Signal integrity is a major consideration when transitioning a quality printed circuit board (PCB) design from prototype to production. This requirement has been necessary because of the emergence of new ICs operating at multi-gigahertz speeds. Digital transmission at such high speeds makes signal losses more dominant rather than reflections.
Earlier, there was a need to control the characteristic impedance of PCB tracks. This requirement came from chipsets operating with switching times in sub-nanoseconds. At any given speed, a long interconnect or trace will exhibit characteristics like that of a transmission line. Here, the predominant characteristic of interest will depend on the switching speed, the line length, and characteristics of the conductor, along with the dielectric properties of the substrate forming the PCB. At such high speeds, long traces without the correct impedance matching could cause reflections, which significantly hindered error-free high-speed performance. However, in majority of the cases, ignoring the losses is usual as they were small enough.
Recent advances in semiconductor technology has a variety of logic families operating at far higher speeds, reaching into the multi-gigahertz range. Now along with impedance matching, it is also important to take into consideration the transfer of maximum energy into the transmission line. Therefore, designers are now more concerned about the amount of energy that is conveying the signal, and rather less on the reflection of the energy at the receiving end.
At multi-gigahertz speeds, materials forming the transmission line start sapping away energy. The core and prepreg absorb energy from the signal in the form of heat. Skin effect reduces the available conductor area for signal transmission, and the resistance of this reduced cross-section results in higher heat losses.
While the primary problems of PCBs at high-frequency operation relate to signal reflection and transmission line characteristics, high-speed operations compound the problems with that of signal loss. Therefore, it is necessary for designers and fabricators to work together for producing cost-effective, efficient, and reproducible PCBs. For this, it is necessary to understand the components relevant to fabrication of high-frequency and high-speed boards, namely:
- Board Materials
- Presence of Vias
- Back Drilling
- Via Plugging
- Heat Dissipating Thermal Vias
Three components make up a circuit board—a non-conductive dielectric substrate core, dielectric laminate layers, and copper foils with the laminate layers as their base. Manufacturers use a variety of materials for making both the core and the laminate layers, and design them to meet specific dielectric constants and other requirements. With high-speed and high-frequency circuits requiring tighter signal integrity, the dielectric properties of the materials become important.
The most common materials for making PCBs is the Flame Retardant type 4 or FR-4, made of woven glass reinforced with epoxy laminate. Although a very cost-effective and easy-to-work-with material for PCBs, with good insulating properties and mechanical sturdiness, FR-4 has limitations where applications require high power, voltage, and heat. Moreover, the dielectric constant of FR-4 changes as the operating frequency increases, resulting in an increase in signal loss. Selecting special materials for high-speed and high-frequency use requires attention to specific material characteristics such as:
Dielectric Constant (Dk): The smaller this figure is, the better. As signal transmission rate varies in inverse proportion to the square root of the dielectric constant of the laminate, high values of Dk tend to reduce the speed of signal transmission.
Dielectric Loss (Df): Also known as Dissipation Factor, a lower Df tends to reduce signal loss.
Mechanical Stability: Tight physical tolerances are very important in high-frequency and high-speed designs, both for manufacturing and use of the PCB. For instance, thermoset hydrocarbon laminate materials are good for mechanical stability.
Moisture Absorption Rate: Circuit board materials should have a very low moisture absorption rate. This is because the electrical performance of high-speed and high-frequency circuits can change drastically with even a small amount of moisture in the PCB. For example, while the moisture absorption rate for FR-4 is as high as 50%, that for PTFE is as low as 2%. Moisture absorption raises the Dk and Df values, impacting PCB performance. For high-speed and high-frequency applications, laminates with moisture absorption values of less than 0.25% are more acceptable.
Thermal Management: It is usual for high-speed and high-frequency designs to deal with higher amounts of heat while in operation. Therefore, materials such as Polyimide while being very robust also have excellent thermal properties, allowing them to conduct heat better. Addition of a ceramic filler material also helps to improve the thermal conductivity of board materials.
Coefficient of Thermal Expansion (CTE): Materials with lower CTE are more robust and better at handling the thermal stresses during PCB assembly.
Conductor Loss: Factors responsible for conductor loss include circuit design, circuit configuration, thickness of conductor, and surface roughness of the conductor. Higher copper surface roughness results in higher conductor loss as compared to that from smooth and low-profile conductor surfaces.
Designers and fabricators select the most suitable PCB substrate materials based on the tradeoff between above material characteristics, manufacturability of the materials, and their cost. High-speed and high-frequency PCB materials are more expensive than regular materials, and more difficult for fabricators to work with, often requiring non-standard fabrication processes.
Presence of Vias
The presence of vias in high-frequency and high-speed boards has always presented problems for designers, as they affect signal integrity significantly. In a balanced impedance-controlled transmission line, the return current flows in the ground plane that the designer places directly under the trace. If there is a via in this path, the return current must find a way to travel through other layers. The extra distance the return current must travel adds impedance to higher frequency signals. Additionally, the via also introduces a discontinuity in the impedance resulting in unwanted reflections.
Fig 1: Vias and Return Paths
The easiest way to minimize the problems vias create is to not use them at all on signal traces. The best way of doing this is to place the receiver as close as possible to the transmitter.
Some vias have a short stub extending past the layer it transitions to, and reflections from the stub creates standing waves with undesired effects. At very high frequencies such as mm waves, this is a significant issue.
Fig 2: Back Drilling a Via to Reduce Stub Length
Designers can mitigate the effects of via stubs by using either blind, buried, or through vias, or by back drilling. In practice, the shortest stubs that fabricators can achieve by back drilling is 5 to 10 mils only
Fine-pitch ICs and BGAs require via-in-pads to breakout individual traces. In turn, these vias need plugging with epoxy to prevent them from wicking solder away from the pad during reflow.
Presence of air bubbles and moisture in the epoxy during the plugging operation affects the impedance the plugged via presents to high-speed and high-frequency signals. Fabricators need to remove bubbles by defoaming the epoxy before the plugging operation. For vias with small aspect ratios, the fabricator may need to use vacuum plugging machines to facilitate the epoxy to fill the via entirely.
Bonding between a plugged blind via and its immediate resin layer is usually weak. If the board has many such vias, high temperatures such as during a Lead-free soldering process can lead to delamination. To prevent this, fabricators must use optimal plugging resins based on the CTE and Tg values of the substrate.
While fabricating, substrate layers have a tendency to absorb moisture. High temperature and pressure the fabricator applies when stacking the substrate layers releases this moisture, which creates a huge pressure on the copper layers, forcing any weak binding between the resin, prepreg, and copper layer to delaminate. The remedy for the above problem lies in rigorously monitoring and controlling moisture absorption during the entire fabrication process.
Heat Dissipating Thermal Vias
Thermal management requires many metalized heat dissipating vias under a heat producing component. These metalized holes carry the heat generated by the component to the other side of the PCB for removal by the dissipation layers.
Special lamination materials that fabricators use for high-speed and high-frequency boards are usually difficult to drill. When the fabricator must drill many heat dissipating via holes in such materials, ordinary drilling methods do not work very well. Drilling bits get hot while drilling many holes at a time, and as there is no way to dissipate this heat fast enough, it melts the hole walls of the substrate. Not only does this compromise the quality of the hole wall, the hole may block up as the drill retracts. The fabricator cannot create proper vias from such blocked holes, threatening the quality of the PCB.
Fig 3: Use of Thermal Vias
For drilling holes in such special material, the fabricator must use special drill bits, increase the suction and vacuuming pressure, and increase the amount of drill cutting. They must also replace the regular Aluminum drill cover with a resin cover, which is more efficient in absorbing the heat from the drill bit. In addition, better lubrication of the drill bit also helps in improving drilling quality
Prior to use, all completed circuit boards must undergo testing. For high-speed and high-frequency multi-layer boards, the testing must focus on solderability and thermal stress. Solderability of such boards must comply with IPC J STD 003B:2007A1, while for thermal stress the boards must comply with IPC TM 650 2.6.8: 2004.