Finishing Those Near-Net-Shape Parts

CNC lathes and machining centers provide the finishing touch for near-net-shape semiconductor housing parts.

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Latronics Corp., Latrobe, Pennsylvania, manufactures ceramic-to-metal, glass-to-metal and metal-to-metal hermetic seals for manufacturers of semiconductor devices. The firm's most widely used products are ceramic-to-metal housings with metal-to-metal lids that enclose and protect customers' silicon wafers from corrosion and mechanical damage. The completed assemblies become such items as silicon-controlled rectifiers, diodes, thyristors, gate turn-off devices and other semiconductor devices, used in applications ranging from locomotives, fork lifts and welding equipment in the industrial sector to air conditioners, electric ovens and washing machines for the home.

Semiconductor devices are hermetically sealed to prevent corrosion that could result in a failure. The housings for these devices are made to exacting standards to ensure seal integrity. Latronics started into the business of making hermetic seals in 1958 and has assembled an impressive array of production equipment—automated metallizing equipment, metallizing furnaces, CNC machine tools, atmospheric brazing furnaces, nickel plating lines, mass spectrometer leak testing equipment, and so forth—to ensure the integrity of its products.

Major components of Latronics' high-power semiconductor housings are the body and lid. The body consists of a high-alumina ceramic shell or sleeve brazed to metal parts (usually stampings) at high temperatures. (The ceramic shell must first be metallized to permit a ceramic-to-metal bond, which explains the metallizing equipment itemized above.) The housing lid is solid copper and must be hermetically sealed to the housing body.

In Latronics' early years, the copper lids—some as large as 5 inches in diameter—were machined from bar stock on CNC lathes. The process was expensive because of the large amount of scrap generated, and it was abandoned. Next, the firm purchased forged lids, but the diversity of its housing sizes and styles created cost, lead time, inventorying and quality control problems, so use of the forged lids was also discontinued.

Although the firm ruled out the conventionally forged lids, it discovered a number of advantages to using near-net-shape parts and began looking for a near-net-shape process more compatible with its requirements. It found what it was looking for in the orbital forging process. Unlike conventional cold forming processes, the orbital forging process applies forming force to only a portion of the workpiece surface at a time. The lower die thrusts the billet against the upper die, which executes an alternating motion known as precession. The action kneads the workpiece material into the die set.

Surface friction and force requirement are low, resulting in a high level of deformation. Components with complex geometries exhibit high accuracy and good fill characteristics in critical areas.

Ron Yurko, vice president, engineering, for Latronics, points out a number of advantages of orbital forging for the firm: "Our housing lids are copper, so the raw material for our orbital forging operation is slugs cut from copper bar. Unlike previous processes, there is no waste. The bar is cut to the slug length that provides just the right amount of material to fill the die.

"With previous processes, we purchased copper bar in many different diameters to more closely match the diameter of the lids we were machining," Mr. Yurko continues. "With the orbital forging process, we can produce all of our lid sizes from only two bar diameters, 1 inch and 1 ½ inches, which makes it easier to buy and store the material."

Another advantage of the orbital forging process for Latronics is that it work hardens the copper bar, increasing its mechanical properties. "If we machined the housing lid directly from copper bar, it is possible that a leak could occur right through the metal," Mr. Yurko explains. "We would catch the defective part in leak testing, however it would have to be scrapped. The orbital forging operation eliminates that danger because it changes the microstructure of the metal to the point where the metal becomes hermetic. That increases our yield."

Space availability is always a consideration at Latronics, so Mr. Yurko appreciates the relative compactness of the orbital forging machine. The shop has two 200-ton machines, which are considerably smaller than the conventional hydraulic or mechanical cold forming machine that would be required to produce in-house the range of lid sizes needed. Also, both orbital forging machines have automatic load/unload, enabling them to run for long periods unattended.

Secondary Machining A Must

For many applications, parts produced by the orbital cold forging process can be used as they come from the machine. However, stringent dimensional accuracy, flatness and surface finish requirements for Latronics' housing lids dictate secondary operations for the forged parts. Typically, operations involve turning and holemaking operations on a CNC lathe and slotting on a machining center.

Latronics operates three turning cells, each with two or three CNC lathes from Wasino Corp. USA (Rolling Meadows, Illinois). The oldest cell is about 10 years old and consists of three G-Series lathes, each equipped with a part-loading carousel and an integral gantry robot for automatic loading/unloading. The machines are arranged in a tight U-shaped arrangement, making it easy for the cell's sole operator to monitor the machines and keep their carousels stocked with workpieces.

The carousel is a staging device that permits the machine operator to load several stacks of workpieces in the machine at one time (as shown on page 33). Each stack is supported on a base plate and contained by three retaining rods that quickly and easily adjust to the diameter of the workpiece being turned. With the part-banking system, the operator can load multiple parts at a time in one or more lathes and attend to other tasks away from the machine.

The gantry robot, which is the second part of the machine's load/unload system, automatically takes workpieces one at a time from the carousel and delivers them to the chuck. A part that requires turning on both sides is turned on the first side, removed from the chuck by the gantry robot, delivered to a part-flipping station, flipped over, and returned to the chuck for machining of the second side. Because the carousel and automatic load/unload features permit unattended operation for extended periods, one operator can usually tend several machines. In fact, each of the Latronics' three turning cells is tended by one operator.

The G-Series machines are equipped with gang tooling instead of a turret. They offer very fast chip-to-chip indexing times, high accuracy (±40 microinches) and easy setup. The gang tooling plate accepts rotary tooling, so the G-Series cell is frequently used for jobs requiring holemaking operations.

A second turning cell consists of Wasino A-12 CNC lathes. These turret-equipped machines are equipped with live tooling and also get jobs that require holemaking and light milling operations.

Latronics' third and newest turning cell consists of three Wasino J-Series CNC lathes, also arranged in a U. Like the other Wasino lathes, the J-Series machines are equipped with part-loading carousels and gantry loaders for automatic loading/unloading. The firm chose the J-Series machines primarily for turning and facing operations on relatively small diameter parts, to round out its CNC machining capabilities.

Almost A Job Shop

Latronics is essentially a batch-type operation. Lot sizes range from 50 to 5,000 pieces, and setup changes are frequent. The firm manufactures products to order rather than to inventory, so it places a premium on flexibility. The semiconductor industry is going through a period of great change, and new products are being introduced at a dizzying pace. Latronics needs to be able to respond quickly to industry changes, and the flexibility provided by its CNC machine tools helps it to do just that. For example, the CNC machines are not only used for production but are also used to produce the machining and assembly fixtures.

A Different Role

For many production facilities, Wasino CNC lathes with gantry loaders are the workhorses of the operation, whittling slugs cut from bar to the size and shape of the finished parts. Latronics uses the lathes as finishing machines to impart the accuracy, flatness and surface finish—that the customer requires—to the housing lids.

"The orbital forging process produces lids that are very close to finish size," Mr. Yurko explains. "We can usually limit the amount of stock to be machined from the lids to 0.020 inch or less. As a result, we have been able to reduce our machining time on the lids by about 60 percent.

"We use diamond tooling to machine the housing lids," he continues. "Previously, we had to lap the parts to achieve our customers' part flatness and surface finish requirements. Now the parts are within specifications as they come off the lathes, and we have been able to eliminate lapping as a production operation. Now it is used only to salvage parts rejected because of cosmetic flaws."

The diamond tooling not only gives Latronics the quality it needs, but it also lasts a long time, which makes for economical tooling costs. The longevity also reduces the need for tooling adjustments—an important consideration for machines that run 24 hours a day and produce thousands of parts per week.

When Latronics first purchased the Wasino lathes, the firm would machine side 1 of all the parts in the run, change the setup, and reload the parts in the carousel to machine side 2. Problem was, the part surfaces must be completely mar-free and the additional handling involved in reloading the parts in the machine created the risk that some would be damaged. "If you're going to pick them up again, you're going to nick some parts," Mr. Yurko insists.

The problem was simply and easily solved by installing a part-flipping station in each machine. With the optional accessory, after side 1 of the housing lid is machined, the lathe's gantry robot delivers the part to the flipping station where it is turned over, picked up again by the gantry robot and reinserted in the chuck for machining of side 2.

"It's been a big help," Mr. Yurko notes. "We not only reduce the risk of damaging the parts by reducing the handling, but we also free up the operator's time for more important tasks. Now we get the parts in and out of the chuck and into a basket for subsequent operations and degreasing without any cosmetic damage.

"We have to be able to machine parts economically to compete with foreign competition, and avoid damaging them in the process," Mr. Yurko continues. "The Wasino CNC lathes allow us to do both. Their integral gantry robots met our part-handling requirements without a lot of changes or modifications. Competitive machines that we looked at would have required a lot of engineering to do our parts. The Wasinos suited our product line perfectly."

After the housing lids are processed through one of the turning cells, some require a slotting operation, which is performed in one of the machining department's two milling cells. One cell consists of two Haas VF-3 vertical machining centers made by Haas Automation Inc., Oxnard, California. The other consists of a Haas VF-3 and a VMC that Mr. Yurko declares is the last such machine made by Wasino. A slotting operation, performed with a slotting saw, is shown on page 32 being performed on one of the Haas machines. Multiple vises or collets are mounted on a plate on the VMC worktable so that multiple parts can be slotted in one machining cycle.

After the slot milling operation, the parts are deburred. "In a semiconductor, a loose burr can cause a device failure that could, for example, stop a locomotive dead in its tracks," Mr. Yurko explains. "We take great pains to make a burr-free part."

An Export Product

Latronics is the only manufacturer of semiconductor housings in the United States. Despite plenty of competition from European and Asian manufacturers, the firm exports 70 percent of its production to world markets. Like many firms, Latronics has felt the effects of the global economic downturn. However, the firm is offsetting the slowdown in product sales by making available its extensive metallizing, orbital forging, plating, brazing, CNC machining and other capabilities to local firms on a contract basis.

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