Cleaning compositions for solid surfaces – auxiliary compositions – Cleaning compositions or processes of preparing – For cleaning a specific substrate or removing a specific...
Reexamination Certificate
2001-01-16
2003-01-07
Webb, Gregory E (Department: 1751)
Cleaning compositions for solid surfaces, auxiliary compositions
Cleaning compositions or processes of preparing
For cleaning a specific substrate or removing a specific...
C510S175000, C510S506000, C134S001300, C134S003000, C134S010000, C134S038000, C134S040000, C134S042000
Reexamination Certificate
active
06503874
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a method for removing flux residue and other surface contaminants formed on the surfaces of microelectronic components during soldering operations using solder paste in electronic module assembly, and, more particularly, to a semi-aqueous solvent cleaning process for removing flux residues that are formed on a chip, substrate, card or other electronic component containing plastic components.
2. Description of Related Art
Semiconductor devices are becoming smaller and more dense with the evolution of new technology. However, increases in circuit density produce a corresponding increase in overall device enhancements and semiconductor manufacturers are therefore constantly challenged to improve the quality, design and other aspects of their products. Some electronic modules carry silicon device chip, surface mount components (SMT) as well as plastic components like plastic ball grid array module (PBGA) on a ceramic chip carrier. For those type components special consideration is required for the assembly fabrication procedures and cleaning methods to remove flux residues formed during solder connections to assure cleaning solvent and process compatibility with all the module materials including plastic components.
Assembly fabrication processes for electronic circuit devices typically use solder connections for electrically joining an electronic device to a support substrate, solder ball and solder column attachment to chip carrier, for ceramic ball grid array (CBGA) and ceramic column grid array (CCGA) connections to organic board or a printed circuit board (PCB), and to attach other SMT components to a PCB.
C4 technology (controlled collapse chip connection), also called flip-chip bonding, is used for example to attach a semiconductor chip to a ceramic substrate. This bonding method involves connecting an array of solder bumps on the chip circuit structure side to C4 bonding pads on the substrate by high temperature solder reflow in a furnace under N
2
or forming gas (5% H
2
with 95% N
2
), typically requiring reflow temperature of 350-365° C. for the 97/3 Pb/Sn alloy in the presence of a rosinbased Alpha 102 solder flux to form a solder bond. The flux residue formed in the C4 joining process is removed in a solvent cleaning process using xylene or chlorinated solvents. Low temperature soldering processes in electronic module assembly operations generally employ low melting alloys or solder paste of which Pb37/Sn63 eutectic paste is widely used. For example, eutectic solder paste is used to attach 90/10 Pb/Sn solder balls and solder columns to a ceramic chip carrier in a ceramic ball grid array (CBGA) and ceramic column grid array (CCGA), to attach solder balls to a plastic chip carrier in a plastic ball grid array (PBGA), to attach PBGA and other plastic components to a ceramic chip carrier, to join surface mount components as capacitors/resistors to a chip carrier, and for mounting of electronic components onto PCBs.
Solder pastes typically comprise a solder powder, for example, eutectic Sn-Pb alloy (Pb37%-Sn63%), melting temperature about 183° C., eutectic Bi-Sn alloy, melting temperature about 135° C., ternary alloy Bi-Sn-Pb and Sn-In along with other additives for wettability, flow properties, and compatibility with screen printing or dispensing methods. Various types of pastes that are used in soldering processes in electronics assembly include: water soluble solder paste flux formulations, no-clean (NC) flux pastes, and non-water soluble paste compositions. The fluxing agents generally used in solder pastes are, organic acids or salts, fatty acids or esters, polycarboxylic acids and derivatives, hydroxy carboxylic acids, organic acid esters, rosin acid and ester compositions, typically abietic acid and its derivatives, and in some cases amines including alkanolamines, aliphatic amines, and aromatic amines.
The presence of flux in solder pastes used for solder connections, for example, in the fabrication of BGA, CBGA, CCGA, SMT discretes, and solder ball and column attachment to a chip carrier for package-to-organic board connections, provides surface wettability of the contacting surfaces during joining which is essential for solder bond integrity.
In the cooling cycle of the thermal profile for joining, the solder hardens and at the same time, vaporized residual flux and its decomposition products deposit on the various exposed surfaces. In a subsequent step, the electronic assembly is then subjected to a solvent cleaning operation to remove flux residue prior to further processing.
The flux residue must be removed from all critical surfaces prior to further operation since it can cause function failure during long term use due to, for example, stress corrosion during exposure to temperature and humidity environment. It is therefore necessary after chip joining and other soldering processes during fabrication of electronic circuit assemblies, that the flux residue be cleaned off (removed) before subsequent operations.
Rosin fluxes are natural products comprising a complex mixture of cyclic hydrocarbon acid fractions which form almost 90% of the rosin flux chemical composition and about 10% neutral fraction constituting their corresponding methyl esters, alcohols, acetate and decarboxylated products. Among the rosin acids which are the major components of the mixture, abietic acid is present up to between about 50 to about 60 percent, dihydroabietic acid about 14%, and tetrahydroabietic acid and dehydroabietic acid being the minor constituents of the rosin acid mixture. These rosin fluxes and other fluxes are known to promote wetting of metal surfaces by their chemically reacting with an oxide layer on the surface of tin and/or lead providing an oxide-free metal exposed surface of high surface energy which thermodynamically should readily wet clean contacting metal surfaces on the substrate and provide reliable solder interconnection.
In the case of water soluble solder paste flux, residue after a soldering operation is generally removed by aqueous cleaning with water alone or water in conjunction with a suitable surfactant. With solder pastes which contain flux and other organic constituents which are not water soluble, flux residue cleaning processes traditionally have employed halogenated hydrocarbons, such as, perchloroethylene, and aromatic hydrocarbons as xylene similar to the rosin flux residue cleaning with xylene in the C4 chip join process. The halogenated hydrocarbon solvents, however, are undesirable due to their harmful effects on the environment and on human health as stated above.
With xylene there are also safety concerns as it is a highly flammable and volatile solvent having a flash point of 85° F. which requires special high cost equipment for chemical safety in manufacturing environment as well as for compliance with rules for air emissions of hazardous air pollutants. In addition to these problems in the use of xylene, perchloroethylene and related solvents, there is a concern as to their viability as flux residue cleaning solvents in instances where the electronic module or ceramic chip carrier may contain plastic components mounted thereon prior to another soldering operation using solder paste, for example, eutectic solder paste for low temperature reflow process (220° C. peak temperature). For example, fabrication of an electronic module with a 42.5 mm×42.5 mm ceramic chip carrier with solder columns involves (1) C4 chip and C4 decaps attachment using alpha 102 rosin flux with high temperature solder reflow at 360° C. peak temp; (2) flux residue cleaning with xylene; (3) solder joining SMTs and a plastic component (PBGA) with water soluble paste with low temperature solder reflow at 220° C. peak temperature; (4) flux residue cleaning with deionized water; (5) solder column attachment using solder paste with non-water soluble flux component at 220° C. peak temperature solder reflow; and (6) flux residue cleaning. A problem associated with suc
Lei Chon C.
Sachdev Krishna G.
Cioffi James J.
DeLio & Peterson LLC
International Business Machines - Corporation
Tomaszewski John J.
Webb Gregory E
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