The first step in making a fault indicator is making the semiconductor chip. Chip manufacturing is a set of very complex, very demanding chemical processes. LEDs and IR emitters share many of the processes developed for non-light-emitting semiconductors except that the basic materials are gallium compounds, rather than silicon.
The chip manufacturing process starts with a cylindrical ingot of the crystalline material grown in a furnace, under high-pressure conditions. While the process is similar to growing silicon crystals, gallium compounds have a more comlex chemistry. Some of the newer materails are grown under very high pressure, up to 1100 psi (75 atmospheres), and the furnaces are installed in concrete bunkers designed to contain an explosion just in case anything goes wrong.
After the crystal ingots are grown, they are ground to a uniform diameter, using microprocessor-controlled equipment. Next, they are sliced, like a salami, into thin wafers. The edges of each wafer are rounded to minimize breakage during the subsequent polishing operation.
Each subsequent step in the chip-making process is a complex, but precise chemical procedure, usually performed under clean-room conditions using hazardous chemicals and very expensive equipment. One key step is creating the pn junction needed to make the emitter work by growing a layer of semiconductor material that matches the crystal lattice of the substrate. This moleculaly thick addition is called the epitaxial layer.
For some materials the epitaxial layer is added by dipping the wafer in a liquid, and the process is then called liquid-phase epitaxy. For other materials the layer is deposited from a vapor, and the process is then called vapor-phase epitaxy. Usually there is a single epitaxial layer, but for some LEDs there are two or more junctions, like a club sandwich. Sometimes all or part of the original substrate is etched away. Metal is selectively vacuum deposited to form contacts or electrodes. Coatings are applied.
Finally the processed wafer is glued to a membrane, and the wafer is cross-cut into tiny chips, which are then tested for forward voltage and light emission. Most often the chips are tiny squares, but we can make rectangles or custom sizes if the customer application calls for something different.
At last the finished chips are ready for the next step in production of our fault indicator -- conversion into an LED. Look at the chart below to learn more about LED materials and what colors each material produces.
The second step is to convert that chip into a finished light-emitting diode. Most Lumex LED lamps use "lead-frame" construction. The "lead-frames" are precision stamped metal, usually steel, and at the beginning of the process consist of a stamping about six inches long containing the leads and internal structure for twenty LEDs, connected together at the bottom and about halfway up the leads. At the top, one lead has a pennant-like shape and is called rhe "anvil"; the other lead is essentially straight and is called the "post." For most Lumex LEDs the lead frame is selectively hot tin or hot solder dipped.
The first operation in making the lamp is to connect the die (the semiconductor chip) to the anvil. This part of the process is called "die bonding," and it is usually done on highly automated equipment, using conductive epoxy.
Then we weld a tiny gold wire from the top surface of the die to the post. Again this is a highly automated porcess, called "wire bonding." This wire completes the electrical connection, but we can`t test the LEDs yet because they are still shorted by the connecting strips.
The third step is epoxy casting, where we place the lead frame in a mold, pour in optical-grade epoxy resin, and oven cure. We`ve now got twenty finshed (but untested) LEDs on the stamping.
Next, we partially separate the LEDs by punching out, or trimming, the middle and bottom connecting strips and one leg. The fancy term the industry uses for this operation is "partial singulation." We can trim the middle connecting strip even with the leads, or leave extensions, called "dam stops."
The final operation in making the lamp is to put the partially separated strips of LEDs into a high-speed automated tester. Singulation, separation of the individual LEDs, is done as part of this process. Up to nine different bins, combinations of color and intensity can be used. All LEDs are sorted by intensity, and yellow (and sometimes green) LEDs by emitted color or hue.
Axial lead LEDs, such as Lumex SSL-LXA223 series, as well as surface-mount devices such as SML-LX0603, are made by a similar process except that the epoxy dome is transfer molded rather than cast.