What Is an HDI PCB? Microvias, Build-Up Orders and When You Need One
Key Takeaways
- HDI replaces mechanical drilled through-holes with laser microvias (down to 0.10 mm) and finer lines, letting a 0.4 mm-pitch BGA actually escape its balls.
- Build-up order (1-nth, 2-nth, 3-nth) counts how many layers of microvias are stacked; more order means more cost but denser routing.
- Via-in-pad with epoxy fill and copper cap wins routing space under BGAs at the cost of extra process steps.
- You need HDI when routing density exceeds 8-10 traces per channel or ball pitch drops below 0.65 mm - otherwise a well-designed 8-layer through-hole board is cheaper.
HDI in one sentence
An HDI (High Density Interconnect) PCB is a board that uses laser-drilled microvias and fine-line geometry instead of conventional mechanically drilled holes to connect layers. The result: more connections per square centimeter, shorter signal paths, and room for the dense packages modern silicon demands.
Microvias vs mechanical holes
A mechanical drill cannot reliably go below 0.20 mm diameter - the bits break, wander, and cost a fortune at small sizes. Laser drilling goes down to 0.10 mm and even 0.075 mm on advanced lines, and drills only one or two copper layers at a time, which is why HDI boards are built in sequential lamination cycles.
That word - sequential - is the key cost driver. Each build-up layer pair means an extra lamination, plating and drilling cycle. More cycles, more money, longer lead time. This is why HDI pricing is quoted by build-up order, not just layer count.
Build-up orders, stacked and staggered vias
1-nth order: one microvia layer pair on top (and/or bottom) of a conventional core - enough for many 0.5 mm-pitch BGAs. 2-nth: two stacked microvia pairs, needed for 0.4 mm pitch and below, or when escape routing is brutal. 3-nth: mobile-class density, typically with any-layer HDI where every layer interconnects by microvia.
Stacked vias sit directly on top of each other - shortest path, most expensive, needs reliable copper filling. Staggered vias offset by a small distance - cheaper and more forgiving, slightly longer routing. Via-in-pad goes further: the microvia sits inside the BGA pad itself, filled with epoxy and capped with copper so the pad stays flat for assembly.
When HDI is worth the money - and when it is not
Honest answer from the shop floor: if your BGA pitch is 0.65 mm or coarser and you can route out with dogleg fanouts, a well-engineered 8-10 layer through-hole board with 4/4 mil lines is dramatically cheaper. HDI earns its cost when: ball pitch hits 0.4 mm or below; escape channels need more than 8-10 traces; board size is a hard constraint (wearables, sensors); or signal integrity demands shorter via stubs - which back-drilling or microvias solve differently.
The classic mistake is jumping to 3-nth order when 1-nth with via-in-pad would route the same part. A good DFM review compares both before you commit money to either.
What we build at Xianglong
We run 1-nth through 3-nth order HDI with 0.10 mm laser microvias, 3 mil line/space, epoxy-filled and capped via-in-pad, and +-8% impedance control on 4-26+ layer stacks. Prototypes ship in 3-5 days depending on order, volume in 10-15. Send your Gerber and BGA footprint - the DFM report will tell you honestly which build-up order your design really needs.
Frequently Asked Questions
Is HDI always multilayer?
Almost always - microvias exist to connect multiple layers densely. A 2-layer board with microvias is rare and usually a special blind-via case.
Does HDI affect cost much?
Yes: expect 1.5-3x the price of an equivalent through-hole stack, scaling with build-up order. The trade is board size, layer-count reduction and package support that through-hole cannot deliver at all.
Can via-in-pad take a BGA directly?
Yes, after epoxy fill and copper capping the pad is flat and solderable - that is the standard construction under 0.4 mm-pitch BGAs to stop solder wicking down the via.
What files change for HDI designs?
Same Gerber/ODB++, but include your stackup intent and the microvia sizes per layer pair. Our DFM checks laser drill registration, fill quality and aspect ratio against those numbers.