Low-melting metal bumps such as solder bumps are used for the mounting of IC chips and the like. Formation of such bumps takes place on the IC chips or on the multilayer printed wiring board.
BGA (ball grid array) is a form of package in which the bumps are provided to an IC chip. This term generally refers to a package in which the IC chip is mounted on a package substrate.
WL-CSP (wafer level-chip size packaging) is a new technology for the high-density mounting. In the WL-CSP, a plurality of IC chip circuits is formed on one wafer and collectively subjected to electrode formation, packaging and burn-in test; thereafter the wafer is cut into IC chip packages.
Collective electrode formation in the WL-CSP may be performed by electrolytic deposition, filling a metal paste followed by reflowing, or placing metal balls followed by reflowing.
The increase of IC chip density has lead to higher density and miniaturization of bumps on the IC chip. Solder bumps have increasingly been studied and been in practical use in the WL-CSP mounting technology because they allow both connection reliability and reduction of processing cost. Specifically, the solder bumps are formed by electrolytic deposition using a mask that is produced by patterning a film of at least 50 μm thickness formed by application or lamination of a positive or negative liquid resist or dry film.
Meanwhile, when the solder bumps are provided to a multilayer printed wiring board, a solder resist layer is formed on the wiring board to prevent fusion bonding between the solder bumps. The solder resist layer is provided with apertures at positions corresponding to electrode pads. To form the solder bumps in the solder resist layer, mask printing of a solder paste is performed. The mask used in the mask printing has a pattern (pattern of apertures) that corresponds to the apertures of the solder resist layer. In carrying out the mask printing, the mask is placed on the solder resist layer so as to align the mask pattern and the apertures of the solder resist layer. The solder paste is then printed through the mask within the apertures of the solder resist layer on the electrode pads; subsequently, the solder paste is reflowed to form solder bumps on the electrode pads.
A variety of studies have been made to cope with reduction of solder bump pitches. For example, JP-A-H06-350230 discloses formation ofapeelable solder dam resist on a solder resist, and JP-A-2000-208911 discloses a method wherein a dry film is laminated on the solder resist layer, apertures are created at positions corresponding to electrode pads, a solder paste is applied followed by reflowing, and the dry film is peeled.
In the aforementioned process of bump formation, the positive resist, although easily peelable in general, causes difficult control of the aperture pattern configuration, often resulting in nonuniform bump size.
On the other hand, the negative resist permits relatively easy control of pattern shape, but is generally difficult to peel because of its photocrosslinking characteristics. In particular, when the solder bumps are formed by filling and reflowing the paste, the resist becomes even more difficult to peel from the board because the crosslinking in the negative resist proceeds to a further extent by the heat applied in the reflowing.
The resist film peeling is generally performed by spray method with an aqueous solution of sodium hydroxide or sodium carbonate. For the dry-film or liquid negative resist, dip method has been increasingly employed with use of a peeling solution that contains a highly polar solvent and an organic alkali. The former method, although inexpensive, often results in residual resist around the bumps having a high density, and causes deterioration of the bumps. The latter method has disadvantages that the peeling solution is expensive, and the metal surface of the bumps and the underlying board surface are corroded when the organic alkali concentration and the peeling solution temperature are increased for improving the peeling efficiency per amount of the peeling solution.