Cleaning and liquid contact with solids – Processes – Using sequentially applied treating agents
Reexamination Certificate
2001-12-21
2004-10-05
Kornakov, Michael (Department: 1746)
Cleaning and liquid contact with solids
Processes
Using sequentially applied treating agents
C510S175000, C134S019000, C134S029000, C134S030000, C134S021000, C438S906000
Reexamination Certificate
active
06800141
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to a method of removing flux residue formed on electronic assembly surfaces and interfaces during high temperature soldering operations using rosin based flux for semiconductor device interconnections in electronic module assemblies. More particularly, the invention relates to a semi-aqueous solvent based method employing high boiling point, halogen-free, and non-aromatic organic solvent based cleaning compositions, in conjunction with water rinse, for removing the flux residue formed on the electronic assembly surfaces during semiconductor device joining to ceramic or organic chip carriers.
BACKGROUND OF THE INVENTION
Electronic assembly processes in microelectronic fabrication typically employ solder interconnections using a lead-tin (Pb/Sn) alloy for electrically joining a semiconductor device to a chip carrier, such as a ceramic substrate or a printed circuit board (PCB). Recently there has also been a focus on Pb-free solder alloys. An example of solder interconnections is C4 (controlled-collapse-chip-connection) technology, also called flip-chip bonding, where a semiconductor chip is attached to a substrate. This involves connecting an array of solder bumps on the semiconductor chip to bonding pads on the substrate by heating the assembly to solder reflow temperature in the presence of rosin based flux to form a solder connection.
In multilayer ceramic (MLC) products, solder bumps on silicon devices are generally Pb/Sn alloys of various compositions which are deposited by evaporation or plate-up techniques. Alternatively, Pb-free solder alloys of the type Sn/Cu/Ag and Bi/Sn are being investigated by the industry to replace Pb based solder alloys.
Solder interconnection processes with high melting solder alloys typically utilize a rosin-based flux, for example Alpha 102-1500 rosin flux (Alpha Metals Flux 102-1500), which is applied on the array of semiconductor device solder bumps or on the solder wettable pads on the substrate. Due to its viscosity and tackiness the flux helps maintain the alignment of the solder bumps to the substrate bonding pads. The flux also provides an oxide-free native metal surface on the Pb/Sn solder bumps on the device side by complexing with the surface oxide layer and thereby exposing the native metal underneath for metal-to-metal contact with the bonding pads on the substrate side. This provides bond integrity and long term reliability of the solder connections.
After alignment, the semiconductor device/substrate assembly is subjected to solder reflow in a furnace under N
2
or forming gas (5% H
2
in N
2
) using a temperature profile with a peak temperature depending on the solder type and composition. For example, with 97Pb/3Sn alloy, a temperature profile with a peak temperature of approximately 350 to 365° C. is used. The processes for flip-chip attachment to a multilayer ceramic chip carrier using solder bumps is disclosed in U.S. Pat. Nos. 3,401,126 and 3,429,040 (Miller et al.), the disclosures of which are incorporated by reference herein.
Rosin flux is also used for solder connections in the fabrication of ball grid array (BGA), ceramic ball grid array (CBGA), ceramic column grid array (CCGA), Surface Mount Technology (SMT) discretes, and hermetic seal band attachment to provide surface wettability of contacting surfaces during solder reflow.
The high temperature solder reflow conditions cause the rosin flux constituents to undergo a thermal transformation generating low molecular weight species which vaporize and are mostly removed in the process. However some of the reactive species, especially the higher molecular weight species, remain on the various surfaces. During the cooling cycle, after the solder reflow, the solder hardens forming solder connections between the semiconductor device and the substrate bonding pads. At the same time the thermally activated residual species from the flux decomposition undergoes cross-linking reactions which result in a resinous/carbonaceous by-product known as flux residue. This flux residue forms on the solder connections and on all other surfaces, including under the chip, on the semiconductor device and substrate that are exposed to the volatile species during solder reflow processing.
This cross-linked flux residue must be removed before subsequent operations can be performed. Failure to clean the flux residue can lead to reliability problems in long term use due to the possibility of stress corrosion when the assembly is exposed to a temperature and humidity environment. It is desirable to remove the flux residue prior to applying and curing an underfill material if such a material is used to encapsulate the solder connections for fatigue life enhancement and protection from the detrimental effect of environmental exposure. Failure to clean the flux residue can lead to voids in underfill coverage and adhesion failure resulting in device function reliability problems.
Rosin flux materials which are derived from the various Pine species are natural products comprising a complex mixture of cyclic hydrocarbon acids which constitute almost 90 percent of the rosin flux chemical composition along with a small fraction of polymerized rosin and about 10 percent of a neutral fraction constituting the corresponding esters, alcohols, acetate, and decarboxylated products. Referring to
FIG. 1
there is illustrated rosin acid structures I-IV, which are the major components of the rosin flux. Abietic acid (I) is the predominant component along with dehydroabietic acid (II), dihydroabietic acid (III), and tetrahydroabietic acid (IV). The rosin flux is known to promote wetting of metal surfaces due to the complexation reaction of the rosin acids in the flux with the oxide layer on the solder surface. The rosin flux also provides an oxide-free exposed metal surface of high surface energy which thermodynamically should readily wet the contacting metal surfaces on the substrate and thereby provide reliable chip-to-substrate interconnection.
Commonly employed methods of cleaning the rosin flux residue left after high temperature solder reflow in the process of device to substrate interconnection involves the use of chlorinated solvents such as tetrachloroethylene and aromatic hydrocarbons such as xylene. More recently solvent compositions for flux residue cleaning consisting of trans-1,2-dichloroethylene (1,2-DCE), fluorochlorocarbons, hydrofluorocarbons (HFC) blends containing 1,2-DCE as the major component, hydrofluoroethers (HFEs), or mixture thereof, have become available as a replacement of tetrachloethylene in flux residue cleaning. These halogenated solvents, however, are undesirable due to associated environmental and disposal issues. This limits their desirability for use in industrial applications.
The use of xylene as a flux residue cleaning solvent also has concerns since it is a highly flammable volatile organic compound (VOC) with a flash point of about 85° F., a boiling point of approximately 135-145° C., a high vapor pressure, and a high evaporation rate. The use of xylene requires high cost explosion proof equipment and chemical safety measures in the manufacturing environment as well as regulatory compliance for air emissions in the case of VOC's and hazardous air pollutants (HAPS).
There are a number of solutions proposed by others which provide alternate organic solvents that are relatively safe and mostly exempt from strict environmental regulations, as well as water-based cleaning solutions and the necessary equipment for alternate organic solvent and water-based cleaning.
Bolden et al., U.S. Pat. No. 5,340,407, the disclosure of which is incorporated by reference herein, describes a process of removing soldering flux and/or adhesive tape residue from a substrate. Bolden uses terpene-based cleaning compositions for flux residue removal from the surface of a printed circuit board, and also for the removal of the adhesive tape residue.
Bakos et al., U.S. Pat. No. 4,274,186, the disclosure of which is incorporated by refer
Lei Chon C.
Riccardi Demian M.
Sachdev Krishna G.
Cioffi James J.
International Business Machines - Corporation
Kornakov Michael
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