Gold-tin laser soldering process refined


FBGs; Gold soldering

Coherent's Laser Processing Center has developed a new method for employing high-power diode lasers to solder gold-metallised telecommunications fibres to gold-plated substrates.

"This new soldering process is particularly important in the telecommunications industry because gold plating has been adopted as an industry standard," said applications manager Tony Hoult. "This has been a difficult procedure due to the complex metallurgical challenges posed by locally heating the gold. This diode laser soldering technique will help a number of telecom component developers through a manufacturing barrier that they've been up against for some time."

Typical complications for soldering gold-plated objects to one another centres around the nature of the gold itself, specifically its high reflectivity and high thermal conductivity. In addition, a gold-tin solder (80Au:20Sn) must be used to join one gold-plated piece to another. The melting point for this mixture is 280°C, a large jump from the 160°C melting point of conventional lead-tin solder. The higher melting point makes soldering gold-plated parts to one another without causing associated damage very difficult.

To overcome these difficulties and produce high quality gold-tin solder joints, the Laser Processing Center employs a 40W NIR Fibre Array Package (FAP) system equipped with a standard 1:1 optics imaging accessory (OIA). To initially prove the feasibility of diode lasers for gold-tin soldering, Hoult placed a number of gold-plated telecom fibers on a piece of gold-plated stainless steel with a 250µm gold-tin preform and applied an 800µm diameter spot from the FAP system.

By strictly controlling the beam's timing, positioning and pulse, Hoult successfully produced defect-free joints with no associated fibre damage.

According to Hoult, a key element in the new gold-tin soldering process is its short (two-second) soldering time. "Since gold-tin solders have a very high 280°C melting point it can create joint oxidation problems if heat is applied for an extended length of time. These problems are drastically reduced by producing the joint in as short a time as possible," he said. "Our technique solders the material in two seconds and without any bulk preheating, a further complication that can create distortion."

Another critical factor in successful gold-tin soldering is precise spatial and temporal beam control. "Coupled with the short soldering time, you must strictly control the precise location of the laser spot and the rate at which you apply the heat," said Hoult.

Coherent's FAP-System for gold-tin soldering is a microprocessor-controlled unit with a standard fibre-array package (FAP) as the system's engine. The design lets users interchange laser diodes at different wavelengths in order to support a wide variety of applications. Moreover, the FAP-System provides the user complete control of the operating temperature and the current of the diode laser, as well as its mode of operation (CW, single shot or multi-shot) via its front panel or computer-controlled RS-232 interface.

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