Step smartly into smaller footprints
Diminutive device packaging--something to "die" for
By Rick DeMeis, Associate Editor
Newton, MA--Largely driven by hand-held telecommunications and portable PC applications, ICs and other electronic components are packing more punch into existing and smaller packages. But with smaller die sizes, increased component density, and more compact packages overall, designers have to consider not only physical effects coming into play in a component but the limits of its manufacturing process.
Take, as an example, Burr-Brown's (Tucson, AZ) 3.10 3 3.00-mm footprint, SOT-23 package, 1.45-mm high, introduced 10 years ago as a three-lead package holding discrete transistors. As with any component, volume production efficiencies have driven down costs, and advances in circuit design and manufacturing processes have boosted performance. The latest eight-lead version contains two complete devices, such as op amps.
|Solid-state device size reduction has progressed dramatically over time. Today's op amps, for instance, are about 20,000 times smaller in volume than the first transistor devices of several cubic inches.|
According to Burr-Brown Strategic Planner Howard Skolnik, "The shrinking of integrated circuit die may require significant changes in the basic fabrication process, often with related changes in circuit design, packaging, and manufacturing. Improvements in wafer fabrication, including reduced defect density and sub-micron line widths, have advanced the ability to economically shrink the size of die and overall component dimensions." He adds, "If defect density is not lowered, reducing size will usually result in higher reject rates."
Test finesse. As smaller devices are produced, they must be tested--in high volumes and cheaply, according to Gary Carr, Hewlett-Packard Components Group (San Jose, CA) business development manager. In bringing HP's SC-70 package to market, "the tiny packages had easily deformable leads, and we had to negotiate a learning curve for high-volume handling and test," he notes (DN 8/3/98, p. 51). Testing requires good lead contacts, and force applied to the wrong spot sometimes bends leads. The solution: additional support on the package bottom. And "disciplined process control and training, as well as the latest in pattern-recognition and handling equipment allowed high-yield, repeatable assembly and test," Carr adds.
Reducing device feature size shrinks die dimensions and cuts the power required. Burr-Brown's Skolnik notes, "The smaller and shallower circuit diffusions lower capacitance. Thus less current maintains the same speed and less power is needed to perform an equivalent function."
But smaller feature size also lowers breakdown voltage, dictating a lower supply voltage. While, Skolnik notes, "in a purely digital component this does not necessarily imply a reduction in performance. Rather, it can mean higher speed and a further reduction in power consumption. However, with analog components, lower supply voltage can impose limitations on performance, including input and output voltage swing, resolution, and accuracy.
"Tiny packages force other compromises on the designer," Skolnik adds. "To attach the IC die to the lead frame of a SOT-23, the designer is forced to internally connect one of the die pins to the 'flag' or central area of the lead frame." This connection defines pin function, but causes the pin to take on different functions when used with various fabrication processes (N- or P- substrate, for example). "Therefore, designing pin-for-pin compatible replacement parts may be complicated in future product generations," he concludes.
And with diminutive packages, the smaller the surface area of the die and package, the more difficult to dissipate power. Die temperature increases, reducing performance and reliability. Thus, notes Skolnik, in products requiring higher power, heat transfer, not feature size, may be the limiting factor in package reduction.
Smaller still. As for the future, Skolnik notes new packaging schemes, such as chip-scale packaging (CSP), driving overall package size down to only slightly larger than the chip itself. Conventionally, an IC die is mounted in a case and bond wires connect IC terminals on its top, active surface to the package leads. With CSP, solder "bumps" are placed on top of the active surface terminals. This package is then soldered in the inverted position, directly to the PCB. He cites concern about taking this attachment technology into conventional PCB environments. Specifically: limited inspection capability and PCB flux residue that could contaminate the IC interface."
Are CSPs compatible with current industry processes? Yes, according to National Semiconductor (Santa Clara, CA), first to market with its 1.45-mm square Micro-SMD dual op amp. Key technologies, according to Doug Simin, senior product engineer, include a stress-absorbing passivation layer on the chip to which the solder bumps are attached. The other side of the package is encapsulated in an epoxy-type coating. "The customer just has to prep the board as with standard surface-mount components. Then pick-and-place and standard reflow soldering can be used," Simin says. And the surface-tension physics of the package and the presence of solder on both the board and bumps allow self-aligning of the packages during reflow. Offsets can be three times those for flip-chip (no board solder) processes.
With contacts between the board and package, x-ray, not visual, inspection is needed to verify connection integrity. John Thomas, national director of marketing for standard analog products, says prospective customers note this is not a concern, because "they already have the right equipment and assembly expertise in place."
Finally, Burr-Brown's Skolnik feels CSPs "are perhaps the end of the evolutionary line for packaged parts, with direct chip attachment to the circuit board being next."
Small but tough
By Rick DeMeis, Associate Editor
Not only circuits but other components comprising electronics packages have had to shrink to keep up with smaller devices or the demand for more performance in a given volume. Take relays for instance. When Aromat (New Providence, NJ) designed its AGN telecommunications signal relay, it had to meet the Bellcore surge requirement of 2,500V between contact and coil, in a 10.6 x 5.7 x 9.0-mm package. That's roughly half the size of its previous relay for the task.
Designers using the company's magnetic-analysis software came up with tight magnetic circuit configuration that reduces flux leakage and EMI from nearby components. The electromagnet coil in the circuit was contained in a "double-molded" package for electrical isolation and reliability.
According to Kyoji Ueda, relay design engineer, to achieve higher isolation between electromagnet coil and contact, the entire coil is covered by resin--which eliminates surges through air to meet Bellcore surge resistance. Reliability is also enhanced because the coil insulation cannot outgas to degrade contact surfaces over time. The double molding takes place when this coil and the permanent magnet that the relay armature will pivot upon are then encased by another resin to mold the "body block" (see figure). Thus, in effect, the relay is formed from three components: case; body block with the coil encased; and the terminal block holding the moving armature. Ueda notes performance is consistent since the package is very rigid to resist deformations due to thermal and mechanical stresses.
On the terminal block, the rectangular hinge spring tab, spot welded to body, is shaped to flex in the plane of the tab to resist vibration and shock. The seesaw armature balance and the flexible hinge allow the relay to maintain contact position under shocks up to 75g. And only above 100g will the relay be permanently affected.
For more information on relays by Aromat (E):Product Code 4620
Don't blow circuit protection
By Brian Crannell, Business Development Engineer, Littlefuse Inc., Des Plaines, IL
Proper selection of circuit-protection guards against the effects of overload conditions. Such occurrences can result from excessive voltages, component failure, or an accidental circuit shorting.
You basically have two protection choices: the traditional one-time-use fuse and the resettable fuse. Polymer-based positive temperature coefficient (PTC) resettable products limit potentially damaging current to a safe level. Unlike heating a wire to its melting temperature to open a circuit, the PTC's nonlinear increase in resistance resulting from a jump in temperature is reversible. This transition is the "trip point." Thermal equilibrium keeps resistance high until power is removed, "resetting" the PTC by allowing it to cool.
Computer plug-and-play applications are an example where resettability makes sense. Here, peripherals can be connected and disconnected with the power on and may cause current overloads. Thus, resetting without replacing a fuse is particularly attractive.
There are other differences between PTCs and fuses that affect choosing the best circuit-protection device:
PTC resistance is about twice (sometimes more) than that of fuses with similar current ratings.
Time-current characteristic (speed of response) of a PTC is much slower than for a Slo-Blo fuse.
Maximum PTC operating temperature is around 85C; the useful upper limit for a fuse is 125C. Both require derating for temperatures above 20C, PTCs more so.
PTC high-resistance leakage current ranges from a hundred to several hundred mA; a fuse has zero leakage.
Typical PTC interrupt rating is 40A; fuse interrupts range from hundreds up to 10,000A at rated voltage.
PTC maximum operating current is about 11A; fuses can exceed 20A.
PTC voltage rating is usually under 60V; fuses rate up to 600V.
But PTCs are also similar to one-time fuses in that:
The hold current or the maximum current the PTC can sustain for 4 hours minimum, at 20C, without tripping to the high resistance state corresponds to the rated current of a fuse.
The trip current or the minimum current, at 20C, that will cause the PTC to switch to the high resistance state corresponds to the 200% overload opening point for a fuse.
Voltage rating is the maximum voltage that can be safely applied to the PTC. The short circuit current is the maximum the PTC can withstand at rated voltage. Both concepts are the same for fuses.
Selecting the best protection device for an application requires matching the circuit parameters to the product specifications: operating current, voltage, temperature, required opening time, interrupt rating, fault current, resistance, and whether resettability is desirable.
Standard chip size , thin-film Surface Mount Fuses
Maximum operating voltage, parts maybe used at voltages equal to or less than this value.
Opening time is in relation to other forms of protection. A fast device will typically operate within three seconds at 200% of rated current .
For more information on circuit protection (E):Product Code 4621
To speak with a Littlefuse product engineer, call 800 999-9445
Electronic-design modules get on the 'net
Multi-disciplined, multi-site design teams need electronic-design automation (EDA) tools that facilitate enterprise-wide integration. With Viewlogic Systems' (Marlborough, MA) Library Studio, designers can publish internally created design modules for re-use, boosting a design's productivity. Managed and accessed over a company's intranet by global design teams, such modules, which are vendor neutral, can be searched for use in other projects, saving development expense. Library Studio supports a "correct by construction" methodology based on company standards, and provides data validation to maximize module library accuracy. Thus, selection of incorrect parts is reduced.
Products to watch
The Universal Retention Module (URM), for holding Intel Pentium IITM, CeleronTM, and MendocinoTM processors, features a patent-pending folding arm support system. The configuration eliminates height constraints when packaging and shipping motherboards. Modules are compatible with the company's Slot 1 millimeter card edge connectors. Boardlocks are integrally molded into the URM base, reducing cost and eliminating tolerancing problems when using existing purchased boardlocks. AMP Inc. (E), P.O. 3608, Harrisburg, PA 17105; FAX (717) 986-7575.
RP Series standard power supplies are fully compliant with both the CE low-voltage directive (LVD) and EMC directives for extremely low radiated and conducted emissions. Power supplies are available in high-density 1,000 and 750W (12 x 8 x 2.5-inch), and 500W (12 x 5 x 2.5-inch) packages. These output ranges are geared for powering microprocessor-based computer, telecom, and industrial applications. Options include output sequencing, auto restart, and current share. A VME signaling option eases VME standard compliance without having to include such functions at the board level. Lambda Electronics (E), 515 Broad Hollow Rd., Melville, NY 11747; FAX (516) 293-0519.
With high bandwidth (900 MHz), good common mode rejection ratio (10,000:1), and low noise (16 nV//Hz), the AP034 active differential oscilloscope probe is suited for wireless and datacom design, and disk drive design and failure analysis. Via a ProBusTM interface, the probe becomes part of the oscilloscope. Sensitivity and offset are controllable from the probe, oscilloscope, or via remote commands (GPIB or RS-232). No external power supply is needed when the probe is used with one of the company's digital oscilloscopes. LeCroy (E), 700 Chestnut Ridge Rd., Chestnut Ridge, NY 10977; FAX (914) 578-5985.
The multimodul(R) MK21 rotational speed monitor for use in hazardous environments features intrinsically safe input circuits. The device monitors pulse sequences from NAMUR sensors in hazardous locations. Two setpoint outputs have independently adjustable on and off switching points to indicate over and underspeed conditions to control equipment. Different color LEDs indicate device status and a front LCD indicates actual speed, measured in user-defined units. The display also furnishes programming prompts during set-up. TURCK (E), 3000 Campus Dr., Minneapolis, MN 55441; FAX (612) 553-0708.
SpreetaTM is a surface plasmon resonance sensor for real-time, direct sensing of chemical and biological substances. The matchbox-size device includes a CMOS chip for signal quantification. Applications include food and beverage processing, water quality assessment, environmental monitoring, and hazardous materials management. With the proper encoding, Spreeta can detect properties such as sweetness quality and substances ranging from proteins to explosives. Coupled with one of the company's DSPs, the sensor can provide measurement and analysis in settings from the factory floor, to restaurants and homes. Texas Instruments Semiconductor Group (E), SC-98088A, Literature Response Center, Box 172228, Denver, CO 80217; FAX (303) 297-0447.
RT Series tactile switches have a conductive rubber dome contact that allows for 0.05-inch travel, which, the company says, is five times greater than traditional metal-dome tactile switches. Available in through-hole and SMT packages, RT switches are made of high-temperature plastics that can withstand infrared soldering processes. Actuation force is 120 or 130 gm. The switches are less than 7mm square with a 5 mA at 12Vdc contact rating. Applications include telecommunications and medical products. C&K Components (E), 57 Stanley Ave., Watertown, MA 02172; FAX (617) 926-6846.
Wearable computer in prototype production
Boeing (Huntsville, AL) has awarded Irvine Sensors (Costa Mesa, CA) $1 million in contract add-ons for the Advanced Humionics Platform (AHP)--a wearable, voice-activated system of integrated electronics and sensors, aimed at being part of future soldiers' clothing. Using its unique Neo-stack packaging technology, Irvine Sensors is developing a "computer in a cube" for the Independent Processor Module (IPM), the system's computer core. The IPM, about the size of a deck of cards, is to be integrated with a battery, microphone, mini-camera, and eyeglass display. Neo-stacking (different sized chips in the same stack) will integrate mass memory storage, an Intel StrongARMTM microprocessor, interconnection logic, and 1394 networking bus in a single cube. Four such cubes per IPM will give the unit workstation capability. The AHP program is managed by the Army's Soldier System Center (Natick, MA) for the Defense Advanced Research Projects Agency.