Die Bonding in an Optoelectronic World -- PCB assemblers venturing into optoelectronics will encounter new substrates and package designs that call for uncommon bonding equipment capabilities.Some industry experts envision the day in the not-too-distant future when printed circuit board (PCB PCB: see polychlorinated biphenyl.
in full polychlorinated biphenyl
Any of a class of highly stable organic compounds prepared by the reaction of chlorine with biphenyl, a two-ring compound. ) real estate will be commonly populated pop·u·late
tr.v. pop·u·lat·ed, pop·u·lat·ing, pop·u·lates
1. To supply with inhabitants, as by colonization; people.
2. with chips having both electrons and photons racing around in them. However, until that day arrives, original equipment manufacturers (OEMs) and electronics manufacturing services Electronic manufacturing services (EMS) is term used for companies that design, test, manufacture, distribute and provide return/repair services for electronic component and assemblies for original equipment manufacturers (OEMs). (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 Dealing with light (photons). See photon and photonics. 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 her·met·ic also her·met·i·cal
1. Completely sealed, especially against the escape or entry of air.
2. Impervious to outside interference or influence: sealed packages for stability; for example, laser diodes 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 an FR-4 board that is then over-molded. Another example is an optoelectronic transmitter mounted on a PCB (Figure 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 Noun 1. fiber optic cable - a cable made of optical fibers that can transmit large amounts of information at the speed of light
fibre optic cable
transmission line, cable, line - a conductor for transmitting electrical or optical signals or electric power , microelectronic-mechanical system (MEMS (MicroElectroMechanical Systems) Tiny mechanical devices that are built onto semiconductor chips and are measured in micrometers. In the research labs since the 1980s, MEMS devices began to materialize as commercial products in the mid-1990s. ) mirror arrays, and even micro-opto-electronic-mechanical system (MOEMS See MEMS. ) fluid switches. Their involvement will be at every level, from dies to packages to transmitting/receiving fibers (Figure 2).
Now and in the future, the heart of a photonic circuit is the laser diode. Some examples are edge-emitting laser diodes and vertical-cavity surface-emitting laser The vertical-cavity surface-emitting laser (VCSEL; [v'ɪxl]) is a type of semiconductor laser diode with laser beam emission perpendicular from the top surface, contrary to conventional edge-emitting semiconductor lasers (also in-plane diodes (VCSELs). These devices require a bonder with capabilities and design features that most traditional PCB assemblers This is a list of assemblers. Hundreds of assemblers have been written; some notable examples are:
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 pre·form
tr.v. pre·formed, pre·form·ing, pre·forms
1. To shape or form beforehand.
2. To determine the shape or form of beforehand.
1. that is then heated to reflow (1) The process of heating and melting the solder that has been screen printed onto a printed circuit board in order to bond chips and other components to the board. Surface mount chips (SMT) use the reflow method. Contrast with wave soldering. See also reflowable text. to make the attachment.
However, some packagers are using epoxy epoxy
Any of a class of thermosetting polymers, polyethers built up from monomers with an ether group that takes the form of a three-membered epoxide ring. The familiar two-part epoxy adhesives consist of a resin with epoxide rings at the ends of its molecules and a curing , 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 is absolutely essential to protect these devices from crushing or overheating Overheating
An economy that is growing very quickly, with the risk of high inflation. 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 on top of the ceramic, and finally a VCSEL (Vertical Cavity Surface Emitting Laser) Pronounced "vixel." A type of laser diode that emits light from its surface rather than its edge. A VCSEL's circular beam is easy to couple with a fiber, and due to its surface-emission architecture, can be tested 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 A positive displacement meter is a type of flow meter that requires the fluid being measured to mechanically displace components in the meter in order for any fluid flow to occur.
A diaphragm meter, with which most homes are equipped, is an example of a positive displacement meter. valve. This type of system delivers a Hershey "candy kiss Noun 1. candy kiss - any of several bite-sized candies
candy, confect - a rich sweet made of flavored sugar and often combined with fruit or nuts
molasses kiss - a candy kiss that resembles toffee " 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 Software that translates assembly language into machine language. Contrast with compiler, which is used to translate a high-level language, such as COBOL or C, into assembly language first and then into machine language. 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.
To perform laser diode epoxy attachments, certain features are useful on the bonder. For example, the bonder's pickup tool may be equipped with a dual head; one picks the laser diode from its carrier and the other is for the epoxy transfer tool.
Vision capabilities are needed to achieve the alignment and placement accuracy required to precisely attach the tiny laser devices. Several design features should be considered for the viewing system. First, a beam splitter A beam splitter is an optical device that splits a beam of light in two. It is the crucial part of most interferometers.
In its most common form, a cube, it is made from two triangular glass prisms which are glued together at their base using Canada balsam. that simultaneously superimposes the transfer tool tip image and site landing image at high magnifications during the alignment step is useful. Second, a stereo zoom microscope with fiber optic illumination allows the operator to inspect the attachment process and check alignment in real-time.
If VCSELs are being placed, misalignments of these chips may occur. For example, the VCSEL's emission point can become misaligned mis·a·ligned
misa·lignment n. when the bonder's pickup tool comes down to pick the VCSEL up out of the carrier because the vacuum on the tool head is turned off. However, if the vacuum were turned on, it could cause the VCSEL to jump up and become misaligned. Also, VCSELs in gel packs can become misaligned when the pickup tool pulls them away from the carrier's sticky bed.
However, misalignments can be completely avoided by choosing a bonder with a precising station. The precising station allows the VCSELs to be removed from the gel paks with an unheated tool without damaging the gel paks. Subsequently, the VCSELs can be handled with a preheated tool before placing them on the submount. The precising station also holds the VCSELs firmly in place using vacuum, so, when the pickup tool lifts each VCSEL, its emission point is not moved off-center from its alignment with the bonding system's overlay image.
VCSEL Viewing Challenges
To properly handle all forms of laser diode bonding, another vision feature is helpful on the bonder. Certain applications require more alignment/placement capability than is achievable using just a beam splitter and scope. One example is placing epoxy or gold/tin on top of a round TO header and then attaching a VCSEL. The problems that will be encountered in this type of application are virtually insurmountable without using either a special viewing feature on semiautomatic laser bonders or the automatic pattern recognition capability found on more expensive fully automatic bonders.
Specifically, the operator cannot see the VCSEL's emission point after the VCSEL has been picked up out of its carrier because the pickup tool holding the die blocks the view. To align the emission point to a specific feature on the submount, the operator must know where the emission point is on the laser diode. Normally, the diode's emission point is aligned to the center of the TO header. In this case, a reference mark must be provided that can be superimposed su·per·im·pose
tr.v. su·per·im·posed, su·per·im·pos·ing, su·per·im·pos·es
1. To lay or place (something) on or over something else.
2. over the header's shoulder so that the positions of the shoulder and the emission point on the VCSEL can be viewed simultaneously.
However, sometimes the emission point is located off-center on the laser diode because space is needed on the laser chip for the landing/bonding pad. A further complication is that, when the laser chip on the wafer is cut, a 10- to 15-micron variance in the shape of the cut chip may occur.
In addition, the TO header on which the laser is to be located is a machine-stamped piece of metal, so its tolerances can vary around the TO header's shoulder. If the TO header's shoulder is out of round by some amount, the emission point placement accuracy will be adversely affected. Even if the TO header is perfectly round, its diameter may vary substantially.
One solution would be a bonder that "remembers" where the emission point is before the VCSEL is picked up out of the carrier and that also compensates for the out-of-round shoulder/diameter variance. Expensive automatic bonders have pattern recognition systems that look at the chip while it is sitting in the carrier, memorize mem·o·rize
tr.v. mem·o·rized, mem·o·riz·ing, mem·o·riz·es
1. To commit to memory; learn by heart.
2. Computer Science To store in memory: the emission point's location and coordinate that with the landing site on the submount. Many PCB assemblers venturing into optoelectronics are not going to want to learn the ropes in this new field using an automatic machine and adding such capability to a semiautomatic bonder may not be a viable option.
To solve these problems, a relatively inexpensive video image marker could be added to a laser bonder. The video image marker consists of a keyboard, a control box and x-y knobs for adjusting the system's magnified electronic images. The video image marker creates a fixed video overlay The placement of a full-motion video window on the display screen. There are various techniques used to display video on a computer's screen, depending on whether the video source has been digitized or is still in analog NTSC format. of cross-hairs for centering the VCSEL's emission point. Different video patterns can then be superimposed on the submount on which the VSCEL is to be mounted. An example would be a circle overlay image of the outside diameter Outside diameter is the diameter of the addendum (tip) circle. In a bevel gear it is the diameter of the crown circle. In a throated wormgear it is the maximum diameter of the blank. The term applies to external gears.1
1. of the top of a TO header.
In operation, the laser diode is picked from its carrier tray and placed on the bonder's precising station. The image marker's crosshairs are aligned to the diode's emission point, and the diode is picked up in preparation for placement on the TO header. The marker's circle overlay is aligned to the header (Figure 4). With the emission point and header both now properly aligned, the operator can precisely place the laser diode on the header.
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 contaminated,
v 1. made radioactive by the addition of small quantities of radioactive material.
2. made contaminated by adding infective or radiographic materials.
3. an infective surface or object. .
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 designs in today's optoelectronics 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.
IPC (1) (InterProcess Communication) The exchange of data between one program and another either within the same computer or over a network. It implies a protocol that guarantees a response to a request. . (2001). Proceedings of IPC National Conference on Optoelectronics. Toronto, Ontario, Canada, May 3-4.
Weiss, S. (June 2001). Photonics and volume manufacturing: The automation crisis. Photonics Spectra Photonics Spectra is a monthly business-to-business (B2B) magazine published for the engineers, scientists and end users who develop, commercialize and buy photonic products. , pp. 97-110.
Don Moore is the president of Semiconductor Equipment Corp., Moorpark, CA; e-mail: firstname.lastname@example.org.
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