Flip Chip On Flex Interconnect Guidelines

Figure 1: Close-up of flip chip on flex substrate.  Precise die-to-substrate alignment, exacting solder reflow, and complete adhesive underfill are critical for thermal reliability in FCOF assembly.

Interconnect Materials
Three basic interconnect schemes are currently in vogue.  Each scheme has significant material issues, and all require precise placement and handling.

With controlled collapse chip connection (C4), pad spacing is limited to 0.01-in. or 0.012-in. centers, with the pads typically in an array format.  The 95-percent lead/5-percent tin solder bumps do not melt during the attachment process.  Solder paste screened or stencil printed onto the flex substrate wets to the bump during reflow, which occurs at 308°C.  Upon reflow, the solder bumps on the flip chip and the land pads on the substrate self-align, allowing some degree of misalignment during placement.  A drawback to C4 is the limitation of solder printing preventing the attachment of integrated circuits (ICs) with pads on 0.004-in. to 0.008-in. centers.

If the polymer flip chip attach method is used, the contacts may be closer because the epoxy can be stencil printed to finer pitches.  For example, epoxy bumps of 0.002-in. diameter have been produced on 0.007-in. centers and on 0.004-in. centers under ideal conditions.  However, achieving finer pitches involves a tradeoff because epoxy does not metallurgically wet to the chip’s pads or the substrate’s lands.  As a result, no self-alignment occurs during the reflow process; therefore, placement must be exact.

A solder bump and reflow attachment process recently developed circumvents the drawbacks to the above two methods.  This process negates the need  for stencil printing and provides the wetting/reflow action that results in the bumps and pads self – aligning.  This new method can flip chip attach IC’s to perimeter pads as small as 0.002-in. diameter on centerlines down to 0.004 on.

This approach uses plated 60-percent tin/40-percent lead eutectic solder bumps on the IC and gold-plated lands on the flex.  The gold-plated lands are compatible with polymer flip chip where silver-filled epoxy is stencil printed onto the gold land areas.

Regardless of approach, temporary or permanent stiffening must be employed to ensure that the flex remains flat during attachment and reflow; otherwise, yields will be low.  After reflow and curing of the underfill epoxy, the flip chip site is very robust, and the stiffening may be discarded.

Pick and Place
Proper selection of pick-and-place equipment is key to developing an FCOF assembly process and achieving targeted yields in production.  The pick-and-place system should offer high placement accuracy; versatility in independently controlling key parameters such as time, temperature and pressure over wide ranges; and easy repeatability or modification through stored program control.

Semiautomatic flip chip bonders can play an important role during prototyping, production startup and volume-production runs.  Regarding prototyping, the semiautomatic machine’s greatest asset is the versatility it provides in changing form one setup to another.  Pertaining to volume production, four semiautomatic machines may run four different product designs for the same cost of running one fully automatic placement machine dedicated to a single product design.  However, in high-volume situations where placement at maximum speed is needed, automatic machines are more suitable.

In choosing a machine to place flip chips on flex, look beyond equipment capable of placing packaged IC’s with external leads on 0.015-in. centers onto PCBs with good yields and throughput.  Typically, these machines have x-y placement accuracy ratings of 0.002 in. and theta rotational control of 0.2°.  Figure 2 illustrates the results of accumulating the above placement accuracy and theta rotational accuracy tolerances when working with the 0.003-in. square bump and 0.004-in. pad pitch typical with FCOF.  In Figure 2, Bump A will short to the adjacent land.

Figure 2: Effect of accumulated tolerances in placement for x, y, and theta.

The vision system on the pick-and-place system is a critical element for achieving proper alignment during the placement process.  In an ideal system, the flip chip is held in a vacuum collet while the flex substrate is placed on the traversing workstation and then moved into approximate position under the chip.  The system should have a cube beam splitter-viewing system with 20 to 200X magnification that automatically extends out to a position between the pick up head and substrate.  The cube beam splitter should present a simultaneous view of the bumps on the flip chip and the land pads on the flex substrate (Figure 3).

Figure 3: View of flip chip bumps on flex land pads.

Adequate lighting of both the flip chip bumps and substrate bond pads is also crucial to rapid, precise alignment.  A system should have separate, adjustable illuminators – one for chip bumps and one for bond pads.  Fine movements of the workstage, precise incremental movements of the viewer and a wide range of zooming with the camera lens are also crucial qualities.

Instead of the operator viewing the superimposed alignment of the bumps and pads, the alignment regiment may require the operator to use fiducial recognition.  In this case, the vision system looks at two or more registration marks that have been added along the periphery of the substrate’s artwork.  Note that many flex circuits do not have enough area to include the extra artwork; adding fiducials runs counter to using space-saving flip chip technology.

User-defined fiducials may be used for locating flip chip bumps and flex pads prior to placement.  A distinctive shape, which is already part of the artwork, is inputted via a computer-aided design (CAD) drawing so that it becomes part of the fiducial library.  The pattern of the chip’s bumps and flex pads are drawn, then added to the fiducial library.

A feature normally found only on automatic pick-and-place systems, pattern recognition is another vision option for finding the placement location.  Any distinctive artwork feature, such as traces or pads, is learned by the vision system and used to locate the component placement position.  Pattern recognition is very user-friendly and versatile.

With a semiautomatic system equipped with a cube beam splitter, the operator initiates the placement cycle once the bump and pads are in alignment.  The viewer retracts and the pickup head holding the chip lowers to gently place the chip  onto the flex bond pads under a preprogrammed load.  To ensure good reflow, each of the chip bumps must be touching a bond pad on the substrate.  When solder is used, applying only a small amount of bond load and then retracting is necessary.  However, for some applications the chip needs to be held at a higher bond load.  Upon placement, the assembly is ready for underfill and then the reflow in an oven.

Fluxing
Upon flip chip pick up and vision recognition, but prior to placement on the flex pads, another very critical stage in the assembly process occurs: the fluxing operation.  The amount applied and type of flux used is extremely important.  Cleaning after reflow is different because the gap between the chip and the flex and between adjacent bumps is small.  Excessive residue from the flux can impede the underfill process, cause voids in the underfill, or interfere with the adhesion of the underfill resin to the chip on flex.

The flux used must have low ionic content to prevent corrosion or parametric changes to the IC.  A very benign, ionicly pure rosintype flux that leaves minimum residue after reflow should be used.  A sparing amount of flux – just enough to lightly tack the flip chip in place during transfer to the reflow operation – should be applied.  A motorized rotary flux station works well for this purpose.  The station’s disk rotates under a squeegee so that a level, controlled thickness of flux is left on the disk.  The flip chip, held by the pick – and – place system’s pickup head with the bumps facing down, is lowered into the flux so that only a very thin, consistent amount of flux is applied to the bumps.

Underfill
Underfill is an essential step in FCOF assembly.  Note that, once the IC has been underfilled, rework and inspection are not possible.  Underfill is required to protect the IC from external contaminants, alleviate stress on the bumps and add robustness to the attachment area.  An underfill resin should be dispensed on the sides of the flip chip after the chip and flex have been heated to 80°C.  Capillary action causes the resin to wick between the flip chip and flex and exit out the other two sides.  The underfill resin of choice is an epoxy with 70 – percent alumina filler.

Semiautomatic dispensers equipped with high – resolution video monitors may deposit precise amounts of epoxy in the form of dots or continuous beads.  Depending on the design, the operator uses a camera and crosshair alignment to zero in on the target site shown on the monitor.  To dispense in a straight line, the operator engages the x- or y-axis brake – depending on the direction of travel desired – which prevents one of the lines of travel from being used.

An automatic dispensing system feeds the palletized flex to a preheat stage and then to the heated dispense station.  A pattern recognition camera finds two diagonal corners of the flip chip and indicates where the dispense head should place the epoxy bead.  Although a pressure – dispense syringe may be used, a positive displacement pump is more accurate for dispensing a consistent amount of resin.

Underfilling is a time – consuming process, requiring three hours at 150°C to cure.  The curing may be done in a convection oven or in a small belt or vertical oven.  New snap cure underfill epoxy can be cured in five to 15 minutes, which allows in – line conveyor cure

Conclusion
Assembling flexible circuits with flip chips can be extremely challenging.  However, when carefully implemented, the technology can further optimize product designs where small size, low weight and high performance are driving forces.

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