26 March 2014
Die Bonding in an Optoelectric World
by Don Moore, President, Semiconductor Equipment Corporation
Circuits Assembly, 2001
|Some industry experts envision the day in the not-too-distant future when printed circuit board (PCB) real estate will be commonly populated with chips having both electrons and photons racing around in them. However, until that day arrives, original equipment manufacturers (OEMs) and electronics manufacturing services (EMS) providers will have to contend with the technology as it exists today. What they now encounter is a labor-intensive, low volume and high cost world where very few photonic devices are being packaged and assembled using automatic equipment because standard processes and packages do not exist.
This "other world" consists of odd substrates and package designs with tiny, delicate photonic devices inside them. Frequently, the photonic devices are housed in hermetically sealed packages for stability; for example, laser diode inside transistor outline (TO) headers. The mounting structures to which the optoelectronic devices are connected to form circuits may be flex circuitry, FR-4 boards or several other interconnect materials. One example is a telecom array amplifier on am FR-4 board that is then over-molded. Another example is an optoelectronic transmitter mounted on a PCB (Fig 1).
Because of the nature of today's optoelectronics market, PCB-oriented OEMs and EMS providers must become familiar with a wide variety of substrates as well as both active and passive devices such as transmitters, receivers, lenses, individual laser diodes, laser diode arrays/bars, photodetectors, fiber optic cables, microelectronic-mechanical system (MEMS) mirror arrays, and even micro-optoelectronic-mechanical system (MOEMS) fluid switches. Their involvement will be at every level, from dies to packages to transmitting/receiving fibers (Fig 2).
Now and in the future, the heart of a photonic circuit is a laser diode. Some examples are edge emitting laser diodes and vertical - cavity surface-emitting laser diodes (VCSELs). These devices require a bonder with capabilities and design features that most traditional PCB assemblers are not accustomed to using.
Currently, gold/tin alloy is the most specified bonding material for attaching VCSELs and edge-emitters to submounts in telecom applications. This alloy is applied either via plating onto the submount or via placement of a gold/tin preform that is then heated to reflow to make the attachment.
However, some packages are using epoxy, particularly for computer communications applications, because it simplifies and speeds up the attachment process and is less expensive. For example, a laser diode can be epoxy-attached to each TO header sitting in a tray on the bonder's placement station, and then all of the epoxy can be cured simultaneously.
Once standard processes and packages for optoelectronic assembly are implemented, epoxy bonding should gain even more favor because it can facilitate the automation process that follows the development and startup low-volume production phases for new designs. The message: Choose a bonder that can run both alloy and epoxy attachment processes so that the bonder's use can be maximized.
Laser Diode Bonding
A laser diode's grain-of-salt size and delicate nature create unusual challenges for OEMs and EMS providers. Precise computer control of the bond loads and temperature profiles are absolutely essential to protect these devices from crushing or overheating during the bonding process. The bonder's Z motion must apply a repeatable, very low bond load to the device to avoid crushing or even stressing it.
Another consideration that may affect the choice of a bonder is its ability to stack components. For example, the application may call for epoxy to be deposited first on a TO header, then an isolating piece of ceramic placed in the epoxy, then epoxy put top on the ceramic, and finally a VCSEL placed into it.
If epoxy is to be used, the size of the epoxy deposit must be smaller than is achievable with a standard needle dispensing system with a positive displacement valve. This type of system delivers a Hershey "candy kiss" deposit, which is too much material for VCSEL applications. Because of the large deposit, the material flows out around the edges when the device is placed, moves up onto the emission area of the laser device and obstructs the emitted beam of light (Figure 3).
Today, many optoelectronic assemblers attempt the epoxy attachment process using a needle dispensing system. However, these assemblers typically lack extensive semiconductor packaging experience. To succeed, the assembler must have a bonder equipped with epoxy transfer tool technology, which was commonly used in the semiconductor industry 20 to 25 years ago. This technology allows the bonder operator to precisely deposit extremely small volumes of epoxy on the submount.
Laser diode bonding requires a sturdy bonder platform/gantry and precise slides and rails. Z-motion control should allow precise, repeatable die bonds at low loads. For gold/tin bonding processes involving either edge-emitting lasers and VCSELs, the bonder must be able to hold the die at a specific, consistent load during both bonding and cool-down cycles so the die is not crushed or stressed. In epoxy attachments, a specific, consistent load is necessary to ensure that the epoxy thickness between the two surfaces is as uniform as possible.
If gold/tin alloy is used, a scrubbing capability may be helpful. Scrubbing eliminates any voids that may be present between the interconnecting surfaces. The amount of scrubbing needed depends on the diode size; the smallest diodes generally do not need any. The bonder should be equipped with a head that returns the scrubbing action precisely to the starting position to maintain the original alignment. Depending on die size, the amplitude and frequency of the scrub may need to be varied. When working with edge-emitters, great care must be taken not to scrub in such a way that the bonding material piles up on the diode's front and back crystal facets, or they will become contaminated.
The bonder's software program should be versatile enough for both eutectic and epoxy applications. All operating functions should be controlled with a real-time software system that provides process verification via closed-loop feedback.
To take on the challenges and reap the rewards offered by the exploding optoelectronics market. PCB-oriented OEMs and EMS providers need to rethink the traditional assembly process. the substrates and package design in today's optoelectronic assembly call for uncommon bonding equipment capabilities. With an appropriate bonder, OEMs and EMS providers can gain experience and compete in this new and exciting industry.
|As appeared in Circuits Assembly - October 21|