PCB Fabrication

How to Choose the Right Materials for HDI PCB Fabrication

HDI PCB fabrication

HDI PCB is short for high density interconnect printed circuit boards. It means the circuit boards with higher layers (>=8 layers), small drills (<=0.2mm) and small trace width and spacing(<=0.1mm). Besides, blind and buried vias usually exist on HDI printed circuit boards. Such kinds of PCBs are usually used in high-tech electronic products that have complex functions, such as cellphones, computers, CPUs, and many kinds of main control boards. In this article, we will talk about the right material that should be used in HDI circuit boards. And the characteristics, processes, applications, and future of HDI PCBs.

Base Laminate Material for HDI PCBs

HDI PCBs provide higher component density and more complex designs than traditional boards. As spacing gets closer and dielectric layers get thinner, the selection of materials is vital for stable performance HDI PCB fabrication. Here are some key considerations when selecting materials for your HDI printed circuit board design:

  • High Tg FR-4: Several rounds of lamination may be needed in HDI circuit board production, so high TG (>=170) laminates must be used, such as Isola Isola FR406, FR408, IS420, IS410, 370HR,185HR, Shengyi S1170G, S1000-2, ITEQ IT180T., etc.
  • High-Speed FR-4: Since HDI PCBs are widely used in the telecommunications industry, so high-speed materials are commonly used, such as Isola I-SPEED®, TERRAGREEN® 400G, TACHYON® 100G, ASTRA® MT77, I-TERA® MT40, Panasonic MEGTRON 6(R-5775), MEGTRON 7(R-5785), TUC TU-872 LK, ThunderClad 1, ThunderClad 2, ITEQ IT-968TC, T-988GTC, etc.
  • PTFE & Reinforced PTFE: Besides excellent electrical properties, PTFE also enjoys superior chemical resistance. So PTFE materials are commonly used in electronic devices with very low loss, such as Rogers RT/duroid 5880, RO3003, Aron 85N, 33N, 35N, 84N, Ventec VT-901, Taconic RF-30A, TLC-32., etc.
  • Polyimide: It has excellent thermal stability and low Z-axis expansion for this high-performance polymer. Polyimide material is usually used in HDI rigid-flex circuit boards. For example, popular brands like DuPont Pyralux AP, as well as Panasonic Felios Polyimide materials like R-F770 and R-F775.

HDI PCBs have thinner layers of insulating dielectric separating copper traces, so the electrical characteristics of these materials must be of the highest standard.

HDI PCB Fabrication

HDI PCB Fabrication

What Kind of Conductor Materials Are Used?

In particular, with trace widths and spaces under 50 microns in dense HDI PCB fabrication designs conductor integrity is critical. Common options include:

  • Copper: The standard conductor used for PCBs can be applied to HDI as well. However, some fabricators require electrodeposited copper to achieve tighter trace geometries. Using an organic solderability preservative (OSP) to treat copper prevents oxidation.
  • Gold: Gold always has very good conductivity. So immersion gold surface finishes are commonly used in HDI circuit boards, which will also create a very smooth surface which is very good for surface amount assembly.
  • Nickel: Before gold plating in surface treatment process, a layer of nickel will be applied on the PCB pads. It will be helpful in formation of good soldering joints in PCB assembly.
  • Silver: Silver’s conductivity is very good-slightly thinner traces can be used than with copper. But it is more prone to migration and oxidation problems, and a lot more expensive. Solderability can be maintained by using Immersion silver finishes.

All You Need to Know About Via, Plating, Solder mask

For high-reliability HDI PCBs, from the dielectrics, vias, and plating to solder mask, every material choice affects performance. Select the best option for your design requirements by working closely with your PCB manufacturer. As long as the right materials are chosen, dense interconnects with fine features and stable electrical properties can be made.

Via and Plating

Layer interconnects in HDI PCB fabrication designs require blind and buried vias. Via hole walls are copper-plated, but must be gold-coated first. Typical via-wall materials include:

  • Electroless Copper: The most common method is for the catalytic electroless copper to be plated first, followed by a thicker electroplate. Provides excellent hole wall continuity.
  • Conductive Composites: Hole walls can be screen printed with carbon black or metal-filled polymers. More complex process, but has higher conductivity than plated copper.
  • Direct Metallization: Palladium catalysts are used to directly activate the dielectric surface before copper plating. Eliminates electroless copper.
  • Blind and buried vias: Blind and buried vias must exist in HDI PCBs, even staggered vias and stacked vias. VIPPO (via-in-pad plated over) technology is also commonly used on via in-pads.
  • Solder Mask: Solder mask protect traces but allows soldering on pads. For HDI PCB fabrication, the mask must maintain insulation integrity at very tiny clearings. In general, photo imageable solder masks work better than their liquid counterparts. LPI epoxies or polyimides are common choices.

Signal Integrity Considerations You Need to In Account

Signal integrity is extremely important but also difficult to assure in HDI PCB fabrication designs where traces are very closely spaced. The choice of materials can be used to reduce crosstalk, signal losses, and impedance discontinuities.

  • Low-Dk, low-Df dielectrics reduce signal attenuation and dispersion in the eye leads which produces cleaner eye patterns. PTFE or LCP is preferred over FR-4 as a materials.
  • The alternating arrangement of ground-signal-ground trace geometries on the burglar alarm transmission line helps maintain impedance and protect against cross-talk.
  • Thicker copper layers reduce conductor losses, but poor thermal expansion match between thin dielectric and thicker conductor layers can cause stresses during fabrication. This is why build-up HDI processes generally use thinner copper.
  • Plane layers of copper make signals resistant to noise and cross-talk. 1-2 mil thick cores with copper planes are better than thinner laminates.
  • Thanks to micro via structures, there are no stub connections introducing reflections. These small microvias also make possible continuous ground planes.
  • Applications like 5G, automotive radar, and satellite communications demand antennas on HDI PCB fabrication designs where low-loss dielectrics are a must.

Thermal Management in HDI PCBs

High density interconnect PCBs tend to have higher power densities, so thermal design plays a significant role. Material choices can help mitigate heat issues:

  • Metal-core substrates, insulating metal-coated products or ceramic-filled materials of high thermal conductivity can all speed up heat spreading.
  • Interleaved between HDI PCB fabrication buildup layers are thermally conductive dielectric or prepreg materials to improve vertical heat conduction.
  • Thermal vias carry heat from inside layers to the board surfaces, where it can better escape. Smaller micro vias allow higher density.
  • Thermal pads with exposed faces make direct contact between components and heat sinks or thermal frames possible. Exposed areas prefer high thermally conductive dielectrics.
  • Embedded heat pipes or vapor chambers are filled with special working fluids, which spread and dissipate heat laterally through evaporation-condensation cycles.

Mechanical performance of HDI PCB Materials

With very thin dielectric layers prone to warpage, HDI PCB fabrication requires careful material selection for mechanical stability:

  • Because of their high modulus, temperature resistance and dimensional stability in dielectrics such as polyimides are superior. FR-4 presents more warpage problems.
  • Laminates reinforced with glass fiber resist thermal expansion and mechanical stresses better than unreinforced materials.
  • To minimize warpage, the buildup dielectric CTE should match those of the layers below.
  • High glass transition temperatures (Tg) increase the mechanical stability at elevated assembly or operating temperatures of core materials.
  • Good elongation and fracture toughness materials are better able to withstand flexing or shock stresses before cracking.
  • Thermal and mechanical stresses between the die, component, and board surfaces must be borne by a sticky. Epoxy films are generally better than paste adhesives.
High Density Interconnect PCBs

High Density Interconnect PCBs

Reliability Factors That You Shouldn’t Miss

  • For long-term reliability in harsh environments, HDI PCB fabrication materials must withstand moisture, heat, and fatigue stresses:
  • By using low moisture absorption materials such as polyimides or reinforced laminates, the hygroscopic swelling that causes cracks in thin dielectric layers can be prevented.
  • Some dielectric materials can withstand the reflow soldering temperature profiles used for BGA packages without delamination or decomposition problems.
  • In case of electrical arcing, flame-retardant dielectric materials will not burn. High-Tg resins retain FR properties at high temperatures.
  • High Tg and modulus materials that are dimensionally stable can resist temperature cycling as well as power/thermal shock. Here, copper-invar-copper cores are at their best.
  • Ductile copper foil and plating reduce the possibility of cracking under dynamic or mechanical flexural stresses.
  • Neither immersion silver, ENIG nor OSP surface finishes have tin whisker problems.

Designers who take account of materials technology in their choice of solutions concerning signal integrity, thermal dissipation, mechanical stability and long-term reliability can get more out of high density interconnect PCBs for advanced PCB assemblies that offer improved performance. Designers and fabrication partners must collaborate deeply to choose the best materials.

Advanced Manufacturing Processes

Realizing the fine features and tight tolerances of HDI PCBs requires advanced fabrication techniques:

  • Laser Direct Imaging: Laser direct imaging provides finer resolution patterning of solder masks and resist layers than traditional lithographic processes. It allows more compact trace spacings and smaller vias.
  • Plasma Etching: With plasma etching, the copper can be more precisely etched for narrower line widths and spaces. This is especially important in traces under 20 microns wide.<|> It also avoids the problems of chemical etching.
  • Thin Dielectric: Build-up dielectric layers are generally used in conjunction with thin core layers down to 2 mils. Thinner dielectric enables finer lines and spaces.
  • Special Lamination: With successive layered lamination and metallization, build-up processes for multilayer HDI PCB fabrication are not limited by the constraints of conventional laminate-based methods. As many layers as can be added. Sequential lamination and curing allow for tighter thickness tolerances between layers, reducing feature inconsistency.
  • Selective Plating: Through selectively electroplating circuit patterns (foregoing the etching step), SAP fine line sem additive processing makes it possible to fabricate tight trace geometries.
  • Blind and Buried Vias: Laser or plasma drilled blind and through-hole vias connect the buried layers with unparalleled dimensional accuracy.
  • Microvias: Smaller micro vias than 50μm in diameter can be reliably fabricated with laser drilling, photo definition, or plating through hole techniques depending upon density requirements.
  • Direct Metallization: Organic solderability preservatives can also replace etch-back processes with direct metallization techniques.

Quality Control Checks That Need to be Performed

With small features prone to defects, HDI PCBs demand robust processes and quality control:

  • PCBs are tested for defects immediately following each fabrication step by automated optical inspection (AOI).
  • A layer-to-layer registration analysis indicates that the buildup layers are well aligned with each other, and interconnections can be counted on.
  • To ensure quality, cross-sectioning and microsectioning techniques show us Microvia continuity as well as trace geometry.
  • In this small clearance region between high-density traces, voltage resistance faults are checked by dielectric breakdown testing.
  • TDR is used for detecting localized impedance differences- hences which can degrade signal integrity if the traces or spaces differ in width.
  • Laminate and etch compensation help to compensate for predicted process imbalances like dielectric shrink or etch-back.

Using precision processes, strict inspection criteria and strong design guidelines as well as advanced compensation methods enables manufacturers to mass-produce low-cost HDI PCB fabrication boards with tight tolerances. Close customer collaboration is key.

HDI PCB and Assembly

HDI PCB and Assembly

What Innovative Applications of HDI PCBs Hold

Continual technology advances expand high density interconnect PCB capabilities and enable finer features:

  • New lasers, such as picosecond pulse lasers with extremely fine via drilling and little heat-affected zones.
  • Plasma dicing can replace blade dicing to reduce stresses on thin, fragile circuits during singulation.
  • Inkjet printing is being worked on for direct deposition of dielectric and conductive materials onto substrates.
  • Traces as small as 5 microns wide can be outlined by fine-line photolithography similar to that for semiconductor fabrication.
  • In this two-photon polymerization technique, focused femtosecond laser pulses 3D print dielectric and conductive microstructures.
  • Inductors and capacitors incorporated into the HDI buildup layers not only save space, they also improve electrical performance.
  • Electroless composite copper, for example, are large area metallization techniques that allow advanced RF designs and integrated antennas.

Leveraging advanced technologies allows HDI PCBs to continually improve in interconnect density, electrical properties, reliability, and yield rates for the assembly process. They can also achieve higher fabrication efficiency.

Leveraging HDI Technology in Daily Life

Once viewed as specialized technology, HDI PCB fabrication techniques are now widely accessible to enable sophisticated PCBs:

Now, HDI capabilities are no longer just for advanced manufacturers. There are many reputable fabrication partners, providing quality high density interconnect PCB Fabrication services at competitive prices.

Today, HDI processes originally developed for high-frequency RF designs are used increasingly in commercial wireless and consumer electronics that require superior interconnect densities.

HDI makes it possible to integrate more functions into a smaller space while maintaining reliability for such applications as automotive electronics, medical equipment, and industrial electromechanical systems.

Owing to exponential growth in the power of computing, increasingly advanced tools for modeling and design as well as fabrication are constantly being added to HDI’s capabilities. If designers have the right tools and HDI expertise then engineers are more willing to accept dense, high-performance PCB designs. With the fabrication of HDI maturing, yields are improving by leaps and bounds. As costs continue to drop dramatically the use of HDI PCB fabrication is spreading rapidly.

Inkjet and 3D printing, as well as semi-additive processes in general, attempt to simplify HDI fabrication even further with yet finer features. With constantly improving technology access, high density interconnect PCB provides transformative benefits spanning industries and applications.

What The Future of HDI Holds

But as HDI fabrication processes were honed and engineers gained experience in doing high-density interconnects, the technology looked set to establish itself not only within a certain segment of the market but soon everywhere instead: perhaps HDI really would become that new normal. Today, the power of high density interconnect PCBs is already revolutionizing nearly all areas of electronics.

There will be more dramatic advances in the future due to continuous innovations. For electronics manufacturers, high density interconnect circuit boards provide a bridge to meet market needs by developing ever more capable and compact products.

In conclusion, by using HDI technologies to their fullest extent, designers place themselves in a fine vantage point from which they can steer the future topography of electronics based on what can be incorporated into limited volumes. The cutting-edge electronic devices and systems of tomorrow will no doubt be powered by the HDI PCB fabrication principles honed today.

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