Specifiers of SCSI connectors for disk drives must anticipate mechanical variations among the three mating cable connectors on the host system.
By Lawson Williams
Although a set of standards that relate a disk drive`s small computer system interface (SCSI) input/output (I/O) connector contacts to the host system`s three separate cable connectors - power, control and data (see Figure 1) - is needed, no such set of standards is available today. Currently, there is not even a de facto standard to relate SCSI disk drive connectors to the three mating cable connectors. Nor are there any standards relating the power, control and data cable connectors among themselves. Specifiers of SCSI connectors for disk drives have no control over the three different cable connectors that will be mated to the disk drive in the host system. Nor is there any control over the production people that will assemble the disk drive into the system.
A small form-factor committee is currently working to develop a set of first standards for SCSI I/O disk drive connectors and their three mating cable connectors. The effort will enable connector makers and computer systems integrators to finally agree on a set of acceptable standards for unitized electrical, dimensional and fabrication criteria.
A possible impediment to establishing these standards is that disk drive makers want to remain flexible in developing new products that have short life cycles in dynamic and fast-paced markets. Also, standards committees are not known to move quickly. These factors may retard the development of the needed standards.
At the same time, it takes production of tens of millions of individual female connector contacts each month to keep up with the demand for more personal computers and workstations. That is a huge number of contacts, and the army of stamping machines that cumulatively produces them is unrelated and scattered around the globe. Each stamping machine has its own stamping die that further affects the finished piece. It would be difficult to impose standards on such widely diverse sources of small parts when cost is king. The result is a broad diversity of connector contacts and a broad diversity of labor. The situation is a matrix of connector pin variations vs. labor variations (see Figure 2).
Eliminating Wall Fractures
Specifiers of SCSI connectors for disk drives must anticipate mechanical variations among all three mating cable connectors and variations in the strength and dexterity of the assembly labor force as well. One concern is that an overly robust production worker will try to force a tight-fitting cable connector into place and will fracture the SCSI connector`s housing wall. Wall fractures happen most often in the power pin section because of the mating forces concentrated there. Simultaneous requirements for high-heat deflection (for infrared reflow surface mount operations), dimensional stability, optimum injection mold cycle times and minimum shrinkage requirements of the mold material can contribute to brittleness in the connector housing, and possibly allow it to fracture under excessive stress.
One solution to the wall-fracture problem is using an Encircled Electronic COMpression (EECOM) male connector contact system (see Figure 3). Because of their bifurcated design, it takes substantially less force to mate and unmate EECOM power contacts than with other connecting systems. Each side of a bifurcated EECOM contact is a flexible cantilever. It protects the housing by absorbing excess torque and off-axis loading. EECOM contacts, therefore, reduce stress on insulator walls and reduce the effects of variations among female cable connectors.
Benefits of Straddle Mounting
Another concern for specifiers of SCSI connectors for disk drives is that assemblers seem to like to wiggle, twist and bend any kind of connector body when they mate and unmate it from its socket. Instances have occurred of factory workers wrenching surface mounted SCSI connectors right off the printed circuit board (PCB). Sometimes, assembly line workers lack dexterity and are unable to successfully mate one or more of the three system cable connectors to the disk drive. Uncontrolled mating forces can cause a surface mounted SCSI connector to separate from the PCB. There is not usually a lot of excess real estate on PCBs to enlarge (and strengthen) the connector hold-down pad areas.
Three-dimensional straddle mounting protects SCSI connectors from being pulled off the PCB. There are tradeoffs, however, to their use. Straddle-mounted edge connectors need to be assembled by hand or by some type of sliding robot. Hand soldering the connector`s contacts to small, dense soldering pads on two sides of the board can be time-consuming and inefficient.
For all their shortcomings, straddle-mounted SCSI connectors are here to stay. Their popularity has increased steadily since their introduction in 1995. Just like surface mount SCSI connectors, straddle-mounted SCSI connectors are available off-the-shelf, in millions, at cost-effective prices, but unlike surface mount SCSI connectors, they cannot be pulled from the board.
Plugging the EMI Leak
With Ultra SCSI data streams at 320 Mbps, protecting the sensitive, high-speed data stream from electromagnetic interference (EMI) can also be a challenge. One point in a disk drive enclosure that can be a major EMI leak is the flex-to-board connector pair, which channels bytewise data streams at 320 Mbps from the data amplifiers inside the enclosure to the PCB outside. This is the disk drive`s electronics, where error correction, data formatting, disk drive control and other delicate functions originate just a few millimeters away. Any EMI can corrupt the data stream and wreak havoc.
Unless the internal connector is properly shielded, EMI energy can leak past the PCB and in between the connector pins. Connector body shielding would be desirable, but given the manufacturing attention to cost and size, it would be better if outside shielding could be avoided.
The enclosure-to-board leakage problem can be solved with EECOM connector contacts. Bifurcated male EECOM contacts plug directly into plated through holes in the PCB. Each EECOM contact provides multiple mating surfaces around the hole circumference that reduces ohmic contact resistance. Also, EECOM contacts are physically much shorter than conventional contacts, which reduces inductance. Lower resistance and lower inductance mean lower signal impedance, and contact integrity, therefore, is enhanced.
Also, EECOM contacts allow PCB designers to seal the intercontact spaces with the board`s own EMI-blocking ground plane or power bus (see Figure 4). This maintains EMI integrity and shields the electronics from noise.
Conclusion
Even though backward compatibility for SCSI disk drive connectors has yet to arrive, the unknowns can be substantially reduced when specifying connectors for disk drives. Bifurcated male pins and straddle-mounted edge connectors can be used to minimize physical loads on connector housings.
LAWSON WILLIAMS is senior vice president, Ranoda Electronics Inc., 1095 Duane Lane, Sunnyvale, CA 94086; (408) 830-0888; Fax: (408) 830-0800; Web site: www.ranoda.com.
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Figure 1. Standard three-in-one SCSI combo connectors for disk drives must mate to three different cable connectors in the host system: power, control and data.
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Figure 2. The strength and dexterity of factory workers installing disk drives in PCs is uncontrolled (strong/weak). The lack of standards relating a SCSI disk drive connector to system mating cables means connector fit is uncontrolled (loose/tight). People tend to twist and rock a connector back and forth when mating it to its socket. The tight-fitting/strong-worker quadrant (shaded area) could result in a fractured connector housing and return of the disk drive to the vendor.
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Figure 3. One-piece EECOM connector systems omit the female connector body and plug bifurcated male pins directly into plated through holes in the PCB.
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Figure 4. A 20-pin connector connects bytewise data directly from a disk drive`s read/write heads to the drive`s PCB. Male pins of the EECOM connector (upper left) plug directly into plated through holes in the PCB itself (lower right). The PCB`s ground plane surrounds the plated through holes around the EECOM pins and protects against EMI leakage out of the disk drive assembly into the PCB and surrounding area.
SPEC SHEET
End Application: Disk drives
Related Products: SCSI and cable connectors, PCBs, connector housings, connector pins
Main Point: Standards that relate a disk drive`s SCSI connector contacts to the host system`s three power, control and data cable connectors do not exist. Specifiers of SCSI connectors for disk drives, therefore, have no control over the three different cable connectors. Nor do they have control over the production people that will assemble the disk drive into the system. These specifiers must concern themselves with assembly of SCSI and cable connectors, including wall fractures in connector housings, and with EMI risks.
