Wire Bonding using Insulated Wire

 a)  Advantages

– Prevents wire shorts and enables cross bonding.

– Uses existing platforms and lowest-cost packaging infrastructure (wire bonding).

– Is suitable for multi-row, ultra fine pitch and area array wire bonding.

– Is suitable for systems in package, stacked dies and side-by-side chip-to chip bonding.

– Enables flexible routing, simplified substrate and packaging of small dies.

– Enables complex package designs and improves the yield of high-density packaging.

– Allows use of the Z dimension and wasted space created by parallelism of bare wires.

– Enables wire bonding with insulated fine gold or copper wire and low-k pad structures.

b)  Challenges  

– Damaged wire with a hole burned in the insulation layer.

– Non-sticking on lead.

– Low-stitch pull readings.

– Frequent machine stoppages.

– “Kissing effect” after temperature cycling.

– Insulation layer pushed out to one side of the wire during moulding.

c)  Solutions and findings

–  Provide a ground adjacent to the free end of the insulated wire with a conducting capillary.

– Add a ground at a spool end of the wire at a far side of the insulated wire path from the free end.

– Form a bump on the second wire bond, offset by a distance of about one half of a diameter of the bump.

– Provide a special capillary movement to achieve mechanical abrasion.

– Use a special surface-treated capillary for effective tearing of the insulation layer.

– Choose a proper combination of insulation thickness, melting and coating temperatures, capillary and wire bonding parameters.

– Use shorter EFO gap and lower-EFO current compared to bare wire to achieve optimal FAB quality.

–  Provide a coating material that easily cracks only at the stitch bond with available bonding parameters.

– Apply high-initial bonding force with low-ultrasonic energy, apply high-initial impact during touch-down, and provide slight scrub motions to promote coating removal.

– Use a smaller capillary hole, which does not scrape the coating during looping. – Use a smaller capillary outside radius, which maximizes the capillary area contacting the stitch bond.

 – Use a matte capillary tip.

–  Move the capillary tip with the wire over the stitch bond pad.

–  Roughen the outer surface of the capillary tip.

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