Highly Reliable Substrates and CCL with Aluminium Monohydrate as a Halogen Free Co Flame Retardant

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1 Phosphorus, Inorganic & Nitrogen Flame Retardants Association E&E Workshop Subject ( 議題 ) Highly Reliable Substrates and CCL with Aluminium Monohydrate as a Halogen Free Co Flame Retardant Presenter with biographies ( 発表者及び経歴 ) Carsten Ihmels studied chemistry at the University of Oldenburg where he received his Diploma in 2000 after a one year research period at the University of Cape Town in South Africa. Subsequently he did his PhD in Polymerization Technology at the Technical University of Berlin and joined the Division Functional Fillers of Nabaltec AG in Today, he is responsible for the R&D of thermoset applications and as regional manager in Sales and Technical Services for Japan, Taiwan, and South Korea (cihmels@nabaltec.de). Abstract( 要旨 ) The implementation of the European Directive on Restriction of Hazardous Substances in E&E (RoHS, 2002/95/EC) had a tremendous influence on the PCB industry because it bans products that contain lead (Pb) since July The result is a higher soldering temperature during PCB assembly which impacts on the heat stability of the materials in use, and generated the need for new technical solutions. Many CCL manufacturers have taken the opportunity to study the adoption of halogen-free flame retardants (FR s) when developing new materials to improve the thermal stability of their products. As a new trend the synergistic use of different flame retardants is under close investigation. This paper shows the benefits when utilizing phosphorus based flame retardants like DOPO (9,10-Dihydro-9-oxa-10-phosphaphenanthrene-10-oxide) in combination with highly temperature resistant Aluminium Monohydrate (AlOOH, boehmite ) to generate synergistic effects in CCL/PCB applications. The use of Aluminium Monohydrate enables to gain the required thermal stability of the end product not only during manufacture but as well during service life (e.g. under elevated usage temperatures such as automotive). Crucial parameters like flame retardancy and thermal stability are discussed. Ultimately a higher reliability of the final CCL can be obtained, in some cases with a drastic reduction of formulation costs. Avenue E. Van Nieuwenhuyse 4, B-1160 Brussels - pinfa@cefic.be -

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4 In the past the flame retardancy in brominated systems was reached solely through resin components, whereas today curing agents and fillers with flame retardant effects are used complementary in non-halogenated systems. 3

5 DOPO containing systems are established as the technical standard of resin-based flame retardancy. In addition there are also nitrogenous systems or blends from Phosphorous and Nitrogen. A candidate based on nitrogen chemistry is for example the Benzoxazine-system, which contains excellent dielectrical properties. 4

6 DOPO (9,10-dihydro-9-oxa-10-phosphaphene anthrene-10-oxide) with a phosphorous content of 14 % has technological taken root for years, due to the fact of its high flame retardant effect by similar application quantities like the standard TBBPA. Like the latter, DOPO is a reactive flame retardant, which is directly tied to the polymer backbone und should therefore exhibit a low tendency to migration. However, because of the relatively low production capacities the availability is limited and the market price is quite high. Though, currently further capacities are being created, especially in Asia. Due to its reaction with the epoxy group of the resin, it also influences the resins properties negatively. Through the addition of the DOPO-molecule the epoxy is partly saturated, which causes a significant reduction of the glass transition temperature. Under the modern processing conditions of leadfree soldering, this is extremly undesirable. Futhermore the brittleness of the resin is raised significantly. In addition to that, the DOPO, like many phosphor-based flame retardants, has a strong tendency to bind water, which in turn has a negative effect on the water absorption tendency of the laminates. Ultimately the high price of this system is an obstacle for the commercialization of such products. In consequence of this insufficient price-performance ratio of these first generation halogen-free laminates, they could never be established in the market. Thus the market share of halogen-free raw materials in the 90s and 2000s was under 10 %. Since the banishment of lead from all soldering baths and the correspondingly higher temperatures in the soldering process such halogen-free systems haven an even weaker position. 5

7 In present non-halogenated systems curing agents and fillers with flame retardant properties can also be used in addition to the resin aspect. These multiple combination possibilities give the developer considerably more freedom to optimize his formulations technically, as well as in terms of costs, for different main applications. 6

8 Besides the wide-spread aluminium hydroxide, the thermally stabilised products magnesium hydroxide (MDH) and boehmite (AOH) rate among the mineral based flame retardants. Whereas all 3 products are available synthetically, only the MDH exists naturally in a for the mining sufficent purity. Magnesium hydroxide isn t compatible with the epoxy resins used in circuit boards because of its high alkalinity and is therefore not used in for industrial purposes. 7

9 The advantages of the application of AOH instead of ATH will be explained in the following slides 8

10 A comparision of the thermal stability of AOH versus ATH. Temperature in this experiment was ramped up at 10 K/min to 250 C and kept constant at this temperature level which equals to realistic temperature during lead free soldering. It is clearly visible that the ATH starts to decompose quickly just after reaching temperature above 200 C while AOH is not reacting to such low temperatures at all. 9

11 Under the common conditions of lead-free soldering with periods in minute range above it s decomposition temperature ATH will disintegrate quickly. Moreover the critical release of water is intensified, since normally there isn t only one but many thermal strains during the production process of circuit boards especially during the assembling. 10

12 In the following the consequences of the water release will be elaborated 11

13 This page demonstrates the reason for the failure of PCBs especially after multiple thermal stress during the assembly process. Different thermal expansion of the various materials used in a CCL can cause very different kinds of failures that can result finally in the total damage of the PCB. That is why today under conditions of lead-free soldering the control of the thermal expansion is extremely important. 12

14 Even in the case an ATH containing PCB survives the assembly process without a visible failure: The water released by the decomposed ATH stays in the resin matrix and can condense in the micro cavities along the glass fibers. Here the water can act as electrolyte of the electrochemical reaction of a filament growth (CAF) from anode to cathode causing a short circuit and failure of the electronic device. This process can occur as well in absence of water but is massively accelerated in its presence. Thus, the usage of ATH can result in reduced life time of the electronic device as the released water catalyzes the formation of CAF. 13

15 So the use of ATH also leads to elevated production costs through lower yields because of delamination and unreliable products with a reduced life span. 14

16 A description of the cost effectiveness across different applications. Applications with the highest reliability are particular fruitful. The high thermal stability of the raw materials used is necessary, in order that a PCB can survive the high loads during the manufacturing process of an electronic component ( survivability ). A high thermal load during the production respectively more important during the utilization influences the durability of the component in its end-use application ( reliability ). The demands on the quality of the applied materials depend on the end-use application, whereby the used materials affect the reliability and durability of the final product. The adjacent illustrations should clarify this relation. The survivability during the soldering process has priority for mass products with a relatively short life, while the requirements for the telecommunications industry, automobile industry and military implementation are much higher. Highest requirements concerning the reliability are important for the sectors aviation, astronautics and army, because a high durability at least 20 years is expected. Consequently some circuit board manufacturers demand from their suppliers an entirely absence of aluminium hydroxide in laminates, in order to avoid serious manufacturing and quality troubles. Besides the high thermal loads during the manufacturing process, hightemperature stable circuit boards are ideal in applications with an enhanced longterm use temperature. This can be of peculiar interest in the automotive industry, where the reliability of the electronic in applications in the engine bay is affected due to the high temperatures (long-term use temperature of 150 C and counting). Highest reliability of the systems during high long-term temperatures is increasingly in demand in modern power electronics (for example electromobiles). Excellent electrical, mechanical and economical results could also be achieved for high temperature requirements circuit boards with specific temperature stable boehmites. 15

17 During soldering the PCBs are omitted to very high thermal stress. This causes the various materials and components to expand differently and especially the epoxy resin expands much stronger than the glass or copper. By the usage of fillers in the resin matrix the resin content can be reduced strongly and consequently the expansion of the resin can be reduced strongly to prevent barrel cracking and circuit breakage. Boehmite has excellent filler characteristics and resin compatibility and offers even at high filling levels good resin flow properties that lead to high laminate adhesion and copper peel strength after lamination and metallization. Utilizing Boehmite enables to produce outstanding CTE values as low as 30 ppm in FR4 or even below 10 ppm in case of packaging materials (values from room temperature to Tg). In contrast to other fillers like silica or alumina, the flame retardancy function of Boehmite comes on top of this outstanding filler performance. 16

18 The flame retardant effect of the AOH is notably lower than of the other metal hydrates due to the lower content of crystal water. Considerable researches have shown, that boehmite exhibits synergistic flame retardants attributes together with other FSM, like metal hydrates and phosphorous FR. This synergistic effect is especially strong in combination with DOPO. This effect is demonstrated in the example above, in which an epoxy novolak is equipped with different flame retardants to achieve UL94V0 classification. In this connection the necessary overall content of the pricy DOPO could be reduced at about 45 % by the use of a combination of APYRAL AOH30 with DOPO. In consequence of the application of the phosphoric flame retardant DOPO, the Tg of the Epoxy was reduced severely. Boehmite, with its above mentioned advantages, contributes to reduce the necessary amount of halogen-free, organic flame retardants dramatically, without losses by the flame retardants effect. Besides the advantages of the AOH, with its excellent filler qualities on the temperature stability and thermo-mechanic qualities, the reduction of the P-FSM also influences the cost of the overall chemical formulation. As a consequence the application of the boehmite allows a considerably cost reduction. The raw material expenses of the resin formulation could be reduced by approximately 40 % in the above mentioned example. Halogen-free CCL allows cost reductions up to 20 %. The combination of excellent technical qualities and low expenses open halogen-free systems for the first time new market possibilities. 17

19 The application of AOH is regardless appropriate for high thermal loads (multiple soldering) and extreme operating conditions due to the high T- stability up to 340 C. The excellent filler attributes permit the implementation of very low CTEvalues and high coating bonds and copper adhesion. In consequence of the partly replacement of high-priced organic FSM due to the synergistic flame retardants with P-FSM, higher Tg and radically reduced chemical formulation cost are possible. In case of fire the known benefits of halogen-free systems like reduction of smoke, which allows superior getaway conditions and the prevention of toxic and corrosive gas generation are valid further on. The latter reduces the fire damages. Due to the wide product range including very small particle size down to 300 nm, ultra-thin applications (such as substrates for semiconductor applications) are possible. 18

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