More than ever before, board-to-board connector makers are now feeling the big squeeze that has been shrinking electronic products since the first IC saw daylight. With contact pitches down to 0.4 mm for some high-density board-to-board connectors, vendors must confront the physical limits of conventional construction.
To win the design assault on inner space and fit inside the growing array of miniature, handheld computing, communications and consumer equipment, makers of these high-density connectors are cutting the distance between contact centers and lowering interboard distances to an amazing 0.9 mm. For these vendors, the marching orders are severe: Fit ever-wider data paths into less space, maintain a rugged construction with less material, accommodate automated assembly and maintain signal integrity in the face of rising clock rates.
Typically, carrying out those orders means going from through-hole to surface mount-in some cases ball-grid array-attachment, abandoning conventional pin and socket arrangements for beam and blade construction, and reducing current and cycle-life specifications. Moreover, to fight crosstalk and maintain signal integrity, connectors carrying high-frequency signals are touting impedance-matched contacts, ground planes and shielding. Here, the need for additional ground pins has added pressure to shrink contact pitch.
To meet today's fast-changing requirements, some board-to-board connector vendors are redoubling efforts to fill customer demands for information-sometimes through the Web. "Things are moving at warp speed today," remarked Bob Farnum, president of Comm Con Connectors Inc. (Durate, Calif.). To keep pace, the company maintains a "full-blown Web site with e-commerce" that lets designers request samples and place orders. Others are finding ways to customize connectors to meet special needs and clear delivery bottlenecks.
Although 0.1-inch (2.54-mm) pitch continues to dominate the board-to-board connector market, the move to fine-pitch versions-2 mm or less-has picked up speed. Among the recent and most aggressive fine-pitch board-to-board connectors to appear are the series 6250 (bottom contact) and 6252 (top contact) from Elco Corp., a division of AVX Corp. (Myrtle Beach, S.C.). Both of the zero-insertion-force, right-angle, surface-mount connectors carry six to 30 contacts on 0.5-mm pitch and cut an impressively low 0.9-mm profile. Contacts carry up to 40 mA at 50 V. Typical price in lots of 10,000 is about 7 cents per contact.
Even tighter contact spacing, a 0.4-mm pitch, graces the JAS series parallel board-to-board connectors from JST Corp. (Waukegan, Ill.). Board stack heights range from 4 to 4.5 mm, depending on the number of circuits (anywhere from 80 to 100). To handle torsional force, the plug housing comprises two separate parts, while solder tabs add "peel" strength.
Contacts are rated at 300 mA, 30 V, and prices range from 68 cents to $1.18 each in volume.
Also, the company's JMD series of parallel and perpendicular board-stacking connectors feature 0.5-mm pitch contacts with 5.5- to 6-mm stack heights and 20 to 100 contact positions, depending on the style. Contacts can handle 500 mA at 50 V and volume prices range from 68 cents to $1 each.
For its part, Molex Inc. offers 0.5-mm-pitch, surface-mount board-to-board connectors with stacking heights from 1.5 to 6 mm and 20 to 100 gold-plated, leaf-style contacts rated at 500 mA. The high-temperature housings withstand reflow soldering and prevent flux wicking. Smaller sizes include a friction-locking system for higher retention. A 60-contact connector with a 3-mm stacking height and solder tabs sells for $5.40 per mating pair in volumes of 10,000.
Fine Stack and Fine Mate series board-to-board connectors from AMP Inc. (Harrisburg, Pa.), both low-profile 0.5-mm pitch surface-mount designs, allow parallel board stacking as close as 1.5 mm (for Fine Stack) and 4.5 to 6 mm (for Fine Mate). Both span 20 to 80 contact positions and include solder pegs to improve peel strength.
Similarly, Samtec USA (New Albany, Ind.) offers its 0.5-mm pitch HMT series headers and mating HMF series sockets, with 20 to 80 contact positions. They can stack parallel boards to within 5 mm. The company's SMT 0.6-mm-pitch connectors for stacking parallel circuit boards push board spacing down to 4 mm.
Reflecting the increased acceptance of such fine-pitch board connectors, Fujitsu Takamisawa America Inc. (SUNNYVALE, Calif.) has expanded its FCN-290 series of 0.5-mm stacking connector to carry 30 to 120 contacts. The minimum stacking height between boards is 2.5 mm and contacts are rated at 300 mA and 200 Vac, 300 Vdc. A 70-pin connector sells for $1.15, in lots of 10,000.
Fine-pitch board-to-board connectors from 3M Electronic Products Division (AUSTIN, Texas) include 50-position sockets and headers and, for Type I and Type II PC cards, 68-position straddle-mount sockets, all on 1.27-mm pitch. In the third quarter, the company plans to introduce 0.8-mm "hermaphroditic" board-to-board connectors-meaning that mating connectors are identical-to simplify inventory.
For the highest-density connections, vendors are plying surface-mount arrays and ball-grid arrays (BGAs), also on 0.5-mm pitch. For example, Samtec's CSB and MSB area-array socket and terminal, respectively, pack 20 or 30 pins each across eight rows. Board-to-board spacing is 3 mm.
Still higher contact counts are available with FCI/Berg Electronics' (Harrisburg, Pa.) MEG-Array, a system of 0.5-mm-pitch, BGA-based board-to-board connectors offering 240, 300 and 400 contacts. Board-to-board spacings range from 3.4 to 8 mm.
Where low profile takes a higher priority than density, some vendors offer one-piece, so-called Z-axis connectors. Sitting on one board, the connector makes contact with simple gold pads on the other.
Critics, however, say that when additional fasteners, and possibly manual assembly, are needed to attach the boards, costs are higher. Still, such steps can be precluded by smart packaging. Among the one-piece connectors being offered are Elco's series 9158, with dual rows of 1-mm-pitch contacts and stack heights between 1.5 and 5 mm, and Samtec's SEI series, also with 1-mm pitch, and with 1.65-mm stack heights.
High frequencies and fast rise times cause their own problems for fine-pitch, high-density board-to-board connectors. In particular, good signal integrity demands low inductance and controlled impedance, and EMI/RFI suppression may need shielding.
For just such cases, Fujitsu Takamisawa America has introduced its FCN-260(D) matched-impedance connector designed for differential data signals. (The company says it performs respectably in single-ended systems as well.)
Forged for data rates up to 2.5 Gbits/s, the connector keeps crosstalk to less than 1 percent for pulses with rise times as short as 100 ps. Contact pitch is 0.635 mm, and differential-signal impedance is 100 ohms. By the third quarter, Fujitsu should release its high-density FCN-086 series, with up to 175 controlled 52- ohms contacts, all on 2-mm pitch, across five or seven rows.
A microstrip structure-separating two rows of signal contacts by a center power ground plane-is at the heart of AMP's Mictor family of 0.5-mm pitch, 50- ohms board-to-board connectors. Available in parallel stacking and right-angle board configurations, and with 38 to266 positions, the connectors include a discrete bus every 12.7 mm, which can be used for power or ground.
Gil Bassak is a freelance technical journalist living in Ossining, N.Y.
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