Why HDI?

Reasons to implement HDI:
  • Surface real estate
  • Signal isolation
  • Reduced signal antenna (less noise)
  • Different component footprints top to bottom
  • Robust PCB manufacturing and long-term board reliability on µBGAs (≤0.5mm)



0.5mm BGA device

Types of HDI

Thru via (Single Lamination) Build Sequence

HDI Build Sequence

Case Study: Available Drill Spans Using Combined HDI Elements

Sequential LAM Blind Vias

Reasons to use blind vias:
  • Real estate – frees up surface routing
  • Different component footprints on top & bottom side
  • Reduce signal stubs/antennas
  • Isolation – keep nets separate
  • Clean ground loops for RF
Constraints:
  • Registration (thru drill to blind vias)
  • Extra plating on board surface
  • Must be filled (prepreg or resin-filled - can be via-in-pad/Plated Over Filled Via (POFV)
  • Extra processing time for sub lam portion

A blind via does not pass through the entire board, and has access to only one external layer

Microvias

Reasons to use Microvias:
  • Real estate – essential for pinout of most microBGA devices (<0.65mm)
  • Clean ground loops for RF and capacitor pads
  • Mechanical rivet for extra pad stability
  • Thermal dissipation (Cu-filled microvias)
Constraints:
  • Extra plating on board surface if resin-filled for via-in-pad (POFV)
  • Aspect ratio must be less than 1:1 (best manufacturability when ≤0.85:1)
A microvia is a low-aspect ratio blind via
Larger blind vias can also be controlled-depth drilled

Buried Vias

Reasons to use buried vias:
  • Real estate – frees up surface routing
  • Isolation – keep nets separate
  • Essential for pinout of most microBGA devices (<0.65mm)
  • Buried vias in combination with microvias give several drill span options (example on next slide)
Constraints:
  • Registration
  • No direct test point access for ICT
  • Must be filled (prepreg or resin-filled)
  • Adds plating to outermost internal layers
  • Extra processing time for sub lam portion

A buried via provides connection within internal layers, and has no access to external layers

Robust PCB Manufacturing and Long-Term Board Reliability on µBGAS

  • One layer of 4 mil microvias dogboned in 10 mil pads (30% cost adder) or via-in-pad (additional 10% cost adder)
  • Standard 10/20 thru vias (no cost adder)
  • M4-5 mil trace & space (no cost adder)
  • Consistent, high yields
  • Standard drill to Cu on internal layers - No CAF risk

  • Many device white papers suggest a 6 mil thru drill in a 10 mil BGA pad (6 mil drill is a 20% cost adder)
  • Resin-filled vias needed for via-in-pad (10% cost adder)
  • Traces between pads are 3.0 mil trace / 3.3 mil space (yield reduction by up to 50%, inconsistent yields lot to lot)
  • Minor soldermask misregistration will expose trace edge (shorts under device causing assembly rework or scrap)
  • Drill to Cu on internal layers too tight for CAF (increased potential for field failures)

Future of HDI - Miniaturization

Resin filled Vias Processing Sequence

  • Resin-filled vias are encapsulated completely in the solderable pad
  • Pad will look like a normal BGA or SMT pad when viewed from the board surface
  • Soldermask can be tented or clearance, same as can be applied to solderable pads
  • Resin filled vias add extra plating to the board surface which needs to be etched through
  • Designs with resin-filled vias require extra spacing (.005” min spacing) for best manufacturability

Stacking HDI Structures




Reasons to stack HDI structures:
  • Real estate – frees up surface routing
  • Pinout of dense microBGA devices (≤0.5mm) for reliability and manufacturability
Constraints:
  • Must be filled (resin or Cu filled/plated shut)
  • Stacking microvias on buried vias can be a reliability concern on boards >.062” thick (> .093” is greatest risk) due to Z-axis thermal expansion
  • No concern with stacking microvias on top of each other
  • Staggered microvias should be .008” min from edge of microvia to the edge of the buried via

Design Rules

Footnotes:

Starting Cu weight on all plated layers to be 0.33oz

* Larger pad size on the thru vias helps with registration if real estate allows

** Aspect ratio is total laminate and Cu thickness (including starting foil thickness)/min hole size diameter

***Use the center of the drill/pad for stacked structures

Cost

Cost Optimization