International Business Machines Corporation

Alexis Bitaillou

Hormazdyar M Dalal

Kenneth Michael Fallon

Gene J Gaudenzi

Georges Henri Robert

Frederic Robert Pierre

Kenneth Robert Herman

Michel J Grandguillot

A method and structure for a solder interconnection, using solder balls for making a low temperature chip attachment directly to any of the higher levels of packaging substrate is disclosed. After a solder ball has been formed using standard methods it is reflowed to give the solder ball a smooth surface. A layer of low melting point metal, such as, bismuth, indium or tin, preferably, pure tin, is deposited on the top of the solder balls. This structure results in localizing of the eutectic alloy, formed upon subsequent low temperature joining cycle, to the top of the high melting solder ball even after multiple low temperature reflow cycles. This method does not need tinning of the substrate to which the chip is to be joined, which makes this method economical. It has also been noticed that whenever temperature is raised slightly above the eutectic temperature, the structure always forms a liquid fillet around the joint with copper wires. This liquid fillet formation results in substantial thermal fatigue life improvement for reduced stress at interface; and secondly, provides an easy means to remove chip for the purpose of chip burn-in, replacement or field repairs.

Method for forming reflowed solder ball with low melting point metal cap

Reflowed solder ball with low melting point metal cap

Disclosed is a method of forming solder terminals for a pinless module, preferably for a pinless metallized ceramic module. The method is comprised of the following steps: forming a substrate having a pattern of conductors formed onto its top surface and preformed via-holes extending from the top to bottom surface; applying a droplet of flux at at least one of said preformed via-hole openings of the bottom surface of said substrate to fill said via-holes with flux by capillarity and form a glob of flux at the bottom openings; applying a solder preform, i.e. solder balls on each glob of flux to which it will adhere, the volume of the preform being substantially equal to the inner volume of the via hole plus the volume of the bump to be formed; heating to cause solder reflow of the solder preform to fill the via-hole and the inner volume of the eyelet with solder; and, cooling below the melting point of the solder so that the molten solder solidifies to form solder terminals at the via-hole locations while forming solder columns in the via-holes. The resultant pinless metallized ceramic module has connections between the I/O's of the module interfacing with the next level of packaging, (i.e., printed circuit boards), that consist of integral solder terminals. Each integral solder terminal comprises a column in the vias of the metallized ceramic substrate, a mound of solder at the top surface of the substrate and spherical solder bumps on the bottom level for making interconnections with the next level of packaging.

Method of forming solder terminals for a pinless ceramic module

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Bretigny sur Orge, France

Alexis Bitaillou