HDI Technology
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)






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
- 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

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






