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Hello! I'm TR of R&D department.
In this article, I’d like to explain about our solder forming process by electroplating in order to cope with the miniaturization and high density of wiring.
What is "Solder" required for mounting of electronic devices
When we talk about "soldering," many people have the image of connecting electronic components and wiring to a printed circuit board using a soldering iron. This is a method called "Solder", in which a low melting point tin-lead alloy is heated and temporarily melted to connect the terminals and wiring of electronic components to the wiring on the printed circuit board side. Nowadays, lead-free solder is used in electronic device products due to environmental regulations. In recent years, in the mounting of electronic devices, the solder forming process by electroplating has begun to be adopted in order to cope with the miniaturization and high density of wiring. In addition, there is an increasing need for low melting point solder to ensure connection reliability.
Responding to lead-free solder
In the 2000s, due to heightened environmental awareness mainly in Europe, the use of lead harmful to the human body was regulated by the RoHS Directive, etc., and lead-free solder was developed, and tin (Sn) , Silver (Ag), Copper (Cu) solder alloy products are widely used.
In surface mounting electronic components on a printed circuit board(PCB), solder ball and solder paste are commonly applied as interconnection methods. Solder balls are placed on a semiconductor chip terminal with a mounter. Solder paste is applied on the printed circuit board by screen printing to form a solder layer. After those processes, the solder is melted by heating in a reflow oven to connect electronic components on a PCB at one time.
As the number of connection pins increases with the miniaturization and higher performance of semiconductor chips, lead-free solder forming technology by the wafer level electroplating process is becoming popular to cope with further miniaturization and narrower pitch of BGA (Ball Grid Array) used for flip chip bonding.
Formation of Cu pillar bumps
With a solder ball, it spreads widely in the lateral direction during reflow. It is necessary to secure the pitch between terminals so as not to cause a short circuit, and there is a limit to narrowing the pitch. For semiconductor chips that require further miniaturization, Cu pillar bumps made by electroplating at the wafer level are used. Cu pillars can be interconnected with a smaller amount of solder without spreading widely. Bumps with electrode diameters of several microns to several tens of microns are called "microbumps" that is applied for connecting between semiconductor chips and fine patterned silicon or organic substrate.
In the Cu pillar bump process, after forming a Ti / Cu seed layer (conductive layer) by sputtering, a photoresist pattern is formed by photolithography, a columnar electrode is formed by Cu plating in the opening area (electrode forming part), and nickel as a barrier layer is further formed. Lead-free solder such as tin-silver (Sn-Ag) is continuously formed on it . These three types of laminated plating are continuously processed by one electroplating tool. If the height (plating film thickness) of the Cu pillars varies widely, it directly affects the connectivity during mounting, so it is extremely important to ensure severe film thickness uniformity in the plating process.
Cu pillar bump formation process flow
Support for low melting point solder plating
As the result of progressing miniaturization of interconnection with the spread of 3D semiconductors (3D-ICs) and 2.5D silicon interposers in which heterogeneous devices are integrated, the problems such as warpage of substrates and disconnection of wiring due to the mismatch of the coefficient of thermal expansion(CTE) between the materials used for the substrates have become apparent. In the integration process, low melting point solder is being considered in order to reduce the reflow temperature.
Candidates for low melting point solder include tin alloys such as tin-bismuth (Sn-Bi) and tin-antimony (Sn-Sb), and Indium (In) that is low melting point metals with excellent expandability and ductility even at low temperatures and excellent heat dissipation. It is being studied under advanced integration of electronic devices and severe temperature conditions on the product usage.
Lastly
What do you think?
We are accepting for designing and manufacturing wafer process solder electroplating tool and the demo evaluation for the installation to our customers.！Please contact us for solder plating!
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