How can I reduce assembly costs ?

Gold Bump Flip Chips

Flip-chip technology is growing at a rate of about 27% per year. The driving force is cheaper, smaller, lower cost substrates. Flip-chip removes the cost of packaging the die, reduces substrate area and assembly costs, whilst at the same time improving thermal management and electrical performance. Mintech offer both traditional stud bumps and our world leading Micro-post (µPost) technology. Stud bumps are applied where a standard flat bump profile is required and micro-post where taller, more-controlled profiles are needed - both technologies being available in either singulated die or wafer form. Recent advances in flip-chip-bonding adhesive technology have simplified processing, reducing both assembly time and temperature. These adhesives are Pb-Free and most cure at around 150～C, which is substantially lower than the temperature required for standard solders. ACA and NCA adhesives are self under-filling and do not crack due to temperature coefficient mismatch.

ACA Adhesive with conductive metal particles approx 8µm diameter

With heat and pressure particles are compressed forming a conductive path

ACA (Anisotropic Conductive Adhesives) are available in paste or film (ACF) form and have conductive balls suspended within the material. The adhesive holds the flip-chip to the substrate and the electrical conductive path occurs in the Z-axis when the opposing conductors are forced together. Data sheets on cure times and temperatures are readily available from a range of suppliers.

NCA Adhesive applied

With heat adhesive shrinks pulling the conductive pads together

NCA (Non-Conductive Adhesives) can be used where the electrical conductive path is made through gold posts onto gold plated pads on the substrate. The adhesive shrinks holding the flip-chip and at the same time compressing the opposing connections together.

Assembly

Both methods are already used in volume production. Using flip-chips and adhesives, substrates can be smaller - the die requiring less space than in its package form. Attaching the chip active face down and making the electrical connections in one operation, removes a number of operations. For conventional Chip on Board, three serial process stages are required - attaching the chip and curing, wire bonding, and glob topping and curing. With flip-chip and adhesive these operations are performed in one single process operation. The adhesives can either be applied in film form, or as a liquid, dispensed locally or over the whole substrate. The process method is to apply heat and pressure - typical curing times being between 6 to 20 seconds, at temperatures between 100～C and 200～C. Assembly cost savings up to 80% have been achieved.