Laminate Materials for PCBs develop quickly
2018-11-23 17:31Writer: qyadminReading:
New communications, networking and computing products continue to push the limits of bandwidth and transmission speeds in printed circuit boards. Ongoing miniaturization of electronic equipment and its circuitry, increasing data traffic rates, the rapid compression of development cycle times are all pushing the PCB industry to improve performance. At the same time, product and printed circuit board cost cannot be ignored. Design engineers are challenged to find solutions for increasing speed and signal integrity in PCBs, while keeping product costs under control. Fortunately, there is a continuing evolution of laminate materials, providing significant improvements in speed and signal integrity, at lower costs.
New materials targeting improved cost as well as improved performance are increasingly being introduced. A number of these efforts are focused on reducing costs at the current performance levels (see Fig 1), and other entries are designed to improve performance while attempting to stay at par with existing price levels (see Fig 2). These performance-enhanced issuances fall into the spaces between traditional class groupings, creating a more continuous suite of laminate offerings. Now, there are more tailored choice options for any specific design application rather than having to select either an over or under laminate available for specification.
The number of companies producing higher speed materials has been rapidly growing. Most higher speed materials came from the US and Japan in the early 2000's. Today, materials are being sourced from Korea, Taiwan, Hong Kong, Singapore, and China. Depending on the specific manufacturer, their experience history and the duration an offering has been in the market, the companies and materials may not have the same type of maturity and application knowledge the industry is accustomed to, Despite this disadvantage, new laminate companies continue to enter the market providing many materials of limited data and background. A large sampling of laminates are plotted for S21 loss at 10GHz in Figure 3, which shows there are many competitive materials today with only incremental performance differences, providing a nearly linear slope of options from top to bottom.
FR4 Class Laminates
When discussing PCB materials, FR4 is often used as a baseline for comparison of material behaviours and especially of processability. FR4 continues to persist into product builds for both its thermal-mechanical strength (a valuable attribute for HDI fabs) and, of course, for its relative low cost. To continue to be in play, FR4 also needs to provide acceptable limits of electrical performance coherent with transmission rates of interest. This has not been a strong attribute of FR4 systems, and Sanmina's experience points to a maximum usable datarate of around 5-6 Gbps depending on the design trace widths and routing lengths. The major issue of using FR4 is usually its loss tangent (Df). But there have been some industry trends mitigating the loss aspect of FR4, extending its potential use.
Pressure on laminate manufacturers by OEMs which started about the time of the then new requirement to withstand leadfree assembly solder excursions, lead to other enhancements and created a sub-class within a class: FR4 resins modified and/or blended in a manner that resulted in some achieving as much as 1/3rd reductions in rated Df compared to traditional FR4s. These offerings continue in the market as they present Df improvements at measurably less cost than mid Dk/Df class entrants, although these modified FR4s do not enjoy much, if any, of the dielectric constant improvements of the mid Dk/Df class.
The impact of halogen-free on the Df of FR4 is another consideration.The change is in the flame retardant to non-brominated replacements that incidentally require lower content within the resin for effectiveness, that being the ability to conform and certify to UL94VO flame. This flame retardant change has measurable impact on Df. Companies have taken note of halogen-free FR4s, with Df ratings in the range of 0.010- 0.016 and at measurably lower cost than many of the mid-Dk/Df class materials. Ultimately, the mid Dk/Df materials have loss tangents available that are lower still, well down into the 0.008 range, but at an appreciable cost premium.
Financial budgets, along with loss budgets, dictate the want and feasibility of using lowered Df FR4's as opposed to opting into a mid Dk/Df material. Cost is always a focal point, so most designs opt for the least costly laminate that will deliver the performance attributes needed for that design application. Using over-engineered material for a design application is often the fault of market offerings forcing designs to over-buy on unneeded attributes in order to acquire the needed ones. As the material market continues to expand its class offerings, using over-engineered materials becomes less necessary. This trend is further demonstrated by the limited but available option of acquiring select leadfree FR4 items on spreadweave glass styles. As the primary benefit of these styles has been mitigation of ZDifferential timing skew, these FR4 offerings are targeting a specific performance aspect, whereas before these premium glass styles were only available bundled with higher performance resin systems.
Lower profile copper options (those smoother than RTF) have not really appeared in FR4s. Sanmina believes this is because greater trace loss benefit can be realised by investing in other, lower Df resin systems rather than using cost premiums to clad relatively lossy FR4 with finely surfaced coppers. Leadfree FR4s are also regularly used in high speed applications as part of stackup hybridisation, reducing cost by being relegated to power, ground, analog and other non-critical layers, while often improving the overall thermal-mechanical performance compared to all high performance material stackups.
Mid Dk/Df Class Laminates
There is currently an even a more dynamic mid Dk/Df market. Mid Dk/Df is defined here as having a dielectric constant measurably lower than FR4, meaning at or less than 4.0 and a Df rating in the range of 0.007-0.013, with a number of entries rated at or below 0.010. This class of materials has been around since the issuance of GE's "Getek" circa 1988, which was followed by Park-Nelco's N4000-13 and its equivalent competitor materials. Nelco also added low Dk/Df options to N4000-13, as did a number of its competitors over the course of time.
Changes to this class started accelerating once ROHS dictates were being adopted. These are highly modified epoxy-based systems, some blended with cyanate
esters, PPEs, PPOs and other proprietary content to achieve their performance targets. The now long, complex molecules were not proving very adaptable to the elevated temperatures of leadfree assembly and their impact on CTE-Z. Modifications were needed and established mid Dk/Df players responded, but only incrementally to protect their existing resin UL qualifications, which would be lost and require a new qualification cycle if UL Labs were to determine a resin had changed to the degree of designating it as a new system. This issue hamstrings the laminators in industry-needed efforts to improve resin systems. Once a resin system formula has been locked down, it takes around a year for a laminator to get that new product through laminate UL, and the multi-months-long fabricator UL qualification process extends that even further. So, time to market and return on investment pressures can run counter to the need to advance a resin system beyond a limited scope despite the need for a fuller re-invention.
Although the market has been able to manage using evolving technology for a few years, HDI has become more complicated: BGA pitches smaller, and plated over via in pad and sequentially laminated build-up boards designs are much more prevalent today. The need to cross that UL line is now evident and several laminate suppliers have responded with new, more robust and performance-improved systems. Not only are they addressing the thermal-mechanical improvements needed, they have also been able to chip away at bettering Df values and have moved into more expansive options of spread weaves and copper smoothness offerings, as well as leveraging new supplier sources for lower Dk/Df glass types.
These changes have enabled another new class within a class, a spate of mid Dk/Df materials measurably above the traditional ones but still situated below the cost and performance absolutes of the ultra premium low Dk/Df class. The traditional mid Dk/Df materials were showing some good use in the 5-10 Gbps space but running short of really supporting the upper end of that range, forcing some designs into over-specifying of materials until these new issues appearing in the market. The new mid Dk/Df+ class is targeting use as fast as 12.5 Gbps depending on trace widths and routing lengths. And this has put pressure on the low Dk/Df class of materials to either reduce their cost as they are or create better performance versions to satisfy the ramping development of 20-25 Gbps and faster designs. Both things are occurring.
Low Dk/Df Class Laminates
To get a better understanding of what is commonly referred to as the low Dk/Df class of materials, we define them as having Dk values at or below 3.7 and Df ratings at or below 0.005. A number of the mid Dk/Df materials can easily make this Dk range if enhanced with low Dk glass, but not while also in the range of Df. These two values together then define a distinct class. The present day standard bearers are Panasonic's Megtron-6 and, to a lesser extent, the Rogers 4000 series of ceramic hydrocarbons. But there are more participants today and many more in the queue and coming. The thermal-mechanical behavior of these materials is always important as they are often relegated to very complex, HDI-type designs and so must endure multiple lamination cycles at leadfree temperatures, which is challenging as it is quite difficult for laminators to concoct complex molecules that deliver very high electrical performance while also being mechanically stable and robust. All the materials of this class have proprietary resin formulations so it is not possible to know all of what may be in their content, although at the farthest end most have measurable content of PTFE (e.g. Rogers Duroids, Taconic, etc.).
Cost always plays a big part in any product design so the PTFE-laden materials are still limited players, although we're seeing some very large platform boards experimenting with PTFE based laminates for the sake of mitigating attenuation over traces on the order of about a meter. But these are 25+ Gbps R&D designs and not what is being built regularly today. Product in the 8-12.5 Gbps space has given Megtron-6 much of its market. As targeted transmission rates have been increasing toward 20-25+Gbps the need for even lower loss materials has become apparent, and most preferably ones that do not have the high cost and processing challenges of PTFEs. A number of these have been introduced in recent quarters and all are under evaluation. Performance claims are just that, claims, making independent testing and measurement always warranted. We are especially interested in seeing how any high-end materials behave as testing clicks through frequencies of, 5, 10, 12.5 GHz or more and how well they survive the rigors of modern HDI fabrication and assembly processes. Also, former background testing is often front and centre today with requirements of CAF, insulation resistance and the like being demand items and significant filters of acceptance for a number of OEM industries.
As a fabricator we are very encouraged to see such a huge flurry of new and promising laminates entering the market and are optimistic laminators will continue providing answers to the pressing needs and demands of signal integrity and speed.