noble metal surfaces, such as gold, silver, platinum, and palladium. Direct contact on copper or nickel is acceptable, as long as no organic surface protective (OSP) coating is present. Most silver-filled conductive epoxies are not compatible with either aluminum or tin-lead coated surfaces. Although the initial contact resistance is low after curing, it will increase significantly with time. Aluminum metal pads on ICs can either be gold-stud bumped or electroless nickel-gold plated to provide a compatible metal surface.
Can this technology be mixed with other surface-mount soldered components?
The traditional polymer flip chip technique works only if no pre-assembled components are present to interfere with the stencil printing process. Ideally, all polymer flip chip components should be assembled and cured before any surface mount soldered devices are placed. Conductive epoxy bumps will survive several solder-reflow cycles. An alternative polymer flip chip assembly technique, known as the NCP (Non-Conductive Paste) process, enables SMT components to be placed and soldered first. This technique uses pre-cured epoxy bumps on the IC together with a pre-applied under-fill to thermo-compression bond the IC to the circuit pads without the need to stencil print conductive epoxy on the circuit board.