Connectors developed for pc-board interfaces have come a long way since the days when I/O density was dictated by the 0.1-in. (2.54-mm) center-to-center spacing of contacts or pitch of commonly available products. Over the last two decades, the demand for high-density interconnects has driven the development of progressively tighter-pitch connectors, culminating in flexible printed-circuit (FPC) connectors with pitches as low as 0.3 mm (Fig. 1).
Typically, these fine-pitch connectors feature short interconnect paths, but not controlled impedances. However, these components usually find a home in portable systems like cell phones, PDAs, and camcorders where the main concerns are the connectors' size and cost, rather than signal speed.
For example, an FPC connector may be used to provide a low-speed interface between a pc board and an LCD or keyboard. Such applications need FPC connectors for their high I/O density and low package height—normally just 2 mm or less.
To achieve 0.3-mm pitch, connector manufacturers have pushed their stamping and molding technologies to the limits. They also have put the onus on flex-circuit manufacturers to deliver matable FPCs with sufficiently tight tolerances in their layouts.
At present, connector vendors aren't eager to contemplate a migration to pitches below 0.3 mm. In part, that reluctance reflects the technical challenges they would face in doing so. It also indicates a lack of demand, or capability, on the customer's part to handle the 0.3-mm pitch. Presently, 0.3-mm connectors are cutting-edge rather than mainstream. For now, 0.5-mm parts, which are more commonly available and offered in more styles, represent a de facto standard for fine-pitch FPC connectors.
Reducing package height is a current priority in FPC connectors. This focus is evident in other styles of board-to-board interconnects too. In particular, low-profile connectors with compression-style contacts are now moving to even smaller z-axis dimensions, while reducing pitch to 1 mm or less.
For compression connectors, which are mostly custom designs, low profiles make them worthy of consideration in portable designs, but not necessarily the same ones where FPC and conventional, miniature board-stacking connectors might appear. Because of their high I/O counts, compression connectors may more likely appear in notebook computers and nonportable applications than in cell phones. But as connector technologies and their applications evolve, it becomes more difficult to determine exactly where a given connector style may be employed.
Moving To Finer Pitches: To increase circuit density from 0.3- to 0.5-mm pitch, connector designers shifted from one row of inline contacts to dual-row staggered contacts. By staggering the contacts, the spacing between adjacent contacts grows to 0.6 mm, making manufacturing the connector and its related flex circuit more feasible in high-volume production.
Moreover, this change lets connector designers bring the connector's tails out in two rows. Naturally, this simplifies the job of laying out the pc board onto which the connector will be mounted. Even so, the tails' tight spacing requires these parts to be surface-mount devices to prevent solder bridges. The spacing also demands care in the board layout.
At the connector's mating end, even greater care is necessary when laying out the flex circuit's contact pads and traces. To ensure a compatible FPC, connector vendors furnish their customers with clear guidelines for FPC layout. (Fig. 2).
To mate with the FPC, the 0.3-mm connector relies on a combination of zero-insertion force (ZIF) and low-insertion force (LIF) contact engagement, which requires the use of a plastic actuator or flip-top on the connector.1 Older versions of the 0.3-mm connector employed a sliding top actuator that's more commonly used in FPC connectors with larger pitches.
Though there are similarities among the various connector vendors' offerings, designers looking for fine-pitch FPC connectors will notice a general lack of standardization. The number of circuits or positions available is one example. Molex indicates a maximum of about 51 circuits; FCI, 57; and Hirose, 61. Conversely, current ratings tend to hover around 0.2 A per contact for the various product lines.
Package heights are another example of variation. So while 2 mm is a common z-axis dimension, some vendors are introducing even shorter versions by continuing to extend the limits of their molding and stamping techniques. In the process, connector makers find ways to shave height off the connector's supporting plastic. They also thin their parts by experimenting with beam deflection (the degree of movement by the contacts) and the actuator (the element responsible for contact engagement and FPC retention force). A recent example from Molex is the company's 54809, a 0.3-mm pitch FPC connector with flip-top actuator that stands 1.3 mm off of the pc board.
The same trend is evident among the 0.5-mm FPC connectors. This arena includes devices like Molex's 1.2-mm high right-angle connectors (54548/54550).
Advances In Board Stacking: FPC connectors aren't the only components going below 0.5-mm pitches. Board-to-board connectors have been moving to 0.4 mm. Although the 0.1-mm difference in pitch sounds minor, it can save 30% in pc-board real estate.
As with the FPC connectors, vendors are working to reduce the connector's profile. For example, AVX's 5602 series is now available with 1.5-mm stacking heights (Fig. 3). However, the company is developing 1-mm versions, with prototypes expected this spring.
In board-to-board interconnects, compression technology provides an increasingly popular option. In lieu of a standard two-piece connector, these one-piece units have spring contacts that mate to pads on a pc board. The single-piece connector can mount to a rigid or flexible pc board using a surface-mount attachment.
Compared to a conventional pin and socket (for flex-to-board and wire-to-board) or blade-on-beam connectors (for board-to-board stacking), the compression contact features a lower profile. Also, assembly is easier because only one connector element needs to be soldered, and registration is simpler than mating blade-on-beam styles.
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