Article pubs.acs.org/IECR
Elucidating How Advanced Organophosphine Accelerators Affect Molding Compounds Chean-Cheng Su,* Chien-Huan Wei, and Chih-Chieh Yang Department of Chemical and Materials Engineering, National University of Kaohsiung, No.700, Kaohsiung University Road, Nan-Tzu District, Kaohsiung, 811, Taiwan, ROC ABSTRACT: Biphenyl epoxy/phenol-aralkyl resins with a high silica content were cured by synthesizing and using a novel organophosphine accelerator, triphenylphosphine-1,4-benzoquinone (TPP-BQ), which exhibits thermal latency. Thermal properties and curing characteristics of epoxy molding compounds (EMCs) that were accelerated by TPP-BQ and triphenylphosphine (TPP) were also studied using a differential scanning calorimeter (DSC). Although an analysis of thermal characteristics revealed that TPP-BQ is inactive at low temperatures, at high temperatures, TPP-BQ increases the curing rate of EMC in dynamic and isothermal curing experiments. Additionally, EMCs containing TPP-BQ exhibited excellent storage stability and swift hardening characteristics, owing to the effect of the thermal latency of the accelerator on the molding behavior of the EMCs. Before gelation, EMC containing TPP-BQ had a lower melting viscosity than that of EMC containing TPP, explaining why the former flowed longer in the spiral flow test than the latter.
1. INTRODUCTION Epoxy molding compounds (EMCs) are widely used as encapsulating materials in mountings of electronic parts, diodes, thyristors, transistors, integrated circuits (IC), largescale integrated circuits (LSI), and very large-scale integrated circuits (VLSI). Molding compounds used to generate electronic devices must be compatible both physically and chemically with delicately constructed devices. Such compounds must not only be molded rapidly at a low pressure but also easily removed from the mold with minimal flash.1−4 In IC design, semiconductor chips have increased while devices have shrunk in size. Highly reliable plastic-encapsulated semiconductor packages are thus required. Previous studies have attempted to improve the reliability of EMCs, which are advanced IC packages, by studying matrix resin modification,2 hardeners,4 coupling agent treatment,5 accelerator,4,6−15 and filler processes.16 For specific and functional packaging in semiconductor devices, many molding compound types are available in the semiconductor industry. General-purpose molding compounds with relatively high flexural strengths that exert relatively larger stresses to the device are promising for use in large and thick packages such as a large small outline package (LQFP) and plastic leaded chip carrier (PLCC). Low to ultralow stress molding compounds are preferred for encapsulating thin packages such as a thin small outline package (TSOP) and thin small outline package (TQFP). However, high-thermal conductivity molding compounds are necessary to encapsulate high-power devices. Molding compounds with a low coefficient of thermal expansion (CTE), small warpage after molding, excellent reflow crack resistance, and low viscosity for the mold underfilling process are necessary for encapsulating semiconductor devices such as a chip scale package (CSP), ball grid array (BGA), and stacked multichip package (MCP). Additionally, molding compounds used for surface mount devices may have a low moisture absorption rate or a high flexural strength at board-mounting © 2013 American Chemical Society
temperatures (or a combination of both) in order to prevent popcorn cracking.17−20 Growing environmental concerns have impacted the latest trends for packaging materials in the semiconductor industry. For instance, semiconductor-packaging materials must be environmentally friendly. Conventional molding compounds include flame-retarding additives such as halogen (Br) compounds and antimony trioxide (Sb2O3). However, using halogen compounds and antimony trioxide involves environmental issues. Alternatively, these elements are replaced with an environmentally friendly flame retardant system which is halogen- and antimony-free, leading to the emergence of the term “green molding compound”.21−23 The Restriction of Hazardous Substances (RoHS) directive and the Waste Electronics and Electrical Equipment (WEEE) directive in the electronic packaging industry have forced molding compound manufacturers to develop green molding materials in recent years. Additionally, green packaging has received considerable interest in semiconductor manufacturing in recent years. For encapsulation of semiconductor devices, green packaging with an environmentally friendly process implies both Br/Sb free and Pb free. Molding compounds require a higher reliability, especially resistance of packages to solder heating. This requirement is owing to that the melting point of Pb free type solder exceeds that of eutectic solder, explaining why the reflow temperature increases from 240−260 °C and necessitates the development of molding compounds to satisfy both flame retarding additive free and high reliability requirements. In typical green molding compounds, flame retardants such as phosphorus-containing compounds, nitrogen-containing compounds, metal hydrate, metal oxide, and inorganic filler replace Received: Revised: Accepted: Published: 2528
September 2, 2012 December 27, 2012 January 23, 2013 January 23, 2013 dx.doi.org/10.1021/ie302347r | Ind. Eng. Chem. Res. 2013, 52, 2528−2536
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conventional flame-retarding additives such as halogen compounds and antimony trioxide.6,7 EMCs typically include a curing accelerator (catalyst), which accelerates the curing of resin and increases the number of molding cycles for mass production. Storage stability, physical characteristics, and reliability of the encapsulated semiconductors diverge widely with the species of curing accelerator used. Capable of controlling initial polymerization or curing, thermally latent catalysts are used in packaging. In EMCs, typical accelerators are imidazole,7,8 amines,9−11 organophosphine,6,9 urea derivatives,12,13 or Lewis bases and their organic salts.14,15 However, most accelerators tend to reduce the pot life or moldability of molding materials, owing to their ability to initiate reactions at extremely low temperatures. Therefore, an effective hardening accelerator must have a thermal latency that promotes the rapid curing of resins when heated to a particular temperature; in contrast, latent accelerators are inert at low temperatures.4,6 Exactly how curing accelerators affect the physical characteristics of EMC have been thoroughly studied.2,4,15 In these works, EMCs with triphenyl phosphine (TPP) have optimal physical properties when cured, subsequently improving the reliability of the encapsulated semiconductors. However, the curing reaction of the EMC is significantly accelerated at a low temperature; in addition, a high melt viscosity during molding. EMC that contains TPP has a short pot life. Therefore, an improved organophosphine accelerator with thermally latent characteristics is urgently required for use in EMCs. Notably, the EMCs must have superior storage stability, latent reactivity, and low melting viscosity during molding. A recent trend has witnessed the formulation of green molding compounds with inorganic filler and resins with high C/H ratios for use in EMCs to replace conventional flameretarding additives for the green packaging process of IC devices. Correspondingly, capable of retarding flammability and resisting reflow cracking, EMC with biphenyl resins and highly loaded fillers is a green encapsulating material for semiconductor device packages. The highly loaded filler is essential to produce reliable packaging materials for microelectronic devices, owing to its high flame retardation, high thermal resistance, high moisture resistance, favorable mechanical properties, and low thermal expansion coefficient of EMC. 4,4′-Diglycidyloxy-3,3′,5,5′-tetramethyl biphenyl epoxy with a low viscosity exhibits excellent adhesion, high toughness, and feasibility of high filler loading. However, 4,4′-diglycidyloxy3,3′,5,5′-tetramethyl biphenyl epoxy has an extremely low reactive, explaining why EMC based on this biphenyl type epoxy incurs molding problems. Therefore, in this work, a novel organophosphine thermally latent accelerator, triphenyl phosphine-benzoquino (TPP-BQ), is synthesized with variously sized TPP-BQ particles. This accelerator is promising for use in high filler-loaded EMCs based on the biphenyl epoxy. Exactly how the latent accelerator affects the activity, thermal behavior, rheology, storage stability, and molding characteristics of EMCs is also investigated.
Table 1. Formulation of Epoxy Molding Compounds composition epoxy hardener accelerator
filler coupling agent release agent colorant
raw materials 4,4′-diglycidyloxy-3,3′,5,5′-tetramethyl biphenyl phenol-aralkyl resin triphenylphosphine (TPP) or triphenylphosphine-1,4-benzoquinone (TPPBQ) fused silica glycidoxypropyl trimethoxysilane ethyleneglycol ester of montanic acid carbon black
parts by weight 100 84 2
1550 7 7 4
phosphine (TPP), was obtained from Hooko co. and triphenylphosphine-1,4-benzoquinone (TPP-BQ) was synthesized in the authors’ laboratory. The chemical structure of the catalysts is described in Scheme 1. The filler was fused silica Scheme 1. Chemical Structure of (A) TPP and (B) TPP-BQ
with a mean particle size (D50) of 20 μm (Tatsumori Co.). The coupling agent was glycidoxypropyl trimethoxysilane (ShinEtsu Chemical Co.). The release agent was the ethylene glycol ester of montanic acid (Hoechst Co.). The colorant was carbon black (Cabot Co., CM 800). 2.2. Preparation of EMCs. The materials weighed at the ratios given in Table 1 were thoroughly kneaded using a tworoll mill, in which the cold roller was operated at 15 °C; in addition, the hot roller was operated at 120 °C. After mixing, EMC was cooled and pulverized. Each sample was stored in a refrigerator at 4 °C. 2.3. Characterization. Calorimetric measurements were made using a differential scanning calorimeter (DSC) (PerkinElmer PYRIS I) equipped with an intracooler. Isothermal and dynamic-heating experiments were conducted in a nitrogen flow of 50 mL/min. In dynamic curing, the sample was heated at a rate of 10 °C/min from 0 to 250 °C. The isothermal curing reaction was performed at six temperatures (130, 150, 165, 175, 185, and 195 °C). The reaction was considered complete when the isothermal DSC thermogram had leveled off to the baseline, which generally took around 1 h. The total area under the exotherm, which was based on the extrapolated baseline at the end of the reaction, was used to calculate the isothermal heat of curing, ΔHIo (J g−1). After the curing reaction was completed in the calorimeter, the sample was cooled to 40 °C. After curing, the samples were scanned at 10 °C/min from 40 to 250 °C to determine the residual heat of reaction, ΔHR (J g−1). The sum of both the isothermal heat (ΔHIo) and the residual heat (ΔHR) of the reactions was taken as the total heat of curing (ΔHT). The isothermal conversion at time t was defined as αI(t) = ΔHI(t)/ΔHT. The obtained Tg values were taken as the temperatures of the onset of glass transition (at which the specific heat changed) in the DSC thermograms. The spiral flow was measured according to ASTM D-3123. The mold was placed in a 10 ton transfer molding press, clamped in place, and allowed to equilibrate at 175 °C for 10 min. An 18 g portion of molding compound was pressed into cylinders, which were preheated in a microwave for 30 s to a
2. EXPERIMENTAL SECTION 2.1. Materials. Table 1 presents the formulation of the epoxy molding compound (EMC). The epoxy resins used were 4,4′-diglycidyloxy-3,3′,5,5′-tetramethyl biphenyl (JER Co., YX4000H) and diglycidyl ether of brominated bisphenol A (Sumitomo Co., ESB 400). The hardener was phenol-aralkyl resin (Mitsui Chemical Inc., XLC-2L). The catalyst triphenyl2529
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mean temperature of 92 °C, and then placed inside the transfer pot. The compound was then forced into the mold cavity at a predetermined pressure in the ram. The fact that the molding temperature in the 10 ton transfer molding press had reached 175 °C was confirmed. A 15 g portion of EMC was put into a pot, and the molding was begun immediately. After molding for 50 and 90 s, the stopwatch was begun as the heating plate was moved down to the bottom of the molding press. The dies were quickly removed and opened, and the needle of the Shore D hardness meter was pressed to obtain a portion of the die 10 s after the stopwatch was started. The hot hardness was measured and recorded. The gelation time of EMC was measured using the hot plate method (JIS-K-5909). A surface thermometer was used to indicate that the hot plate temperature of the gelation time tester reached 175 °C. The EMC was measured using a JSRtype curelastometer. Melt viscosity was determined from the maximum flow velocity when 2 g of EMC was forced out (10 kgf/cm2) from the metal nozzle (1 mm I.D. and 10 mm long) of the capillary flow tester (Shimazu Ltd.), which was heated at 175 °C. For evaluation of the storage stability of EMCs, molding compounds were stored at 20 °C for a long time. The spiral flow of the EMCs was produced every other day until the flow length was reduced to zero.
TPP accelerator had a higher degree of curing than EMCs with TPP-BQ accelerator did in the mixing process, which results from the TPP-cured EMC having a high reactivity in the kneading. During preparation of EMC, TPP-BQ-cued EMC had a lower reaction rate than that of TPP-cured EMC at the preparation temperature. The total heat of reactions (ΔHT) was determined as the total area under the thermogram, based on the extrapolated baseline at the end of reaction. Figure 1 shows all of the characterization temperatures of the curing of EMCs, including the exotherm peak temperature (Tpeak), the initial exotherm temperature (Ti), the final exotherm temperature (Tf), and the range of the exotherm peak (ΔT = Tf − Ti) in the dynamic curing experiment. In the DSC thermograms, the exotherm peak temperature and the initial exotherm temperature of EMC that contained triphenylphosphine (TPP) are lower than those of EMC in the reaction accelerated by triphenylphosphine-1,4-benzoquinone (TPP-BQ). After the EMCs had been fully cured, both the system that contained TPP and that which contained TPP-BQ had almost the same final exotherm temperature. Additionally, the EMC in the reaction accelerated by TPP-BQ had a higher initial exotherm temperature (Ti = 106 °C) and a higher temperature of exotherm peak (Tpeak = 193 °C) than EMC in the reaction accelerated by TPP (Ti = 80 °C and Tpeak = 143 °C). This result showed that EMC containing TPP-BQ exhibit superior reaction stability at low temperatures. The acceleration of the reaction of EMC containing TPP-BQ is weak at low temperatures but strong at high temperatures. These results indicate that TPP-BQ thermally latently accelerates the EMC curing reaction. Moreover, samples with various organophosphine accelerators had similar heats of reaction (ΔH ≅ 187 J g−1) and degrees of dynamic curing of EMCs. A general equation for the curing reactions of EMCs is as follows.4,6,24−27
3. RESULTS AND DISCUSSION 3.1. Thermal Behavior Analysis. Figure 1 shows differential scanning calorimeter (DSC) thermograms for the
r=
dα dt
(1)
Where α is the conversion and r is the rate of reaction. EMCs that contain TPP and TPP-BQ accelerators were cured at six fixed temperatures130, 150, 165, 175, 185, and 195 °C. A kinetic analysis was performed using eq 1. Only the rate curves for samples at the temperatures 130, 150, 175, and 195 °C were plotted to avoid crowding the diagrams. Figures 2 and 3 plot the rate curves for the curing of EMCs catalyzed by TPP and TPP-BQ, respectively. At low temperatures (130 and 150 °C), the rate of curing of EMCs containing TPP exceeded that of EMCs containing TPP-BQ; in contrast, EMCs containing TPPBQ were cured at a higher rate than those containing TPP at high temperatures (175 and 195 °C). These findings indicate TPP is a better catalyst than TPP-BQ in the curing of EMCs at low temperatures. However, TPP-BQ accelerated the reaction of EMCs more than TPP did at a high temperature, and EMCs containing TPP-BQ were relatively inert at a low temperature. Notably, for IC encapsulation, the general biphenyl EMC transfer molding temperatures range from 175 to 185 °C. During molding, EMCs containing TPP-BQ are less active before the temperature reaches the molding temperature. The experimental results indicate that TPP-BQ serves as an ideal accelerator in biphenyl EMCs. Figures 4 and 5 show the glass transition temperatures (Tgs) of the organophosphine-cured EMCs, which were cured at various isothermal temperatures. In the TPP-BQ-cured EMCs, the same Tg (129 °C) was obtained for the completely cured
Figure 1. DSC thermograms in the dynamic mode of EMC with TPP and TPP-BQ.
dynamic curing of biphenyl epoxy molding compounds (EMCs), using various organophosphine thermally latent accelerators at a heating rate of 10 °C/min. The materials weighed at the ratios given in Table 1 were thoroughly kneaded using a two-roll mill, in which the cold roller was operated at 15 °C; in addition, the hot roller was operated at 120 °C. Following mixing, the glass transition temperature (Tg) of the EMCs was 15 and 23 °C for TPP-BQ-cued EMC and TPPcured EMC, respectively. This finding suggests that EMCs with 2530
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Figure 4. Glass transition temperatures of the TPP-BQ-cured EMCs at various isothermal temperatures. Figure 2. Plot of the reaction rate versus time for EMCs with TPP at four different temperatures: (a) 130, (b) 150, (c) 175, and (d) 195 °C.
Figure 5. Glass transition temperatures of the TPP-cured EMCs at various isothermal temperatures.
transition temperature of epoxy system is directly proportional to cross-linked density. As the curing progresses, the Tg value increases until it reaches the curing temperature when the material vitrifies. Moreover, the Tg value does not readily increase any further when the reaction rates drop markedly once a material becomes glassy. As EMC cross links, the glass transition increases. Above results suggest that the TPP-BQcured EMCs have a higher cross-link density than TPP-cured EMCs when the EMCs are completely cured at high temperatures. Each experiment was replicated five times. Table 2 presents the mean residual heat of reaction (ΔHR), isothermal heat of reaction (ΔHIo), total heat of curing (ΔHT), and isothermal conversion (αI). Precision of the results correlates with the accuracy of calorimetry (
