Novel conductive adhesives for surface mount applications

Electrically conductive adhesives (ECAs) have been explored as a tin/lead (Sn/Pb) solder alternative for attaching encapsulated surface mount components on rigid and flexible printed circuits. However, limited practical use of conductive adhesives in surface mount applications is found because of the limitations and concerns of current commercial ECAs. One critical limitation is the significant increase of joint resistance with Sn/Pb finished components under 85°C/85% relative humidity (RH) aging. Conductive adhesives with stable joint resistance are especially desirable. In this study, a novel conductive adhesive system that is based on epoxy resins has been developed. Conductive adhesives from this system show very stable joint resistance with Sn/Pb‐finished components during 85°C/85% RH aging. One ECA selected from this system has been tested here and compared with two popular commercial surface mount conductive adhesives. ECA properties studied included cure profile, glass transition temperature (T_g), bulk resistivity, moisture absorption, die shear adhesion strength, and shift of joint resistance with Sn/Pb metallization under 85°C/85% RH aging. It was found that, compared to the commercial conductive adhesives, our in‐house conductive adhesive had higher T_g, comparable bulk resistivity, lower moisture absorption, comparable adhesion strength, and most importantly, much more stable joint resistance. Therefore, this conductive adhesive system should have better performance for surface mount applications than current commercial surface mount conductive adhesives. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 399–406, 1999     

Characterization of reworkable high-temperature adhesives for MCM-D application

The need to have a high-temperature adhesive that can withstand temperatures in excess of 350°C for MCM-D silicon substrate process application, yet which can be reworkable at slightly high temperature ∼ 400°C for the removal from the glass pallet, is important. A novel, reworkable, high-temperature adhesive based on polyimide–amide–epoxy (PIAE) copolymer was developed and investigated using modulated differential scanning calorimetry (MDSC), thermal gravimetric analysis (TGA), Fourier Transform Infrared Spectroscopy (FTIR) and solid-probe pyrolysis mass spectroscopy (MS). Compared with commercial polyimide–amide (PIA) adhesives, FTIR spectra reveal that the thermally degradative ester groups contribute to the reworkability of the PIAE adhesive at a specific temperature (400°C), yet they remain thermally stable at a lower working temperature (350°C). FTIR spectrum comparison of the residuals of PIAE and PIA are similar after exposure to 400°C. MS spectra of outgassed products identify that the components of radical fragmentation from PIAE are due to polymeric chain degradation at 400°C, while only volatile trace water and N-methyl pyrolidone (NMP) are evolved from the commercial PIA adhesive. TGA results suggest a complementary explanation for the variation of total ion current (TIC) curves on these two adhesives. MDSC curves further verify that the reworkable PIAE adhesive is a copolymer. Furthermore, a reasonable thermal degradation mechanism is presented on the adhesive reworkability. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 997–1005, 1999     

Analysis of holding power in the blends of poly(butyl acrylate) with poly(vinylidene fluoride-cohexafluoro acetone)

Shear adhesion of pressure-sensitive adhesive tapes was evaluated for the blends of poly(butyl acrylate) with poly(vinylidene fluoride-co-hexafluoro acetone). The shear adhesion was determined as the function of the shear strain of pressure-sensitive adhesive tape against elapsed time under the shear stress. Shear adhesion of the blends increased with increasing poly(vinylidene fluoride-co-hexafluoro acetone) content. Experimental shear strain data were characterized with dynamic viscosity, stress and shear rate plot, and a generalized viscoelastic model of shear adhesion. However, the experimental data cannot be expressed with these viscoelastic properties. It is believed that shear adhesion is influenced by the viscoelastic properties and other factors (e.g., friction coefficient between adhesive and adherend or cohesive strength of adhesive polymer). © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 68:727–738, 1998     

Polycondensation and autocondensation networks in polyflavonoid tannins. I. Final networks

A thermomechanical analysis (TMA) study, directly on a bonded joint during the process of adhesive hardening, on the sinergy and interference between polycondensation and autocondensation on procyanidin, and profisetinidin–prorobinetinidin-type polyflavonoid tannins network formation and hardening, confirmed that also at the higher curing temperatures characteristic of the hardening of tannin-based wood adhesives, hardening by polycondensation can be coupled with simultaneous hardening of tannins by autocondensation. Some coreactants appear to depress the tannin autocondensation while still leaving a small contribution of this reaction to the formation of the final crosslinked network. Other coreactants instead appear to enhance formation of the final network by sinergy between the 2 condensation mechanisms, while still others do not show any interference between the 2 types of reaction. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 70: 1083–1091, 1998     

Radiation effects on aluminum-epoxy adhesive joints

Two epoxy adhesive types, Cole-Parmer and Devcon, were used for preparing aluminum-epoxy bondings. The adherend surfaces, of 30 mm in diameter, were prepared using grits of 120, 240, and 320 followed by a final grit of 400, according to the ASTM D897 standard. The curing was set at 72 h at room temperature. The samples were submitted to irradiation for different times in the pool of a SLOWPOKE-2 reactor which produced thermal neutrons, fast neutrons, and γ rays. The tensile properties of nonirradiated and irradiated samples were obtained with an Instron Tester, model 4206. The failure stress, about 11 MPa for nonirradiated samples, had a large decrease after a short period of irradiation and then constantly increased for longer irradiation periods. This may be explained by a predominant effect of crosslinking over chain scissions for higher irradiation doses. The density data and tensile properties of the bulk cured epoxy (Devcon) also supported the above findings. The presence of water on the bonding joints had an effect of exaggerating the irradiation effects on the strength of joints. The use of the adhesive failure modes to group the results into subgroups has permitted the reduction of the spread of the results from the tensile tests. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67: 37–47, 1998     

Adhesives in demanding applications

Structural adhesives are nowadays employed extensively in structural engineering applications where advantage is made of many outstanding attributes, most notably their ability to confer reduced weight, improved fatigue performance together with reduced production and life cycle costs. Increasingly in recent years, attempts have been made to employ bonded structures in harsh environments. In this article an attempt is made to consider the demanding environments which bonded joints are frequently required to resist, focusing principally on hot/wet and high temperature conditions. With the former, the ways in which water can adversely affect joint behaviour are highlighted and the factors, such as nature of adhesive and surface treatment, crucial to optimum joint performance are discussed. Adhesive joints in high temperature environments are discussed in terms of early high temperature adhesives, designed with little thought to processability, together with more recent developments in high temperature adhesives where processability has been ‘engineered’ into the polymer structure.     

Effects of in situ esterification on the performance of carboxyl functionalized rubber‐modified epoxy film adhesives

Epoxy‐based film adhesive formulations were developed with 10 wt % solid carboxyl functional rubber. Due to the high rubber content and resulting viscosity restrictions, the rubber could not be prereacted with the epoxy before hot‐melt filming. Therefore, an esterification catalyst was used to perform this reaction in situ before the epoxy curing reactions. The performance of this adhesive system is compared to that of one without the esterification catalyst. A significant difference in the flow characteristics was observed with incorporation of the esterification catalyst, but only small variations in mechanical performance were found. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 728–734, 2000     

New Developments in Structural Adhesives for the Automotive Industry

Recently developed epoxy paste adhesives, reactive hot melts, adhesive film tape and polyurethane adhesives are presented for structural bonding in the automotive industry. Paste adhesives usually require a precure stage to obtain handling strength of the joints and to guarantee wash-out resistance of the adhesive in the paint baths. This step can be omitted with reactive hot melts and adhesive film tape, which are solid before and after their application. In addition they allow an improved working hygiene. Some mechanical properties of the adhesives are shown such as lap shear strength and peel strength as well as lap shear strength as a function of the bondline thickness. Results of the excellent durability of epoxy one-component pastes, reactive hot melts and adhesive film tape are given from cyclic environmental and salt spray tests.     

Adhesive Bonding to Galvanized Steel: I. Lap Shear Strengths and Environmental Durability

Initial (i.e., unaged) adhesion, as well as adhesion after seven day, 60°C water immersion and six week scab corrosion accelerated environmental exposures, has been assessed for five different one and two-part epoxy adhesives, bonded to three different types of galvanized steel substrates. We have shown that adhesion, as measured by lap shear strength, is specific to the galvanized substrate type. In general, for a given adhesive, adhesion to “hot-dipped” galvanized substrates is harder to achieve and maintain under accelerated environmental exposure than is adhesion to “electroplated” galvanized. Also, for a given type of galvanized steel, the one-part epoxies evaluated generally showed higher initial strengths, as well as better strength retention under environmental exposure than did the two-part epoxies.     

Preparation and characterization of wood polyalcohol‐based isocyanate adhesives

Sugi (Criptmeria Japonica) wood meal was liquefied at 150°C with a mixture of poly(ethylene glycol) 400 and glycerin in the presence of a sulfuric acid catalyst. The resulting liquefaction products were used directly to prepare isocyanate adhesives via mixing with polymeric diphenylmethane diisocyanate without the removal of the residue. The properties of the liquefaction products and the performances of bonded plywood were tested. The results showed that the residue content decreased and the hydroxyl value increased as the reaction time increased. The viscosity and weight‐average molecular weight significantly changed with the reaction time. All the dry test results of the shear strength met the Japanese Agricultural Standard (JAS) criteria for plywood. After a cyclic steaming treatment, however, only the plywood bonding with adhesives from the liquefied wood with a reaction time of 1.5 h satisfied the JAS criteria. The wood failure was very low. The emissions of formaldehyde and acetaldehyde were extremely low. Liquefied‐wood‐based isocyanate adhesives have the potential to become ideal wood adhesives because of their bond durability, safety, and recyclability.