Which PCB Connectors Are Best?
Aubrey Kagan, Engineering Manager, Emphatec
1/17/2014 06:50 PM EST
At the outset of this blog, let me make full disclosure. I used to work for one of the companies that I mention
in this article. Also, my current employer was spun off from, and still has a business arrangement with, that
company. This arrangement has kept me occupied for 22 years. As a result, I have greater access to
Weidmuller versions of the terminals I discuss than to others, so this blog is likely to appear biased in that
regard. Although I will try and stay balanced, it must be said that the contents of this (and all my other
blogs) are my opinions alone, and not those of my employers -- current or previous.
It seems to me that the connection requirements for industrial electronics are different than the other market
sectors, with the possible exception of hobbyist electronics. In the industrial market, integrators take a bunch
of functional modules and combine them with Programmable Logic Controllers (PLCs) and sensors to make
a complete system whose various modules are connected by long lengths of cables. Often, some portions of
the system are installed long before the others and they have to be hooked up on site. Changes in
specifications and plant upgrades also mean that there must be some flexibility in the interconnections.
Industrial products do not approach the high volumes of the automotive and consumer market places and are
much less cost-sensitive, trading this for interface flexibility. Mostly, the whole industrial package is
concentrated and housed in cabinets. They are typically mounted on rails, and the European TS35 and TS32
rails seem to me (sitting on the fringes of the market) to be the most dominant. There are many variations of
terminal blocks that mount on these rails (I may write another blog on those in the future).
There are many approaches to flexible (by which I mean versatile) connections, but it seems to me the
European offerings appear to be slowly taking over the market, despite established approaches like barrier
strips in the North American industry. There are also other common approaches, such as the 0.1" or 0.156"
headers made by Molex, Amp, and many others which require crimp tools. Then there are spade
terminations and their ilk, and you may even remember the low-end Fahnestock clips.
The first issue in making field connections is the service personnel typically have only three tools, a BFS, a
BFH, and a BFW (a screwdriver, a hammer and a wrench -- the "B" stands for "big" and I leave the "F" to
your imagination). The bigger the connectors used, the happier the installers are. The second issue is
terminating the wire, which may also require specialized crimping tools. This is part of what the nascent
industry (in the 1950s and 60s) was addressing, but -- as we shall see -- things have come full circle.
One of the problems is getting high density connections off a board and connected to the field wiring. An
approach was to bring out connections from the PCB via a standard high density cable, and to provide
interposing external terminals to connect to as you can see in Figure 1.
Figure 1. A 37-way D-subminiature to screw terminal adapter. The connectors are double stacked to
improve the density, but are not pluggable.
Variations also allow for industry standard flat IDC (insulation displacement connector) cable to screw
terminal. This kind of cable can be inflexible; it only comes in fixed lengths, and it needs specialized
crimping/soldering equipment. Since it mounts horizontally on the rail, it also uses up rail space, which is
often an issue as a result of under-design. The biggest issue is the isolation and current capabilities of the
From my biased perch, the lion's share of the PCB screw terminal market appears to be owned by two
German companies, which are, in no particular order, Weidmuller (Weidmueller in the USA) and Phoenix
Contact (if you disagree, please feel free to contradict me in the comments below). There are a few more
German companies (Wago and Weco, for example) and some US and Asian (try Degson Electronics)
manufacturers as well.
It used to be easy to tell who made a terminal by its color. Weidmuller's offerings were orange or black,
while and Phoenix's were green. More recently, Phoenix has introduced some in black, and there are now
many mimics of the original shades. I should also mention that there are some higher-end housings that
include connectors that are integral to the package as you can see in Figure 2.
Figure 2. A housing with specialized pluggable connectors.
There is an ever-expanding range of PCB terminals. They started out with non-pluggable screw terminals
(see Figure 1 and Figure 3), and today they can be purchased with different numbers of poles. Furthermore,
some can even be joined together to make terminal blocks of any length.
Figure 3. Non-pluggable screw terminals; note the ability to add clip-in tags to identify each pole of the
The use of fixed terminals means that -- when removing and re-installing a module -- you have to
individually unscrew every terminal and then remember how to reconnect the wires. As a result, the trend is
to pluggable connectors, but there are exceptions, especially when you are using connectors for very high
currents like those in Figure 4.
Figure 4. Connectors capable of carrying 100 amps -- imagine what the tracks look like!
Your choice of which pluggable connector to use is governed by the voltage and current of the signal. Often,
connectors are interchangeable between manufacturers, sometimes down to the plug of one manufacturer
fitting the socket of another. The highest capacity pluggable connector I know (Figure 5) is rated at 54 amps,
but I don't imagine many of you are involved in that ballpark, so I will quickly move on, not the least that it
also exceeds my comfort level by many orders of magnitude.
Figure 5. A really heavy duty (Weidmuller SU/BU-series) pluggable connector with a 0.4" (10.16mm)
Weidmuller produces a series of pluggable connectors denoted as a BL/SL pair (BL for the socket; SL for
the plug). There are four sets of spacings of the poles: 7.5mm, 5.08mm (0.2 inch), 5mm, and 3.5mm, with a
large selection in the number of poles. Each manufacturer has variants on these, but the 5.08mm, 5mm, and
3.5mm seem to be common to all. Probably the most common pluggable connector is the 5.08mm, which -I guess -- is in the sweet spot of the size-current-voltage tradeoff. As a result, there is a dizzying array of
plugs (normally PCB mount) with variations in insertion angle (90°, 180°, and 135°), endplates, screw
reinforcement, double-decker plugs, and even flying lead. The sockets can be just as varied, and some of the
options of both plugs and sockets can be seen in Figure 6.
Figure 6. Several different types of connectors in 5.08mm spacing. A: tension clamp; B: Weidmuller socket;
C: Phoenix socket; D: right angle plug (270°); E: vertical plug (180°); F: flying plug; G: this is our own
patented modification to the socket, allowing the use of clip-in tags on the socket (all others use a selfadhesive label).
You can mix-and-match some plugs and connectors from Phoenix and Weidmuller, just make sure in
advance. It is possible to polarize the connectors, but my experience is that the end result is finicky and
flimsy, so I tend to vary the number of poles in each connector as a differentiator (you will see an example
of this quite clearly in Figure 9).
As density has increased (and we will discuss this in a little while), the size of the screw hardware gets
smaller, reaching its limit at about 3.5mm for plug-in connectors and 2.54mm for terminal blocks. This is
leading to some growth in the market for tension camps and insulation displacement solutions. In the latter,
forcing an insulated wire onto a bifurcated (look it up) pin pierces the insulation, also making for a quick
connection. With the former, you use a small screwdriver or an affixed plastic lever to open a spring loaded
terminal, insert the wire, and release making this a quick approach. There is one variation where you simply
strip a solid core wire and push it into the tension clamp. Because of the BFS syndrome mentioned earlier in
this blog, we have been known to add a suitably sized screwdriver to the BOM (bill of materials).
Figure 7. The 3.5mm connectors are not all that different to the 5.08mm versions seen in Figure 6. The
second orange connector from the left uses insulation displacement connections. Observe the mounting
screws in some of them for when the assembly might involuntarily separate.
The next stage in density is the move to double-decker connections, like Weidmuller's B2L range as
illustrated in Figure 8. The choices drop dramatically, only allowing for vertical or horizontal insertion
angles. Also, the sockets are only available with tension-clamp terminations, although there is one variation
with socket removal levers and another that allows you to screw the plug and socket together.
Figure 8. Samples of B2L and S2L double-decker plugs and sockets. The screwdriver is inserted into the
rectangular slot on the socket on the left to release the spring clamp. The stripped wire is then inserted into
the associated hole along the outer edge.
Phoenix have gone even smaller, down to 0.1 inch and 2.5mm in their COMBICON HD PTSM range,
which employs a tension-clamp technique (Phoenix calls this a spring clamp) and solid wire. The problem
with smaller connections is that the wire diameter, of necessity, becomes smaller as well. Also, the socket
clamps become more suited to solid core wire, thereby losing some flexibility on the loom and causing
increased problems because of breakage due to metal fatigue.
Figure 9. A recent project of mine with a fair selection of BL/SL and B2L/S2L connectors. There are 2 x
5.08mm connectors of different colours; 3-way, 4-way, 8-way, and 14-way 3.5mm connectors, and a 2-way
3.5mm connector with screw mounts. Finally, there is a double-decker 10-way 3.5mm connector. One of the
advantages of plug-in connectors is it is cheaper for functional testing since you don't need a bed of nails.
When you use a multi-stranded wire in a screw-tightened cable clamp, there is a tendency for the wire to
work its way loose when the wire is flexed. The strands get rearranged, and the problem is compounded by
an uneven distribution of the surface area of the wire. There is another issue when strands get caught during
the insertion process and wave about in the breeze posing a danger of shorts or shocks. The preferred
solution is to use a ferrule, which is a soft metal barrel (usually with some plastic insulation) that fits over
the wire and is crimped. The resulting wire termination is then inserted into the socket housing (no loose
ends) and screwed down. The soft metal deforms into the shape of the cable clamp, thereby making a
connection that distributes the current more evenly and does not come apart. You can see a selection of
ferrules and an associated crimping tool in Figure 10.
Figure 10. A "ferruled" wire, a selection of ferrules, and a low-end crimping tool. As you can see, ferrules
also make it easy to daisy chain wire by twisting the ends together and crimping.
One more thing -- because of the mechanical strength required, most of the PCB plugs are through-hole
technology (the smaller Combicon HD Phoenix range mentioned above are one of the "exceptions that prove