Are HDI Boards Economical?

Posted April 10, 2024

The focus of the electronic industry is on HDI or High-Density Interface boards right now. Rightly so, as the trend of the industry is to produce devices with minuscule form factors, for which, boards made with the HDI technology are of paramount importance. However, the most common question that arises is whether HDI boards are economical. Here, Rush PCB Inc. analyses this question from different angles, and the conclusion is a resounding yes, HDI boards are certainly economical. Let us look at the arguments offered.

Layers and Laminations

It is true that boards with multiple laminations are more expensive compared to those with a single lamination. However, when developing a suitable stackup, the number of laminations is only one factor among many to consider for a cost-benefit analysis.

For instance, designers can add multiple layers to make their boards smaller. However, they must also consider the yield, and focus on the lowest installed cost. For a complex circuit, they must assess the cost-benefit of using HDI technology to using a conventional board. For instance, they can compare the use of blind and buried via architecture with an 8-mil hole-to-copper clearance to an HDI board that uses 3-4 mil clearance, has 8:1 aspect ratio holes, and uses fewer layers.

Designers should carefully asses the technical capabilities of their PCB manufacturing partners to ascertain if they are fully equipped to manufacture large order quantities of complex HDI boards. For instance, if their partner is typically specialized in prototype manufacturing of HDI boards, then they will be hard-pressed to produce them in production quantities, resulting in huge wastage. They may readily produce ten numbers of a six-layer design with a 3-mil hole-to-copper clearance. They may struggle to produce twenty numbers of a 12-layer design with the same clearance, managing to supply the quantity in time with a 50% acceptable yield. At this rate, if the designer were to approve them to produce 10,000 boards, the manufacturer would have no choice but to pass on the manufacturing loss to the customer, leading to a price escalation of 50% or more to the customer.

Is HDI Technology Necessary?

HDI technology is most suitable for boards with dense component placements, for example, BGAs on a very tight pitch. With conventional designs, it is possible to make such boards using through holes on a single-lamination. However, it would demand very small holes and very close hole-to-copper clearance. Not only is this design difficult to fabricate, the yield would be terribly low.

However, designing these boards with HDI technology makes it very easy for the designer. The BGA breakout requires 2.5 mil to 3 mil traces within the matrix, with connecting segments measuring 2 mils wide. The designer can use multiple sequential laminations to reduce the total number of layers, and apart from the BGA breakout traces, all traces can be 4-5 mils or even wider.

To make an economical HDI board, it pays to consult the manufacturing partner and seek their advice.

Choice of Materials

For an economical HDI board, it is mandatory to identify the least expensive materials that will satisfy the mechanical and electrical requirements of the application. Designers can consult datasheets and price lists for materials. However, they only provide basic guidance but offer no information about the actual cost of the board fabricated from these materials or a comparison of costs between two types of similar materials. Designers may find two materials with nearly identical characteristics, but they will have no clue that one of them may be more difficult to process as compared to the other. The raw prices offered hold no clue to the ease of manufacturing and the resulting yield of the board. This is where the experience of the manufacturing partner comes into play.

The board manufacturer can, based on their prior experience, identify the material that not only meets the performance requirements but also the ease of manufacturability. The net result of this choice would be an optimum economic balance between performance and manufacturability.

Basically, the manufacturer looks at materials that will be dimensionally stable, capable of withstanding multiple laminations, resistant to CAF or conductive anodic filament growth, and capable of withstanding laser drilling with good consistency.

For high-speed signals, the board with a relatively flat response over the frequency range of interest will allow faithful propagation of signals without distortion. The broad band of frequencies associated with high-speed digital signals are subject to distortion because of impedance variations from changes in the dielectric constant of the board materials with frequency. Another factor degrading the signal integrity is the dissipation factor of the board material.

Therefore, an economical HDI design would choose materials with low dissipation factor and dielectric constant and with little variation across the frequency domain. This choice would allow faster signal propagation while maintaining signal integrity.

Designers must also consider that for a given dielectric constant, a thinner substrate will require narrow traces to maintain an impedance of 50 ohms. Selecting a material with a low dielectric constant will allow keeping traces wide enough for easy manufacturability while maintaining the necessary impedance value.

Factors Influencing HDI PCB Fabrication

For economical PCB fabrication, the material must possess three key qualities:

Dimensional Stability

Heat and pressure during the manufacturing process make all materials shrink and stretch to some extent. If the material movement is predictable, this is not an issue.

Easy Machinability

For HDI materials, this means the capability to withstand laser drilling without problems.

Ability To Survive Multiple Laminations

This is necessary as the fabrication of multiple-layer boards requires multiple laminations

Although we use the word laser-drilling, the activity would be better described by vaporization. When making holes using a laser beam, the process typically directs a highly concentrated laser beam that transfers intensely focused energy onto a specific area. The material at the spot continues to absorb the energy until it starts to melt and vaporize. It is necessary to carefully control the energy delivered to ablate the material at the top without damaging the copper landing pad underneath it.

As different dielectric materials absorb energy at different rates, their manufacturability differs. For instance, while Megtron 6, FR408HR, and N4000-13 SI, are all comparable materials for high-speed digital PCBs, drilling them using lasers is not the same.

The Right Type of Resin

Epoxy resin is the one that this industry most commonly uses. This is on account of its low cost, good mechanical, electrical, and thermal properties, and its excellent adhesion quality both to itself and to metal foils. Epoxy resin is a thermosetting resin that uses hardeners and catalysts to improve its adhesive properties.

However, epoxy resins are flammable. As HDI boards are high-density and dissipate more heat, it is necessary to incorporate flame retardants into the epoxy to reduce its flammability. However, for higher thermal stability, such as in industrial and marine environments, and in organic chip packages, retardants are not useful, and BT-Epoxy resins are used.

Via Types and HDI Board Cost

The cost of an HDI board and its end-product quality is highly dependent on its via quality and plating. Mechanical and laser drilling are the most commonly used processes for drilling vias. Among the two, mechanical drilling is the more conventional technique, and also the most economical for drilling through-holes and blind vias.

Although the capital cost of mechanical drilling is lower compared to laser drilling, its operational cost is higher. This is because mechanical drilling must deal with broken drill bits, which increase the cost due to:

● Work stoppage to replace drill bit resulting in additional operating charge

● Extra metallization of the damaged hole

● Cost of the new drill bit

Therefore, for large-scale production and critical drilling, manufacturers prefer laser drilling.

Mechanical drilling is economical compared to laser drilling if we are considering drilling a single hole or via. However, the economics tends to favor laser drilling when considering drilling thousands of vias with high precision, consistent quality, and high throughput. The right drilling technology remains one of the critical factors in optimizing the economics of an HDI board.

Conclusion

For producing an HDI board economically, one must consider several factors such as material type, yield, and drilling methods. Higher yield in HDI boards can result from using 3-mil traces and better control of the drilling mechanism. Using HDI technology also allows reducing the number of PCB layers. Rush PCB Inc. recommends consulting your manufacturing partner when starting the design. This consultation should identify materials providing an optimum balance of manufacturability, price, and performance.