Through-hole connector resolves surface-mount dilemma

Manufacturing of a modern component-laded printed circuit board (PCB) is an amazing fusion and coordination of diverse technologies. There’s the board as substrate itself, the stencils and masks that enable precise placement of solder paster, and the pick-and-place mechanical system that places components (both ICs and passive ones) on the appropriate lands with pinpoint precision and repeatability, all culminating in most cases in a sophisticated reflow-soldering process.

Most of the loaded components use surface mount technology (SMT) and tiny contacts to their respective lands on the PCB. However, it wasn’t always an SMT world. In the early days of PCBs, the situation was somewhat different. Most of the components were dual inline package (DIP) ICs and passives with tangible wire leads, where their connections went through holes in the board (Figure 1).

Figure 1 Dual-inline package (DIP) was dominant in the early days of ICs and is still favored by makers and DIY enthusiasts; but most devices are no longer offered this way, nor can they be. Source: Wikipedia

Not only did this require costly drilling of hundreds and thousands of space-consuming holes, but component installation was a challenge. The loaded board—with these through-hole components mounted on one side only—went through a wave-soldering process which soldered the leads to the tracks on the bottom of the board.

The advent of SMT

The use of surface-mount technology began in the 1960s, when it was originally called “planar mounting”. However, surface mount technology didn’t become popular until the mid-1980s, and even as recently as 1986; surface-mount components represented only around 10% of the total market. The technique took off in the late 1980s, and most high-tech electronic PCBs were using surface mount devices by the late 1990s.

SMT enables smaller components, higher board densities, use of top and bottom sides of the board for components, and a reflow soldering process. Today, active and passive components are offered in SMT packages whenever possible, with through-hole packages being the exception. SMT devices can be placed using an automated arrangement, while many larger through-hole ones require manual insertion and soldering. Obviously, this is costly and disruptive to the high-volume production process.

The demand for SMT versions is so overwhelming that many products are available only in that package type. SMT makes possible many super-tiny components we now count on; some are just a millimeter square or smaller.

Due to the popularity of SMT, vendors often announce when they have managed to make a former through-hole component into a SMT one. Doing so is not easy in many cases for ICs, as there are die-layout, thermal, packaging, and reliability issues.

There are also transitions for passives. For example, Vishay Intertechnology recently announced that it has transformed one of its families of axial-leaded safety resistors into surface-mount versions using a clever twisting to the leads in conjunction with a T-shaped PCB land pattern (Figure 2). This is not a trivial twist because these resistors must also meet various safety and regulatory mandates for performance under normal and fault conditions while being compatible with automated handling.

Figure 2 Transforming this leaded safety resistor from a through-hole to SMT device involved much more than a clever design as the SMT version must meet a long list of stringent safety-related requirements and tests. Source: Vishay

In other cases, vendors of leaded discrete devices such as mid-power MOSFETs have announced with fanfare that they have managed to engineer a version with the same ratings in an SMT package. No question about it; it’s a big deal in terms of attractiveness to the customer.

What about the SMT holdouts?

Despite the prevalence of, and desire for, SMT devices, some components are not easily transformed into SMT-friendly packaging that is also compatible with reflow soldering. Larger connecters for attaching discrete terminated wires to wiring blocks are a good example. If they were SMT devices, the stress they endure would flex the board and weaken their soldered connections as well as affect the integrity of the adjacent components. Their relatively large size also makes SMT handling a challenge.

But that dilemma is seeing some resolution. Connector vendor Weidmüller Group has developed what it calls through-hole reflow (THR) technology. These are terminal-block connectors for discrete wires that do require PCB holes and through-hole mounting for mechanical integrity. Yet, it can then be soldered using the standard reflow process along with other SMT devices on the board.

One of the vendor’s families with this capability was developed for Profinet applications and supports Ethernet-compliant data transmission up to 100 Mbps (Figure 3).

Figure 3 One of the available families of THR connector blocks is for Profibus installations. Source: Weidmüller

These connector blocks use glass-fiber-reinforced liquid crystal polymer (LCP) bodies to guarantee a high level of shape stability. The favorable temperature properties of the material (melting point of over 300°C) and the in-built pitch space (stand-off) of 0.3 mm (minimum) are well-suited for the solder-paste process. They come in choice of two pin lengths of 1.5 mm and 3.2 mm to precisely match board thickness, all with very tight tolerance on dimensional stability and pin centering (Figure 4).

Figure 4 The connector pin must have the right length and precise centering for reliable contact. Source: Weidmüller

The reflow wondering profile is like the ones required for other SMT components, so the entire board can be soldered in one pass (Figure 5).

Figure 5 The recommended reflow soldering profile for these THR connectors matches the profile of other SMT devices. Source: Weidmüller

Another connector family supports various USB connections (Figure 6).

Figure 6 A range of THR USB connectors is also available. Source: Weidmüller

With these THR connectors, you get the mechanical integrity of through-hole devices alongside the manufacturing benefit of automatic insertion (Figure 7) and reflow soldering. There is no need for a separate step to manually insert the connector and have a separate soldering step. You can also use them for through-hole wave-soldering as well, if you prefer.

Figure 7 Even the larger-block THR connectors can be automatically inserted using SMT pick-and-place systems. Source: Weidmüller

Connectors such as these will undoubtedly lower manufacturing costs while not compromising performance. Once again, it’s a reminder of the vital role and impact of mechanical know-how and material-science expertise to less-visible, low-glamour yet important advances in our “electronics” industry.

Bill Schweber is a degreed senior EE who has written three textbooks, hundreds of technical articles, opinion columns, and product features. Prior to becoming an author and editor, he spent his entire hands-on career on the analog side by working on power supplies, sensors, signal conditioning, and wired and wireless communication links. His work experience includes many years at Analog Devices in applications and marketing.

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