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Process Flow Comparison
Source publication
This paper describes a generic low cost multichip module technology that uses a highly impermeable liquid crystal polymer (LCP) both as an MCM/L substrate material and as part of a near-hermetic assembly to bring electrical I/O out while providing a thermal management path. The approach has a number of advantages over other conventional approaches...
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Citations
... Liquid crystals were found to have satisfied all of the requirements for a base laminate in a thin film circuit, such as resistance to wet etch chemistries and strong adhesion to electrodeposited copper layers. 47 A finish metallization such as tin or lead solder must be applied to a printed circuit in order to protect an electrodeposited copper conductive layer from corrosion; these metallization processes generally require extremely high temperatures and can contaminate underlying layers. Liquid crystal polymer protective substrates were seen to be as robust as more expensive inorganic or transition metal protective substrates in protecting the underlying circuit from contamination, etc. ...
Water soluble conjugated polyelectrolytes (CPEs), which fall under the category of conductive polymers, possess numerous advantages over other conductive materials for the fabrication of electronic devices. Namely, the processing of water soluble conjugated polyelectrolytes into thin film electronic devices is much less costly as compared to the processing of inorganic materials. Moreover, the handling of conjugated polyelectrolytes can be performed in a much more environmentally friendly manner than in the processing of other conjugated polymers because conjugated polyelectrolytes are water soluble, whereas other polymers will only dissolve in toxic organic solvents. The processing of electronic devices containing inorganic constituents such as copper indium gallium selenide (CIGS), is much more expensive and poses much greater environmental risks because toxic metals may be released into landfills or waterways upon cell disposal.75 Because conjugated polyelectrolytes enjoy an assortment of advantages over other materials for the manufacturing of thin film electronic devices, there is globally vested interest in the researching of their properties. Despite the fact that CPEs can serve as efficient electron transport mediums, devices such as organic solar cells cannot realize their highest efficiencies unless the morphology of CPEs is precisely controlled. Charged surfactants can electrostatically and ionically interact with CPEs, and when introduced in specific concentrations, molar ratios, and temperature ranges, will aid in a ‘coil to rod’ transition of the CPE, wherein polymer chains undergo intramolecular transitions to obtain rigid-rod morphologies. The kinetics and thermodynamics of the ‘coil to rod’ transition are heavily dependent upon the type(s) of charged surfactant complexed with the CPE (i.e. on the surfactant architecture). By performing UV/Vis Spectroscopy and Fluorometry on dilute polymer/surfactant solutions, Polarized Optical Microscopy (POM) and Small Angle X-Ray Scattering (SAXS) on high concentration polymer/surfactant solutions, and Differential Scanning Calorimetry (DSC) and X-Ray Diffraction (XRD) on solid-state polymer/surfactant samples, the role of various surfactant architectures on the kinetics and thermodynamics of the ‘coil to rod’ transition was studied. The liquid crystalline physical properties and the extent of solid state crystallinity were also investigated. Through an analysis of the data obtained from these various techniques, it was found that the ‘coil to rod’ transition is progressively favored when the alkyl chain length of a single tailed surfactant is sequentially increased, and that as the concentration of double-tailed surfactant increases, the ‘coil to rod’ transition is negated.
... Two flex substrate materials were used in this work: polyimide and liquid crystal polymer (LCP). LCP characteristics include good dielectric properties to 100GHz, low moisture absorption and very low moisture permeability5678910111213. The coefficient of thermal expansion (CTE) of the LCP can be controlled during the biaxial extrusion of the LCP film. ...
Driven by a growing range of applications in the automotive, industrial, military, aerospace, computer, telecommunication, consumer electronics, and medical electronics industries, miniaturization and the use of flex circuits continue to be of prime interest to electronics manufacturers. The assembly of thinned silicon die (25-100 mum) onto flex substrates provides options for ultrathin, flexible electronics for applications ranging from smart cards to space-based radars. For high-density applications, 3-D modules can be fabricated by stacking and laminating preassembled and tested flex layers and then processing vertical interconnections. This paper describes a low cost, highly manufacturable process developed for flip chip assembly of thinned die to poly-imide flex substrates that eliminates the need for special handling tools and techniques. In this paper, solder bumped thinned die are reflow soldered to the patterned flex using a method that maintains the flex substrate flat during die placement and reflow. Reflow is followed by underfill dispense and cure. The underfill dispense process is critical to avoid underfill flowing onto the top of the thin silicon die and will be discussed. Parts assembled using these processes have undergone reliability testing, a high degree of reliability has been found, and those results are presented.
Liquid crystalline polymer (LCP) substrates offer a number of advantages for high-density packaging. These properties include high temperature capability (>250 o C), low coefficient of thermal expansion (8ppm/ o C), low moisture permeability (comparable to glass), smooth surface, and good high frequency characteristics. LCP substrates can be fabricated as flexible films (2mil thick) or as rigid multilayer substrates. In this paper the processing of rigid and flexible LCP substrates are first discussed including etching, drilling, and plating. Next, the compatibility of LCP substrates with wire bond and flip chip assembly processes and materials are examined. Specific tests include solderability with eutectic Sn/Pb and lead-free alloys, surface insulation resistance with no-clean fluxes, gold wire bondability and flow/wetting of underfills for flip chip assembly. The high temperature capability of the LCP is compatible with the higher reflow temperatures associated with lead–free solders and also allows thermosonic gold wire bonding at a substrate temperature of 200 o C. Solder dips in lead free alloys at 274 o C have shown no delamination of the copper foil. Gold ball shear test results demonstrate average shear strength of 62.4 grams with a standard deviation of 4.3 grams when bonded at 200 o C. . Optical fibers can be molded into the LCP substrate for optical connections and optical fibers can also be molded into the package sidewall for optical connections. Finally, hermetic packages have been fabricated and shown to pass fine and gross leak tests.
Liquid Crystalline Polymers (LCPs) offer a number of advantages in advanced printed wiring board and packaging applications including, low coefficient of thermal expansion, low moisture absorption, low moisture permeability, smooth surface, low dielectric constant and low dissipation at high frequencies, and high temperature capability. This paper examines the processes for manufacturing LCP printed circuit boards, assembly onto LCP boards and fabrication of LCP packages. Specific printed wiring board (PWB) processes examined include lithography and etching, hole formation and metallization, and solder mask and surface finish application. The solderability and surface insulation measurements of test coupons passed industry requirements for printed wiring boards. Flip chip assembly and gold thermosonic wire bonding have been demonstrated. Finally, hermetic packages have been fabricated and shown to pass fine and gross leak tests.
After many years of development, liquid crystalline polymer materials are finally ready for significant use as a printed circuit board material. These materials have a ways had very attractive electrical and physical properties but were a challenge to be made into industry acceptable circuit boards. Through extensive technology development efforts by Foster-Miller to develop biaxially oriented LCP film, producing high quality substrates is now possible and are available from Rogers. As with all new substrate materials, a set of appropriate circuit fabrication processes must be selected to support this new material set. This paper addresses some of these processes.
Summary form only given. The ideal package for direct chip
attachment, from a thermal and mechanical viewpoint is silicon.
Unfortunately, this material is produced in formats no larger than 300
mm in diameter. A research program is currently under way to look at
alternative substrate materials that are well suited to MCM-D/L
processing. Ideally, the candidate would be of a low cost, off-the-shelf
large format (400 mm square) variety, able to withstand the high
temperatures and chemicals commonly used in thin-film processing. This
paper details a program developed by a consortium of large area
processing vendors to evaluate materials as a base for RF and high
frequency applications. Specifics on the materials chosen and methods
used to test them are provided. The first task is to identify possible
candidate base materials that could be used given certain requirements.
Second is to evaluate their resistance to various chemicals. Third is to
determine the materials' dimensional stability. This is accomplished by
running test specimens through thin film processing and determining if
warpage occurs after steps involving extreme temperature cycles such as
dielectric curing. One of the most sensitive indicators of dimensional
instability occurs during photolithography, when the metal or dielectric
pattern is being transferred. If a material were to shrink or expand,
evidence of this occurs when trying to position alignment markers
correctly. Should the materials pass these preliminary tests, actual
test vehicles may be built to determine the material viability under
electrical and reliability testing
