Environment-friendly Halogen-free Materials for PWBs

Environment-friendly Halogen-free Materials for PWBs Yoshiyuki Takeda*/Kenichi Ikeda*/Nozomu Takano** *R & D Group, Electronic Laminates Div., Hitachi Chemical Co., Ltd. **Research & Development Center, Hitachi Chemical Co., Ltd. Ibaraki, Japan Hikari Murai Hitachi Chemical Co. America, Ltd. ABSTRACT Recently, the requirement of the printed wiring boards (PWBs) of environmental harmony type rises rapidly. We have developed the core technologies of halogen-free. They consist of resin system technology and high filler content technology. The point of the resin technology is the development of a new resin system which takes nitrogen into the molecule frame in a large quantity. The point of the high filler content technology is the development of a new filler interphase control system (FICS) which enables the high dispersion of fillers. A variety of halogen-free substrates which can be applied to the diversified needs have been developed by combining these technologies. They are, thin la minate for the multi-layer PWBs, MCF-4000G, build-up material for high density interconnect (HDI), and MCL-E-679FG, high Tg laminate for the advanced plastic IC packages (PKGs) and PWBs. These materials have excellent heat-resistance, and are suitable for lead-free solder as well. The robustness towards temperature, humidity and frequency of those materials are better than that of current materials. The synthetic board design of the environmental harmony type becomes easy by combined use of these materials. We are also developing several multi-layer and build up materials, HDI for high frequency, high speed and high reliability application to meet the PWBs and advanced PKGs requirement for the near future. Introduction Recently, the environmental interests rise rapidly through the world. These discussions have changed the types of materials that manufacturers are allowed to use in their products. Our fundamental policy is Harmony with the environment. Halogenated compounds are widely used as a flame retardant for plastics. In the case of electronic equipment, halogenated flame retardant is used to make copper clad laminates, housings, and devices safer. Because they can extinguish fire, they help to prevent accidental fires. However, the dis cussion had started to regulate the use of materials containing halogen from the environmental control purposes. [ 1] About seven years ago, we began a research on variety of environmentally friendly alternatives to standard PWBs. Our goal was to eliminate halogen compounds from PWBs, yet improve upon their existing performance. In 1997, we succeeded in producing a PWBs in FR-1 grade that met these requirements. [ 2] PWBs are required to be thinner and integrated into higher density wiring pattern as PKGs are expanding rapidly. [ 3,4,5] Recently, the requirement of halogen-free substrates in PKGs is also increasing. Since then, we have been developing several other types of environmentally friendly or green substrate products. They are thin laminates for the multi-layer PWBs, build-up materials for HDI, high Tg laminates for the advanced PKGs and PWBs, and etc. Development Concept There are four concepts common to all of these materials: 1. Flammability satisfies UL94V-0 2. Halogen- free 3. Antimony- free 4. Red phosphorus- free Antimony and Red phosphorus are alternatives to halogens as flame retardant. However, Antimony has potential for safety hazard, and Red phosphorus has potential to burn itself. Thus, we decided to avoid using any of these 1 / 9

compounds. Lead-free solder is expected to be widely used in the near future for the environmental reasons. Lead-free soldering requires higher temperatures than conventional soldering. We have developed three types of new halogen-free materials., thin laminate for the multi-layer PWBs, has the high Tg and good heat resistance characteristics for lead-free solder. MCF-4000G, an insulation film with copper foil, has a low dielectric constant and can be handled well in the lay-up process. MCL-E-679FG, high Tg laminate for the advanced PKGs, has high Tg, high elastic modulus and low CTE. Core technologies of and MCF-4000G Generally speaking, BPA type epoxy resin is used as the base resin of conventional standard FR-4. BPA type epoxy resin itself is somewhat flammable. Therefore, conventional FR-4 needs much volume of flame retardant compounds. We have investigated new resin that has basically high inflammability. The point of this resin technology is the development of a new resin system which takes contained nitrogen in main molecular framework in a large quantity. It also has higher cross-linking structure. The properties of new base resin system that does not include any kinds of frame retardant compounds are shown in Table 1. Table 1. Properties of New base resin system Item Volume of aromatic components in the skeleton Flame retardant class Flammability Maximum combustion time Glass transition temp. (Tg) Condition - UL94 1.6mm TMA Unit wt% - sec o C Properties of cured resin New resin BPA type epoxy 60-70 40-50 V-1 HB 24 >60 150 120 Flexural modulus 25 C GPa 5.9 3.4 Water absorption PCT-20h wt% 1.1 3.8 Flammability of New resin system itself is UL 94V-1 level, that means it has the better inflammability than conventional BPA type epoxy resin. We selected New resin and by adding the several kinds of frame retardant compound, we have developed and MCF-4000G. General Properties General Properties of in comparison with those of conventional FR-4 are shown in Table 2. Item Tg CTE of Z direction <Tg (ppm/ o C) >Tg Tensile modulus (Young s modulus) Flexural modulus Water absorption Condition Conventional FR-4 TMA 140-150 o C 120-130 o C DMA 180-220 o C 150-160 o C TMA ( t 0.8-1.6mm) 40-60 150-220 50-70 200-300 25 o C 25GPa(23-26) 25GPa(23-26) 24GPa(23-26) 23GPa(22-25) 0.02-0.04% 0.05-0.07% E-24/50 + D-24/23 Peel strength 25 o C 1.2-1.4kN/m 1.4-1.6kN/m (18µm) 180 o C 0.6-0.7 0.2-0.3 Flammability UL94 2 V-0 / 9 V-0 Dk 1MHz 4.8-5.0 4.5-4.8

Table 2. General Properties has a higher Tg and a lower Coefficient of Thermal Expansion in Z direction than the conventional FR-4 laminate. This will result in higher through-hole reliability. Dielectric constant of is slightly higher than that of conventional FR-4. Heat Resistance for Lead-Free Solder The most remarkable property of is its high heat resistance. Heat resistance of is shown in Table 3. Layer count 2 Floating test 288 o C Floating 240-260 sec. Floating test 260 o C Floating 288 o C Floating >300 sec. 112-140 sec. 4 D-X/100 + 260 o C 20 sec.dip 3h OK Dipping test PCT + 260 o C 20sec. dip 1.5h OK Thermal stress test Condition 260 o C 10 Floating 5cycle OK 288 o C 10 Floating 5cycle OK Table 3. Heat resistance of for lead-free solder Conventional FR-4 <60 sec. >300 sec. <30 sec. 0.5h OK 0.5h OK 5cycle OK 2-3cycle OK After submitting various materials to a 6-hours PCT followed by heat shock treatment at 260 degrees Celsius, the conventional FR-4 laminate blistered, whereas remained unchanged. Even at 288 degrees Celsius, after PCT of 2hrs, didn t blister. Lead-free soldering requires higher temperatures than conventional soldering. We believe that is the ideal material for the lead-free solder applications. Robustness of Dielectric Property The comparison between dielectric properties of and those of conventional FR-4 are shown in Figure 1. 5.0 4.5 5.0 4.5 Dk 4.0 Conventional FR-4 3.5 3.0 (a) 0.0 1.0 2.0 3.0 4.0 5.0 Frequency (GHz) vs Frequency (at RT) 0.03 Conventional FR-4 Dk 4.0 Conventional FR-4 3.5 3.0 0.03 (c) -40-20 0 20 40 60 80 100 Tempereture ( o C) vs Temperature (at 1GHz) Df 0.02 Df 0.02 Conventional FR-4 0.01 (b) 0.00 0.0 1.0 2.0 3.0 4.0 5.0 Frequency (GHz) 0.01 (d) 0 3 / 9-40 -20 0 20 40 60 80 100 Tempereture ( o C)

Figure 1. Dielectric properties of Figure 1-(a) shows that dielectric constant of being higher than that of conventional type. When frequency increases in the GHz band, the Dk of conventional type decreases, while that of doesn t change significantly. In other words, has greater stability than the conventional type FR-4. Figure 1-(b) shows the s dissipation factor is only half that of conventional FR-4 s. So, delivers comparable stability with lower transmission loss. Figure 1-(c) shows that, as temperature increases, Dk of conventional FR-4 also increases, while that of remains essentially unchanged. We believe this stability is a highly desirable characteristic. Figure 1-(d) shows that has two clear advantages over conventional FR-4. First, it has a much lower dissipation factor than conventional FR-4. Second, dissipation factor of remains stable as temperature increases than that of conventional FR-4. Thus, has well balanced properties and good robustness towards temperature, humidity, and frequency. has been in mass-production and it has been evaluated and being used for PWBs in notebook personal computers, cellular phones etc. MCF-4000G General Properties MCF-4000G is an insulation film with copper and it is used in surface layer of build-up materials for HDI. General Properties of MCF-4000G are shown in Table 4. Item Condition Flammability 4 Layers Elastic Modulus 50 o C 250 o C CTE <Tg Tg DMA Solder Heat Resistance 288 o C, float 2ply Peel Strength 18µm Dk Df 1GHz Table 4.General Properties of MCF-4000G Unit - GPa ppm/ o C o C sec kn/m - MCF-4000G V-0 4-5 0.1-0.2 30-40 150-160 50-60 0.9-1.0 3.6-3.7 0.014-0.017 General properties of MCF-4000G are equivalent or better than those of FR-4. Dk and Df of MCF-4000G are better than those of FR-4. MCF-4000G has excellent laser drilling processability. Core Technologies of MCL-E-679FG for advanced PKG and High density PWBs The PWBs performance has mainly been achieved by the use of organic resin. However, the organic resin does not have good inflammability. Therefore, the less organic content, the better inflammability are. And the combination of the organic and the inorganic materials becomes important to cope with the future s more severe requirements. "Interphase" will be defined to have a certain thickness of interfacial layer. The interphase control technology is very important in order to combine both inorganic and organic materials efficiently (Figure 2). 4 / 9

PWB /High modulus /Low CTE & warpage /Equipment workability /Good hand ring Glass fiber Epoxy resin /High Tg /Low water absorption /High heat resistance /Lowe, tand Filler /Low water absorption /High modulus, hardness /Lowe, tand, CTE /High heat resistance /High radiation Figure 2. Laminate system for PWBs The Filler Interphase Control System (FICS) has been newly developed with the advantage of high volume contents and good dispersing characteristics of the fillers [ 10], obtained by using the original treatment of the interphase between matrix resin and filler. Technical Concept of FICS In case the filler is added to the matrix resin (solution) with high contents, the laminating processability declines and the micro structural defects remain in cured resin. Because, the aggregation of filler and high viscosity of the resin varnish result in increasing moisture absorption and it cause to decreas e both insulation resistance and mechanical strength of laminate [ 11]. This effect becomes more serious in higher contents of the filler, and it is difficult to put the higher filler content composite into practical use even though surface treatment with the conventional coupling agents is applied. Figure 3 shows the technical concept of FICS. FICS Conventional Filler Original Treatment (High Dispersion & Good adhesion)? Low CTE? High modulus? Low water absorption Decrease of organic resin (Inorganic content : 70vol%) Figure 3. Filller Interphase control system (FICS) Aggregation? Increase of water absorption? The decline of the electric insulation & CAF restraining property It has been believed to be impossible to have higher filling ratio (80 vol. %) put into the resin. However, we achieved this by forming a unique interfacial treatment layer which is controlled by the balance of hydrophilic /hydrophobic or the amount of heat-resistance structure. The result in high dispersion of filler, the substrate has good adhesion of intrephase and high heat-resistance. 5 / 9

Figure 4 shows the viscosity of 60 vol. % filled resin varnishes with and without the application of the novel interfacial treatment. According to Figure 4, it is easily understood that the forming of a unique interfacial Varnish viscosity Pa s 3.0 2.5 2.0 1.5 1.0 0.5 Un-treated Treated 0.0 0 10 20 30 40 50 60 70 treatment layer on the filler can reduce the varnish viscosity effectively. Figure 4. Viscosity of vanish Filler content:60vol% Revolution per min. rpm This has successfully provided the laminates (Figure 5) without any defect due to filler aggregation. Glass fiber Filler Cross section of laminate Resin layer Figure 5. Cross section of laminate Figure 6 shows the heating loss in weight of fillers. 100 95 90 Treated (FICS) 85 80 Un-treated 75 70 65 60 200 225 250 275 300 325 350 375 400 6 / 9

Figure 6. Heat resistance of treated filler The temperature of the heating loss rises almost 30 degrees by the treatment of FICS, which can make the composite higher in heat performance. This advantage in the heat resistance is enough to meet the high temperature reflow process with lead-free solder. General Properties of MCL-E-679FG We have developed MCL-E-679FG by using FICS. General properties of MCL-E-679FG is shown in Table 5. Table 5. General properties of MCL-E-679FG MCL-E-679FG (Sample) High Tg FR-4 (Br cont.) GPY (Br cont.) Flammability - UL94V-0 UL94V-0 UL94V-0 X(Tg>)(ppm/ o C) 13-14 14-16 15-16 CTE Z(Tg>)(ppm/ o C) 20-30 45-55 45-55 Z(Tg<)(ppm/ o C) 100-120 230-250 230-250 Tg TMA( o C) 160-170 173-183 170-180 Flexural Modulus Barcol hardness Water absorption Item (GPa) 27-30 22-24 22-24 200 o C 35-45 25-35 25-35 PCT5h(wt%) 0.40-0.50 0.55-0.65 0.60-0.70 Heat resistance 288 o C/20s dip PCT5h PCT1-2h PCT1-2h Solder float 260 o C with Copper > 300s > 300s > 300s Peel strength 18um (kn/m) 0.9-1.1 1.4-1.5 1.3-1.4 Dk 4.8-4.9 4.6-4.8 4.2-4.3 1MHz Df 0.009-0.010 0.014-0.015 0.011-0.012 The total volume of inorganic component in PWB substrate is more than 80 vol. %. The substrates with inorganic component ratio 80 vol. % have the excellent characteristics. MCL-E-679FG is halogen-free and it has the following advantages in comparison with the standard materials for PKG application. 1. High elastic modulus (1.1-1.4 times that of conventional High Tg substrates) 2. Low water absorption (50-75% that of conventional High Tg substrates) 3. Low CTE (x,y,z) (60-75% that of conventional High Tg substrates) 4. Excellent surface flatness (25-50% that of conventional High Tg substrates) 5. High heat resistance 6. Low dissipation factor (Df) Bacol hardness of MCL-E-679FG is shown in figure 7. Water absorption of MCL-E-679FG is shown in figure 8. 7 / 9

Barcol hardness (t0.4) 90 80 70 60 50 40 30 20 FR-4 10 E-679FG High Tg FR-4 GPY 0 0 50 100 150 200 250 Temperature ( o C) Water absorption (wt%) 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 (t0.4) FR-4 High Tg FR-4 GPY E-679FG 0 1 2 3 4 5 6 Treating time (h, PCT) Figure 7 Bacol hardness of MCL-E-679FG Figure 8 Water absorption of MCL-E-679FG MCL-E-679FG has high elastic modulus. And, Bacol hardness of MCL-E-679FG is also higher than that of standard high Tg materials from room temperature to 200 degree Celsius. MCL-E-679FG has low water absorption by PCT treatment. Therefore, it has excellent robustness of warpage property and dimensional stability in PKGs and PWBs manufacturing process. In wire -bonding or flip -chip process, it will show good results. It also has excellent processability for mechanical drilling and laser drilling. It is high heat resistance, we believe it will be the ideal material for the lead-free solder application. Thus, it is suitable for high density and thin thickness advanced PKGs and PWBs. Road Map of Base Materials The synthetic board design of the environmental harmony type becomes easy by combined use of these materials. We are also developing several multi-layer and build up materials, HDI for high frequency, high speed and high reliability application to meet the PWBs and advanced PKGs requirement for the near future. All base materials must meet four criteria: 1. High performance at a low cost 2. Excellent reliability 3. Highest possible speed 4. Lowest possible impact on the environment Conclusion 1. The core technologies of halogen-free flame retardant system have been developed.. 2. The point of the resin technology is the development of a new resin system which takes nitrogen into the molecular frame in a large quantity. 3. The point of the high filler content technology is the development of a new filler interphase control system (FICS) which enables the high dispersion of fillers. 4. Our halogen-free materials are, thin laminate for the multi-layer PWBs, MCF-4000G, build-up material for high density interconnect (HDI), and MCL-E-679FG, high Tg laminate for the advanced PKGs and PWBs.. 5. These materials have excellent heat-resistance, and are suitable for lead free solder as well. The robustness of those materials are better than that of current materials. 6. The synthetic board design of the environment harmony type becomes easy by combined use of these materials. 8 / 9

References [ 1] O.Hutzinger: "Formation of poly -bromoinated dioxins and furans from the pyrolysis of some flame retardant", from a lecture at Fukuoka Conference, DLO8 (1986) [ 2] Yano et al: Development of "MCF-432F" Copper plated multi-layer board of non-halogen paper phenol, Hitachi Chemicals Technical Report No.28, pp.37-40 (1997) [ 3] Flat panel display 1996, Nikkei BP [ 4] Aoki; "Trends in printed wiring board materials for bare chip mounting", Electronics Mounting Technology, 10, 1, 58-64 (1994) [ 5] Aoki et al:trend of the latest application technology of thin, multi-layer circuit board, Circuit Mounting Society Magazine, 10(1)pp.19-24(1995) [ 6] Bfrip:" Workshop on safety of bromic flame retardant",(1994) Hitoshi Nishizawa: Flame retardance of Polymer, Taisei-sha [ 7] Suga et al:lead-free solder map and the scenario for its practical application, 12th Lecture Meeting on Circuit Mounting Technology [ 8] Hitoshi Nishizawa: Flame retardance of Polymer, Taisei-sha [ 9] Aizawa et al: RO resin MCL for high quality multi-layer board, Hitachi Chemicals Technical Report No.26, pp.21-24(1996) [ 10] Takano; "Low CTE & High Elastic Modulus Substrate for Mounting Semiconductor", The 3rd IEMT/IMC Symposium (1999) [ 11] Ota; "Influence of grain size and surface treatment to the acoustic emission characteristics of epoxy resin filled with silica grains", Thermosetting Resin, 16, 4, 177-182 (1995) Note The entry contents of these data base to the experiment results of our company and do not guarantee these characteristic values. The contents may be revised necessary according to new findings. 9 / 9