It is thanks to the current flame retardant technologies, that electronic components, which regulate the complex programming of the plethora of new and evolving products in our environment, can operate safely within regulation guidelines.
All electronic devices now contain printed circuit boards , providing customers with intelligent and interactive services. Present in TVs, computers, mobile phones, washing machines and even refrigerators or coffee makers, they are typically made of plastic laminates with metal circuits, with soldered chips, components, cables and sockets.
There are many types of printed circuit boards, the most common being:
FR-2 is the grade designation for synthetic resin bonded paper, a composite material made of paper impregnated with a plasticized phenol formaldehyde resin, used in the manufacture of printed circuit boards. Until recently, FR-2 sheet was widely used to build low-end consumer electronic equipment.
FR-4 circuit boards are made of woven fibreglass cloth with an epoxy resin binder that is flame resistant (self-extinguishing). With near zero water absorption and considerable mechanical strength, FR-4 is most commonly used as an electrical insulator. The material must retain its high physical values and electrical insulating qualities in both dry and humid conditions. They also have to meet high flammability requirements (UL 94 V-1 a plastics flammability standard or higher). More than 90% of these circuit boards are based on brominated epoxies.
FR-4 High and ultra-high performance
Today’s high reliability battery-powered consumer electronics require highly dependable circuit boards, to maximise autonomy. This is even more important in such ultra-high end applications as computer servers, aerospace and military applications, which require especially demanding electrical properties (dielectric permittivity (Dk), loss tangent (Df). Currently, many of the circuit boards in these applications are based on polytetrafluoroethylene (PTFE). A variety of alternate resin systems are available, which may include brominated or phosphorus-based flame retardants.
Choosing the most suitable flame retardant solution needs to take into account multiple factors. One single criterion cannot be the basis for material selection.
Nowadays not only computers, but also “smart” mobile phones, TV sets, hi-fi systems and even some washing machines incorporate sophisticated microprocessors. Their frequencies are constantly increasing, resulting in demanding electrical performance and excellent signal integrity. Some critical electrical parameters are high-speed bus performance, dielectric permittivity (Dk), loss tangent (Df) and moisture absorption (specifically how it affects the former parameters).
These electrical properties are directly linked with the chemical composition of the polymers used in the circuit boards. The availability of best solutions for printed circuit boards is therefore a prerequisite for continued progress in electronics.
For example, electronic components are usually designed to perform best at a certain dielectric permittivity (3.6 < Dk < 3.9 are common values for the industry-standard brominated epoxies). The higher average dielectric permittivity of alternative formulations currently available (~4.0 < Dk < 5.2) can affect the functioning of highly sensitive electronic components such as microprocessors, with higher risk of failure for the end product.
The ban in 2006 on the use of lead in all electronic and electrical appliances as required by the EU RoHS Directive brought a considerable challenge for the production of printed circuit boards. Instead of lead-containing solder, new alloys are now used to connect the chips and sockets to the conductive layers. These new alloys have a higher melting point (on average 40°C to 50°C higher), which means that the polymer substrates used in printed circuit boards need to retain all their properties at higher soldering temperatures (typically 250°C to 360°C).
Moisture uptake and soldering reliability
Apart from its potential impact on electrical properties, moisture plays a key role in determining whether a material can withstand lead-free soldering conditions. At high temperatures, the vapour pressure of trapped moisture absorbed in the plastic laminate can lead to rapid deterioration (delamination), resulting in the printed circuit board and the attached electronic components being discarded even before useful service life begins. The less moisture present in the material and the lower moisture uptake potential, the better. Thus, moisture uptake and retention of the flame retardant is a major factor in its selection.
Flame retardants are expected to retain or slightly diminish parent resin glass transition temperatures. Glass transition temperatures (Tg), above 170°C and below 200°C are becoming most useful, as higher values often give brittleness to laminates. Brittleness can cause issues with pad detachment, poor drilling capability and more rapid wear on drill bits. Another factor influencing drill performance is the abrasiveness of the insoluble filler components.
Inter-laminar and copper peel strength is important, as high amounts of insoluble fillers can negatively impact inter-laminar peel strength. Copper surface texture and copper-resin adhesion also have a major impact on circuit board performance.
Laminates, as well as each component in the formula, are also being given higher thermal stability, with laminate decomposition temperatures (TGA 5 wt% loss values) - Td - above 350°C desired for higher-end applications.
Use of flame retardants
Standard practice since the early stages of the electronic age has been to use reactive flame retardants in printed circuit boards, i.e. to incorporate the flame retardant directly in the polymer structure. Once this process is complete, the flame retardant is covalently bound to the polymer. Reactive flame retardants provide optimal performance and are a sustainable solution for circuit boards.
These are by far, the most common solution in circuit boards laminates. Brominated epoxy resins are produced by the incorporation of Tetrabromobisphenol-A (TBBPA) as a monomer at the epoxy resin manufacturing stage. Brominated-epoxy-based FR-4 boards are the current industry standard.
They provide the best combination of mechanical properties, thermal stability, moisture uptake, electrical performance and cost effectiveness. Brominated epoxies also have low levels of failure during drilling and assembly operations, especially for multi-layer laminates.
In high and ultra-high end applications where low Dk and Df are required, additive brominated flame retardants perform better than the usual brominated epoxies based on TBBPA. This is especially true for computer servers, aerospace and military applications.
Phosphorous formulations and metal hydroxides
Besides bromine, epoxy resins can also incorporate phosphorus as a flame retardant. The most successful solution to date is DOPO (9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide). Due to the monofunctional nature of its structure, DOPO has to be reacted into specific multifunctional epoxies. About 6-7% of the FR-4 printed circuit boards currently on the market are partly based on this technology.
To meet fire safety requirements, phosphorous resins are used in combination with metal oxides such as Aluminium Tri-hydroxide (ATH) or Aluminium Oxide-hydroxide (AOH) – around 20-30% by weight loading – and sometimes with other additive flame retardants. These formulations have good thermal stability, but tend to face technical challenges, such as higher water uptake, increased brittleness, higher Dk, and higher failure rate at assembly stage, making them unsuitable for high-reliability battery-powered consumer electronics.
Metal phosphinates such as aluminium or zinc salts of diethylphosphinic acid (AlPi, ZnPi) have recently been tested in circuit boards. They are typically used as additives in combination with synergists, such as melamine polyphosphate.
With printed circuit boards, the interaction of metal hydroxides with fire (dilution of combustible gases via water release under thermal stress) can lead to the presence of water in the laminate during soldering operations, potentially increasing the assembly failure rate and affecting electrical properties. In practice, ATH is essentially used in intrinsically less-flammable flexible circuit boards for smaller devices like mobile phones or cameras.
1 Dielectric constant (Dk, or dielectric permittivity) determines the speed at which an electrical signal will travel in a dielectric material. Higher dielectric constants will result in slower signal propagation speed. Loss Tangent (Df, or dissipation factor) is a measure of how much of the power of a signal is lost as it passes along a transmission line on a dielectric material. These are determined by the inherent properties of the components of any specific resin system, although we normally refer to a “relative dielectric constant” because it is dependent on test method and frequency as well as material per-se.
2 Dr. Haley Fu, International Electronics Manufacturing Initiative (iNEMI) Taipei IMPACT conference, October 2009.
3 Glass transition temperature (Tg): The temperature at which the mechanical properties of a laminate begin to change rapidly. Glass Transition Temperature is just what its name suggests:
it is that temperature at which a material changes from a hard, brittle “glass-like” form to a softer, rubberlike consistency.
4 Brittleness: Oxidation is the principal mechanism by which printed circuit boards embrittle and turn brown as they sit or operate at high temperatures over a period of time. Above the Tg this process occurs more rapidly because of greater diffusion rates and more molecular motion.
5 Decomposition temperature (Td): This property varies greatly with the chemical composition of materials, from the mid 300°C range for many epoxy systems to over 400°C for some polyamides. Td is the temperature at which a material begins to degrade thermally.
Everything You Ever Wanted to Know About Laminates...but Were Afraid to Ask, Arlon-Materials for Electronics, November 2008.
6 R. Rajoo and E.H. Wong, Trends & Challenges of Environmentally Friendly Laminates, PC Fab, March 2003.