Surface mount technology (SMT) is an electronic packaging technology wherein the leads of electronic components are soldered directly to metallized pads on the surface of a printed circuit board (PCB). The SMT with leadless ceramic chip carriers (LCCCs) is used to design, fabricate, and assemble affordable, high-speed, high-density electronic modules with reduced size and weight and improved electrical performance. In surface mount devices, the LCCCs are soldered directly onto the fabric composite PCB substrate. New high-performance composite substrate materials must be developed to take full advantage of SMT. Fabricating a PCB that will perform reliably throughout its intended life is also an increasingly important requirement, especially if the goal is to satisfy the high reliability required in military applications. Consequently, SMT is driving the development of PCB substrate materials with improved thermal and electrical properties. In our continuing effort to meet these military demands, we evaluated high-temperature resistant/high-performance acetylene-terminated polyimide composites for use in SMT PCBs. This paper focusses on the processing and on the thermal, thermomechanical, and dynamic mechanical properties data developed for these acetylene-terminated polyimide composites for their potential evaluation as PCBs. The characterization includes such properties as in-plane coefficient of thermal expansion (CTE), out-of-plane CTE, and glass transition temperature (Tg), which determine the solder joint reliability, plated-through-hole (PTH) reliability, and dimensional stability.

The interest in laser processing technology has increased significantly in recent years because of increasing demands for application-specific IC design and fabrication, yield enhancement, circuit restructuring, and prototyping; all of these benefit from an adaptive processing technique using direct energy for improvement of precision, resolution, process automation, and turnaround time. This paper reviews several laser-patterned metallization techniques developed for high-density multichip interconnection applications. Key material and process requirements for developing a viable laser-direct-write interconnect technique on polyimide are addressed.

Two kinds of ion beam etching techniques, Ar gas ion beam etching (IBE) and oxygen gas reactive ion beam etching (RIBE), were used for adhesion enhancement of the PI/Cu and Cu/PI interface, respectively.

Ar gas IBE on Cu followed by deposition of PI film on the modified Cu surface can double the adhesion strength of spin-coated PI on Cu compared with unmodified Cu. The adhesion enhancement is due to increasing the surface contact area of the rough Cu surface to PI. However, a corrosion reaction happens at the polyamic acid (PAA) and Cu interface, resulting in the dissolution of Cu into the PI film and Cu oxide layer growth during curing of PI film. A Cr layer between Cu and spin-coated polyamic acid(PAA) is necessary to keep the Cu from dissolving in PAA and diffusing into PI. Ar IBE modification on Cu followed by deposition of a thin Cr layer on the modified Cu surface is recommended to enhance the adhesion strength of spin-coated PI on Cr.

O2 RIBE on PI followed by evaporation of Cu on a modified PI increases the peel strength of Cu/PI by more than 25 times. The maximum peel strength of Cu on the PI modified by O2 RIBE is almost 70 grams/mm. The increase of adhesion is due to the mechanical interlocking of a unique structure of modified PI - a grass-like structure. This structure occurs because of the inhomogeniety of PI film - the presence of ordered and disordered phases in a PI film. In addition, the Cu/PI interface exhibits stability with thermal cycling and humidity, because the adhesion enhancement is mostly due to a mechanical effect and not a chemical effect. The diffusion of Cu into PI at various temperatures is not significant. Typically, Cu atoms can diffuse up to 300 at 400 °C in 1 hour. Another advantage of O2 RIBE on PI is the capability of enhancing the adhesion of the second PI layer on a first PI layer.

These technologies allow the precise control of the adhesion strength of the PI/Cr/Cu and Cu/PI interfaces by independent variation of the beam energies, ion doses and angle of beam incidence. Also the ion source is easily installed in the same vacuum chamber for Cr and Cu evaporation. This provides low cost and simple operation.

High density thin film interconnects are expected to be widely used for multi-chip module application to accommodate next generation high I/O and high speed integrated circuits. These interconnects typically use polyimide as the dielectric, and aluminum or copper (with protective overcoat) as the conductor. The interconnects are typically built on silicon or alumina substrates. Large film stress occurs due to the high processing temperature required to cure polyimide and due to the mismatch in thermal coefficients of expansion (TCE) between the film materials and substrate materials. This work studies film stress for these materials.

An instrument which measures thin film stress in-situ at temperatures between 25 and 450°C was used to characterize the stress in polyimide, nickel, and copper films. Two substrate materials, silicon and sapphire, were used in order to extract the TCE and elastic modulus for each film material. Three polyimide materials were evaluated. One of the polyimides studied showed complete stress relaxation at temperatures above 300°C and was almost completely elastic upon heating and cooling between 25 and 300°C. The TCE was calculated to be 41×10−6/°C and the biaxial elastic modulus was 4.0×109 Pascal. The nickel had very low stress asplated, however, high tensile stress was observed after 350°C annealing as a result of TCE mismatch. After first annealing, the nickel was almost completely elastic upon cooling and repeated heating and cooling between 25 and 350°C. Copper, on the other hand, was not completely elastic under similar thermal treatments. High thermal stress caused plastic deformation to occur in copper films. The room temperature stress in copper film after 350°C annealing depended on yield strength instead of TCE mismatch. The stress in these materials and its effects on processing and reliability for high density interconnect will be reported.

Most literature on High Density Multichip Interconnect (HDMI) focuses almost exclusively on processing of the organic dielectric. Nevertheless, it is only one of the components in High Density Multichip Modules.

Substrate properties, metal dielectric adhesion, internal stresses in the various films, and many other physico-chemical properties of the materials, can all be affected by neighbouring layers or process parameters improperly identified. Thus, in the course of process development, the real causes of difficulties and observed phenomenas can easily be misconstrued.

This paper reviews some of the relationships between the properties of the thin film metallization and their effects on the dielectric layers. It also points out to some of the difficulties that can occur when treating the dielectric and the metallic layers as separate issues.

Multilayer interconnection structures incorporating a novel polymeric dielectric derived from a bis-benzocyclobutene(bis-BCB) monomer have been fabricated. This paper discusses the processing conditions for the construction of these circuits and describes electrical characteristics of the dielectric layers. The relative dielectric constant of the BCB film was 2.7. Thermal cycling produced no significant change in the conductance of three level metal via chains through two layers of the polymer.

The Research Initiative on Silicon Hybrids (RISH), is one of several National Electronics Research Initiatives created under the auspices of the UK Department of Trade and Industry. Its aims are to investigate the merits of silicon hybrids, evaluate their performance and develop the technology necessary for their implementation. The paper describes the design and fabrication of silicon hybrids using thin film metal-polyimide interconnect. Several chip attach options are considered. The merits of the technology are illustrated using functional circuit demonstrators.

This article concerns physicochemical mechanisms leading to formation of local heterogeneous structures in bulk polymers. Previous experimental work suggests that such heterogeneities are related to ionic conduction and electrochemical attack on encapsulated microelectronics. Six mechanisms are discussed: (a) Macrosyneresis; (b) Osmotic swelling; (c) Liquid-liquid phase separation; (d) Microsyneresis; (e) Polymer fracture during solvent sorption/desorption; (f) Polymer-substrate interactions.

Moisture and ionic impurities play a major role in the failure mechanism of integrated devices. [1] The use of organic polymers as microelectronic coatings offers advantages in improved planarization of metal steps, low temperature processibility, and reduction of mechanical stresses. [2] Cited disadvantages are the high inherent moisture absorption and level of impurities. Polymers introduce levels of moisture and impurities 10–100 times greater than vacuum deposited inorganic insulators.[3] Moreover, the combination of ionic impurities and a high humidity environment will degrade attractive polymer properties such as high resistivity and low dielectric constant. [4]

This paper describes the development of a technique for protecting non-hermetically packaged integrated circuits for use inside the human body. A two layer combination consisting of a thin adhesion-promoting primer of plasma polymerized methane covered with a thicker layer of Parylene®-C has been found to provide excellent protection during long term exposure to simulated body fluids under continuous DC bias. The process for synthesizing the two layer combination in continuous vacuum is described. Results from adhesion and electrical testing of passive test structures are given. A new technique for the formation of small openings in thick polymer coatings for neural stimulation and sensing sites of implantable sensors is described. The technique uses biased thin film resistors formed on the passivation layer to provide localized heated areas which prevent polymer deposition during synthesis.

Polyimide is used extensively in a variety of integrated circuit packaging applications. It is a good dielectric material with excellent planarizing capabilities, but like most polymers, it absorbs moisture. This hygroscopic behavior can lead to reliability problems in integrated circuit packages. The effects of variations in process history on moisture uptake are examined using gravimetric measurement techniques. In particular, the effects of cure schedule and exposure to high temperature/high humidity environments on steady state moisture uptake are reported. Steady state moisture uptake is shown to be a decreasing function of cure temperature. Moreover, the steady state moisture uptake in polyimide is greater after the samples have been “aged” in a high temperature and humidity ambient. Electrical measurements are used to examine the effects of cure temperature on diffusion kinetics of moisture in polyimide. The diffusion coefficient decreases with increasing cure temperature.

A review of research in the author's laboratory on the conversion of reactive liquids to amorphous polymeric glasses is presented. Inter-relationships between reactants, reaction conditions, and subsequent material properties of thermosetting and high Tg polymers are discussed from the point of view of a generalized time-temperature-transformation (TTT) diagram.

Silicone resins and gels have been used extensively for IC protection. Reduction of interfacial moisture and ionic contamination is considered crucial to device reliability.

The present research has revealed an interesting phenomenon. Prolonged lifetime was obtained for comb pattern samples using a non-conventional encapsulation method. In order to increase specimen sensitivity to interfacial moisture, samples were contaminated prior to encapsulation. Three groups of contaminated samples encapsulated with polysiloxane were produced. Group I was encapsulated with a polymer/CaCl2/polymer/substrate structure. The structure of group II consisted of only the polymer on the contaminated substrates. Group III was encapsulated with a CaCl2 embedded polymer/polymer /substrate structure. Following exposure at 85°C, 100−10 amperes after 2,000 hrs, with no visible corrosion. Group II and III samples showed visible corrosipn after 390 to 470 hrs with leakage current of the order of 10−5 amperes. Direct measurement of RH at the polymer/substrate interface and water sorption experiments were done to explain the mechanisms of these packaging structures.

The experimental results suggest that a desiccating multilayer coating can maintain low interfacial water concentrations. During exposure to high external RH, water from the surroundings diffuses through the top layer, producing an interlayer consisting of a solid CaCl2 and a CaCl2-saturated solution. Until all of the salt dissolves, the water chemical potential of the saturated solution defines the water content at the polymer/substrate interface. Hence the RH at the polymer/substrate interface was maintained below that required to dissolve the residual surface contamination for a longer time than by conventional encapsulation. This technique could be useful to reduce the dependence of IC reliability upon pre-encapsulation cleaning.

Polymers used to encapsulate microelectronic devices absorb moisture – it's their nature. But only recently, with the growing use of surface mount technologies, has this phenomenon fostered concern much beyond the traditional chemical (corrosion) issues. The mechanical stability of plastic encapsulated components in use is now affected by moisture and its complex relation to interfacial adhesion and stress distributions within the package under different environmental (test) conditions. Strategies to defend against moisture-associated problems involve control of the application environment (removing moisture), improving package structural designs (increasing robustness), and managing materials choice to reduce moisture sensitivity. Each strategy has limitations, both physical and practical (economic), and all currently are being pursued. This paper will review the technical challenges, with particular emphasis on the state of affairs with regard to material development and application.

It is proposed that the stresses in a thin polyimide film can be analyzed using the solution to the problem of a vibrating membrane. The polyimide membrane is supported on a copper substrate and excited with a small speaker. The membrane vibrates uniformly in response to its resonant frequencies. The vibration pattern will be recorded using holographic interferometry. The biaxial stress in the film is calculated from the measured characteristic frequency, the density of the material, and the mode of vibration. This technique may also be used to determine the Poisson's ratio of the film by combining uniaxial and biaxial stress analyses.

Polyimides generally possess excellent thermal and mechanical properties, making them attractive candidates for high performance applications. To be useful for microelectronic applications, however, these materials must also be good insulators, as well as be readily processable.

The incorporation of flexible polysiloxane segments into the polyimide backbone structure has been shown to yield soluble, processable copolyimides with good thermal and mechanical properties. In addition, the siloxane component imparts a number of other significant benefits for electronic applications. These include reduced water sorption, surface modification, good thermal and ultraviolet stability, and resistance to degradation in oxygen plasma environments. For polar polyimide systems, siloxane incorporation will also reduce the dielectric constant. The use of other less polar, more hydrophobic monomers will consistently yield soluble systems with lower dielectric constants as well.

In this work, a series of high molecular weight, soluble polyimide homopolymers and segmented polysiloxane-polyimide copolymers were synthesized by a solution technique. The solution procedure, conducted at lower temperatures (˜170°C) than the classical bulk thermal imidization (300°C), has been shown to yield polyimides of enhanced solubility. In order to further enhance processability, molecular weight was controlled through the incorporation of monofunctional reagents such as phthalic anhydride and maleic anhydride, yielding nonreactive or potentially reactive endgroups, respectively. A series of maleic anhydride terminated imide oligomers with varying molecular weights were synthesized based upon the hexafluoropropane linked dianhydride and bisaniline diamine. In their oligomeric state, they exhibited enhanced solubility compared with their linear high molecular weight analogue. As these monomers were relatively nonpolar and hydrophobic, they afforded polyimides of low dielectric constant and a low level of water sorption. After thermally crosslinking the endgroups, the advantages of insoluble network systems could be realized. Particular advantages for electronic applications include thermal and dimensional stability over a wide temperature range, good mechanical properties, and chemical resistance. Structure-property characterization, including water sorption, dielectric constants, solubility behavior and thermal/mechanical properties will be reported.

Unsaturated and saturated oligomeric siloxanes were polymerized by laser irradiation in the UV range without the use of photoinitiators. The obtained silicon layers possess high purity and non-corrosive effects.

To evaluate the influence of permeant and temperature exposure on the state of stress in polyimide coatings, uniaxial experiments on thin polyimide films held at constant length were performed. The uniaxially constrained poly(4,4′,-oxidiphenylene pyromellitimide) or PMDA-ODA films were exposed to methylene chloride while the stress was monitored. It was found that the stress was reduced to zero from a stress as high as 40 MPa by the swelling with methylene chloride. Attempts to remove the methylene chloride at room temperature with a nitrogen purge resulted in a stress increase of only 8 MPa. However, the remaining stress of an additional 20 Mpa was recovered when the sample was heated to 350°C and returned to room temperature. Temperature exposures of 150°C and 200°C resulted in a stress increases of only 14 MPa and 17 MPa, respectively. These observations suggest a strong coupling between the stress state, temperature and equilibrium swelling by permeants.

Five classes of spin-on dielectrics are discussed: polysilicates, polysiloxanes, polysilsesquioxanes, polyhydrocarbons, and polyimides. The emphasis is on the first three materials because very few polyhydrocarbon spin-on materials are available for study while spin-on polyimides sorb significant amounts of water. The polymer chemistry of the silicates, siloxanes, and silsesquioxanes is linkage through Si-O-Si-O bonds. Polyhydrocarbons are primarily linked through C-C bonds. As the name implies, polyimides are linked through (usually aromatic) imide groups. Because of their polar nature, polyimides, polysilicates, and polysiloxanes have dielectric constants >3, while the large hydrocarbon content of the remaining materials results in a dielectric constant of <3. Films of polysilicates are relatively brittle and thus are limited to a thickness <1 μm. On the other hand, films of polysilsesquioxanes, polyimides, and polyhydrocarbons can be readily deposited to a thickness >1 μm. Polysilicates have high temperature (>900°C) stability and do not react with O2; most polyhydrocarbons decompose above ˜350°C and oxidize readily; some polyimides are stable to 500°C; polysiloxanes and polysilsesquioxanes readily survive 425°C in N2. (Films of the latter two materials are converted to silicates upon heating at high temperatures in O2 or steam.) The infrared spectrum of a 425°C-cured silicate film shows the presense of Si-OH bonds and water, but after a 900°C anneal, the spectrum is almost identical to that of thermal oxide. The spectra of films of polysiloxanes or polysilsesquioxanes do not exhibit Si-OH absorption. Water retention in films is deleterious to the electrical and mechanical (swelling) properties of the dielectric and to making good aluminum-to-aluminum electrical contact through submicron vias. The water leads to a mobile charge (H+) phenomenon that is readily detected by a Triangular Voltage Sweep technique. Except for polysilicates, solutions of the other spin-on materials have reasonably long (weeks-to-months) shelf-lives. Polysilicate solutions are sols and tend to gel; the time to gelling is a function of solids content and solvent. Even though they exhibit shortcomings, polysiloxanes appear to be best suited as interlevel dielectrics in multilevel metalization schemes.

From a processing point of view polyimides are attractive materials for use as interlayer dielectric and as passivation coating on ICs. For plastic encapsulated devices they can also improve on the thermomechanics, thereby enhancing yield and reliability. We simulated the thermomechanical behaviour of such a device with finite element calculations. Based on these simulations the commercial product PIQ-L100 was chosen as a model polyimide to study some important physical and chemical parameters. The results from investigating internal stress, molecular orientation and imidization kinetics can be used to optimize polyimide processing and to obtain the desired coating characteristics.