ECWC 2014: The Base Materials Session
A highlight of the second day of the 13th Electronic Circuits World Convention (ECWC) in Nuremberg, Germany, was the session on base materials, with presentations exploring the realities of fire retardancy and flame retardant additives, the introduction of a new low-loss laminate, and a study of the mechanical properties of resins. The session was a introduced and moderated by Dr. Viktor Tierderle of RELNETyX.
Alun Morgan, director of OEM Marketing for Isola Group Europe, gave the first presentation, a well-researched paper entitled “Fire Retardancy – The What, Why and How,” which made a critical examination of all aspects of fire retardancy, from the need for fire safety, through the functions of flame retardants, the chemical and physical mechanisms of reactive and additive flame retardants, halogenated and halogen-free, to toxicological, environmental, and legislative issues.
“What are flame retardants and why do we need them?” Morgan began with some scary video clips showing how rapidly a fire could take hold and explained that flame retardants were compounds added to materials during or after manufacture to inhibit or suppress the combustion process. On average there were more than 4,500 fatalities annually in the EU-27 as a result of fires; accounting for 2% of all fatal injuries. The primary purpose of a flame retardant was to reduce the risk of fire related fatalities and the presence of flame retardants in otherwise combustible materials had the effect of either preventing the fire from developing altogether or slowing down the build-up phase of the fire and extending the escape time.
Flame retardants could work by either physical or chemical action. There were three classes of physical action: Cooling, formation of a protective layer, and dilution. Chemical action could take the form of reaction in the solid phase or reaction in the gas phase.
Halogenated flame retardants were the most widely used in printed circuit board materials, the effective halogen being bromine, chemically incorporated into the resin as tetrabromobisphenol-A (TBBPA). The most popular non-halogenated flame retardants were phosphorous-containing compounds of which the most widely used was dihydro-oxa-phosphaphenanthrene-oxide (DOPO), again chemically incorporated into the resin. Morgan described the chemical breakdown mechanisms which enabled these materials, halogenated and non-halogenated, to interfere with the combustion process.
“Halogen-free” was somewhat of a misnomer, although in a PCB materials context it effectively meant “free of halogenated flame retardant.” The IEC definition of halogen free permitted 1,500 ppm total halogens, with no more than 900 ppm of either chlorine or bromine. Halogens were always present to some extent in epoxy resin, as residues from chemical precursors and catalysts used in its manufacture.
There had been many studies into the toxicology of flame retardants. Although it was obviously important to protect people from death or serious injury by fire, there was concern that the materials used to provide that protection did not present risks to human health or the environment. TBBPA, the common brominated flame retardant in PCB materials, had been studied exhaustively and there had been no evidence of risk to human health from toxic gases resulting from combustion, and no clear scientific justification for restricting the use of halogenated flame retardants. Studies in relation to REACH and RoHS legislation had not raised concerns about the continued use of halogenated flame retardants, although ultimately consumer choice would continue to be a significant factor whatever the science indicated.
Morgan concluded by quoting Neils Bohr: “Prediction is very difficult, especially if it's about the future,” but did predict that fire safety would remain a primary requirement for electronic and electrical equipment and to ensure fire safety there would always be a need to incorporate flame retardants into plastic systems in electronic and electrical equipment that would otherwise pose a significant fire safety risk.
Morgan’s presentation had been principally focused on flame retardants chemically incorporated into the epoxy polymer. Carsten Ihmels from Nabaltec described the use of metal hydrides and oxides as functional fillers, to improve the flame retardancy, thermal stability, thermal expansion, thermal conductivity, and processability of thermosetting resins.
Ihmels suggested the use of combinations of functional fillers and organic flame retardants to achieve the considerable advantages of a highly flame retardant polymer formulation with a good balance of cost, electrical, thermo-mechanical, and processing properties. He discussed the effects of combining metal hydrates and other fillers together with various halogen-free organic flame retardants on parameters like flame retardancy, thermal stability, thermal expansion, thermal conductivity, and processability.
Aluminium hydroxide had historically been popular as a halogen-free flame retardant filler for PCB laminates, decomposing at elevated temperature to release water and consume energy. But it was unstable at lead-free soldering temperatures and its use could lead to reliability problems such as delamination and CAF.
Boehmite, a crystalline form of aluminium oxide hydroxide available in a wide range of particle sizes, had a much higher decomposition temperature and could be used as a filler in thermally demanding applications to enable cost-effective manufacture of highly thermally stable aluminium hydroxide-free prepregs and laminates with excellent mechanical and thermal properties. Being a mineral filler, Boehmite could be used to dramatically reduce Z-axis thermal expansion of the laminate, and by partial or total replacement of commonly used fillers like silica or quartz, below-Tg CTE values as low as to 30ppm could be achieved. Additionally, using boehmite as a filler offered significant improvement in thermal conductivity for applications like substrates for LED lighting. The technical advantages of higher heat resistance combined with high reliability at lower cost opened new market opportunities for halogen-free materials in automotive, military and avionics. Even high quality consumer electronics could be successfully produced at competitive cost with high yield.
The focus of the session turned to new base materials as Lin Lin, from the Technology Development Centre at Panasonic, introduced a newly-developed ultra-low-loss laminate for high-speed and high-frequency PCBs.
The exponential trend in information and communications technology, for example the projected 61% compound annual growth rate in global mobile data traffic, necessitating the high-speed transmission of large-volume data using the minimum of electrical energy, had created a demand for halogen-free low-loss laminates suitable for fabricating high layer count PCBs with high heat resistance, low CTE, high thickness accuracy, and suitability for lead-free soldering
Of the many laminates previously developed for use in high-speed and high-frequency applications, only woven-glass ceramic-filled PTFE materials had the right dielectric properties to meet performance requirements at GHz frequencies. But PTFE materials were difficult to laminate so their use was limited to low layer-count constructions, so Panasonic had set out to develop a new-generation material with equivalent dielectric properties but better heat resistance and processability, suitable for building high layer-count multilayers.
To achieve the desired dielectric loss characteristics, they had based their new material on a low-Df thermosetting polymer, a modified polyphenylene ether, with a low-Df, low-Dk hardener and a low-Df inorganic filler. The high crosslink density gave low CTE and good temperature resistance. When combined with a low-Dk glass cloth and a low- profile copper foil, the result was a novel high-performance material with excellent stability of dielectric properties within a wide frequency range, with ultra-low transmission loss and lower temperature drift than ceramic-filled PTFE. And the laminates and pre-pregs were eminently suitable for the fabrication of high layer-count HDI multilayer constructions. The new material was expected to be widely applied to next-generation high-speed and high-frequency applications such as network equipment, base stations and high frequency parts. The material had high Tg, low moisture absorption and good heat resistance, making it compatible with lead-free soldering processes.
Not all electronics-grade resins were used in laminate manufacture. The final presentation of the base materials session gave an insight into alternative applications of resins at the next level, in impregnating, encapsulating, and potting of transformers and modules. Jens Buerger from ELANTAS Beck, specialists in the protection of electronic components in the automotive, industrial, medical, and avionics sectors, reported the results of a study of the impact of linear coefficient of expansion on temperature shock resistance.
There were three main categories of electronic protection: thin-film coatings applied by dip or spray were mainly used for moisture protection of PCBs. Thick film coatings, typically applied by dam-and-fill, swirl or dip, were used for major protection of PCBs from chemical attack and vibration. Encapsulation or potting, by dispensing or vacuum potting, gave chemical, mechanical and vibration protection, as well as sealing the assembly.
Typical encapsulation materials were epoxy, polyol or silicone resins, with fillers to improve mechanical properties, thermal conductivity and flame retardancy, and additives for such purposes as reducing viscosity, degassing, improving adhesion, or humidity protection. The temperature shock resistance of these materials was a function of coefficient of thermal expansion, glass transition temperature, and elastic modulus. Frequently, the customer requested a material with a low elastic modulus, but also wanted a low coefficient of thermal expansion, which tended to be conflicting requirements. Additionally there were market requirements for materials with glass transition temperatures outside the operating temperature range, which further limited the choice of available material an could result in significant increase in cost.
The low elasticity of metal packages compared with plastic packages could provoke early failure of the potting compound, or the module itself, under thermal shock conditions. Buerger described testing procedures, programme cycles, temperature ranges, rates of change of temperature, and hold times, which could meaningfully evaluate the reliability of potting compounds under thermal cycling and thermal shock conditions. He also reviewed the causes and sources of processing defects, many of which could be traced back to the effects of poor storage conditions on the usable shelf life of the material.