Microvias

The miniaturization trend – more complex technology that needs to fit in a smaller surface – affects not only the electronic product but the circuit board itself. Such demands drive the need for more advanced printed circuit boards and the use of microvias.

What is a microvia hole?

According to the new definition within IPC-T-50M a microvia is a blind structure with a maximum aspect ratio of 1:1, terminating on a target land with a total depth of no more than 0.25mm measured from the structure’s capture land foil to the target land.

An illustration of a PCB microvia hole | NCAB Group

The IPC-6012 also defines the structure of a Microvia.

  • The Microvia is a blind structure with a maximum aspect ratio of 1:1 between hole diameter and depth, with a total depth of no more than 0.25 mm, when measured from the surface to the target pad or plane.
  • Typically NCAB considers the dielectric thickness between surface and reference pad to be 60 – 80um.
  • The diameter dimensions of the microvia have a  range of 80-100 microns. The typical RATIO is between 0.6: 1 to 1: 1, ideal 0.8: 1

Why are microvias important?

Electronics products are becoming increasingly complex and this increase in density means that we face challenges of miniaturization both in terms of the electronics product and the circuit board itself – all while the functionality increases.

We are seeing smaller components and smaller features and these, combined, with the shrinking real estate means that the higher density of the circuit board calls for finer track and gap and increasing number of interconnections.

The result is that we have an increasing number of holes with increasingly smaller diameters, yet no longer can the density allow for traditional plated through via holes. Such demands drive the need for microvias – laser drilled holes that are smaller mechanically drilled holes and connect selected layers (e.g. 1-2) rather than passing through and connecting all layers.

Trends in PCB:s - more electronics in smaller devices | NCAB Group
One of the drivers in the field of electronics currently is miniaturization, where we’re seeing smaller units having to accommodate an increasing amount of electronics. This is also affecting the PCB’s design, with for example more advanced solutions such as HDI, flex and flex /rigid cards having to be used in some cases.

HDI structures

The IPC-2226 defines the structures of HDI by type, there are three types and we can observe in the images highlighted.

Type 1 structures

HDI PCB type 1 structure with microvias | NCAB Group

Those that contain a single layer of microvia on one or both sides of the core. It uses both microvias and through holes for interconnection. It is important to keep the ratio 0.8: 1 for the microvia. This structure uses ONLY blind holes NO passage to buried holes.

Type 2 structures

HDI PCB - type 2 structure with microvias | NCAB Group

Are similar to type 1 in that they contain only a single layer of microvia on one or both sides of the core. It uses both microvia and through holes for interconnection, but unlike type 1 these structures also use buried holes.

Type 3 structures

HDI PCB - type 3 structure with microvias | NCAB Group

Are the most complex and demanding for factories, they contain AT LEAST two layers of microvia on one or both sides of the core. As for type 2, it also uses through holes in addition to blind and buried vias.

Design guidelines for microvias

Below are extracts from our HDI design guidelines showing some details for type I, II and III constructions. However, one important point to note regarding type III construction is that stacked structures should be limited to 2 layers of microvia and, where possible, avoid stacking on buried holes.

HDI PCB - design guidelines type 1 Microvia | NCAB Group
HDI PCB - design guidelines type 2 Microvia | NCAB Group
HDI PCB - design guidelines type 3 Microvia | NCAB Group
Features (dimensions um)RECOMMENDEDADVANCED
AMicrovia size / diameter10080
BCapture land325 (class 2)
350 (class 3)
250* (class 2)
250* (class 3)
CTarget land300 (class 2)
325 (class 3)
250* (class 2)
250* (class 3)
DDielectric L1 – L2 microvia60-8060-100
EMicrovia center to PTH edge380300
FOuter layer space10076
GInner layer space10076
HMicrovia to buried hole375300
IBuried hole to PTH450430
JPitch – internal microvia (different net)425325*
KPitch – outer microvia (different net)525 (soldermask web)
425 (no soldermask)
325*
LPitch – staggered microvia400225
MDielectric for internal microvia60-8060-100
NBuried hole to buried hole450350*
OBuried hole size / diameter250150
PMicrovia to microvia300220
Buried via target landBuried via + 250Buried via + 250*
QSkip via microvia size / diameter300200
RSkip via capture land500 (via + 200)400 (via + 200)
SSkip via target land600 (via + 300)500 (via + 300)
TDielectric L1-L3 skip via200160
USkip via to copper on L2250150

* For design tighter than the given value, please consult your local NCAB technical contact to discuss specific projects case by case.

Stacked or staggered microvia constructions?

Although we continue to see both stacked and staggered designs (see image below), there has been an IPC test committee established to look into the reliability of stacked microvias for high performance products.  

Failures, post manufacturing, have been reported over recent year and the problems appears to manifest itself at the bottom of one microvia and the metallurgical junction of another via or copper layer. What is known is that this trend is seen in complex stacked microvias, but not in staggered vias.

Illustration of PCB with stacked and staggered via | NCAB Group

The data compiled so far seems to suggest that stacked microvias of 3 or more layer of microvia are much more likely to experience failure than staggered via structures. It is worth pointing out that the numbers are low in terms of % failure, yet when compared to similar failure rates in staggered via holes there is a marked difference.

The rule of thumb, or recommendations at this point in time seem to be that the stacked structures should be limited to 2 layers of microvia and where possible avoid stacking these on a buried via hole. If the design calls for a third layer of microvia, then this third layer shall be tracked away from the 2 microvia stack and completed as a staggered design.