Screw Terminal Reliability?

[Mason T]

Hello Everyone,

Screw terminals seem to be the standard way to interface with flight computers. They are not known to be the most vibration resistant connections. There are plenty of spring loaded terminals out there that have better vibration resistance. Is there a reason the industry stays with the screw terminals? Have there been any failures traced back to these terminals coming loose? Do you have a schedule to check all connections?

My question comes from being newish to the hobby and having a background as an Electrical Engineer in industrial and consumer electronics.

 

[rharshberger]

Hello Everyone,

Screw terminals seem to be the standard way to interface with flight computers. They are not known to be the most vibration resistant connections. There are plenty of spring loaded terminals out there that have better vibration resistance. Is there a reason the industry stays with the screw terminals? Have there been any failures traced back to these terminals coming loose? Do you have a schedule to check all connections?

My question comes from being newish to the hobby and having a background as an Electrical Engineer in industrial and consumer electronics.

probably size is the main reason, screw terminals can be reasonably compact when mounted directly to the altimeter board. As for failures related to the terminals coming loose, most of the ones I have seen were one of two situations, A) the flier did not tighten the terminal screw securely or B) its possible to trap the wire in the terminal without actually tightening down on it much and a gentle tug will not pull the wire free. Both situations can be checked with a firm pull (as much as you think the wire can stand).

 

[tfish]

The more connections there are , the more failure points there are.

I connect my ematch directly to the altimeters.

Tony

 

[cls]

I agree with Tony, I am a confirmed Wire Twister. It's in DSM V, look it up.

better to go direct. when I can't, I make a J hook in the wire, double check the screw clamps down on both strands. never had a failure, there. every other kind though, don't ask.

 

[cwbullet]

Wire twist and screw switches for me. It depends on the size of the rocket and if I have a screw switch handy.

As far as terminals, I will always use the screw terminal. They have never failed for me. Why would I ever just solder a wire to the board? It is a loss of convenience.

 

[wonderboy]

Assuming you are referring to the screw terminals on the altimeter itself (not user installed terminals on av-bay lids, etc): I think it's just a matter of a cheap, simple, and compact connection system. I've never had a wire pull out from the altimeter screw terminal before. If you consider all sorts of environment where these may be used, I think a rocket is a relatively vibration free environment. You'll have a few sudden accelerations acting on the system, but no long term constant vibrations like you'd get on a car or motorcycle. These environments can really stress connection systems especially if you hit natural frequencies/harmonics of the connection components, and expose them for long periods of time.

I continue to use the screw terminals on all my altimeters, but you can easily desolder them from the board and directly solder you wire connections to the board if you want to eliminate the possibility of a loose screw terminal causing failure mode. Note you'll then move to a wire fatigue failure mode, depending on your soldering skill and strain relieving you do on the wires. (which in my opinion is also a very low risk on a rocket, others will beg to differ).

 

[Banzai88]

I've used screw terminals exactly like those on our altimeters in electronics for 35+ years in ships and aviation support equipment. Properly terminated wires and torqued terminals, the connections will remain secure for decades, even in high G and high vibration environments.

If these terminals fail in casual rocketry, it's more likely a user error than a fundamental fault of the connection system.

 

[Steve Shannon]

I've used screw terminals exactly like those on our altimeters in electronics for 35+ years in ships and aviation support equipment. Properly terminated wires and torqued terminals, the connections will remain secure for decades, even in high G and high vibration environments.

If these terminals fail in casual rocketry, it's more likely a user error than a fundamental fault of the connection system.

This!

The fact is for rocketry we get away with wiring practices that would never be allowed in industry such as twist wires together and tinning leads before inserting into a euro terminal. We get away with it because the period of vibration is extremely short compared to what occurs in industrial uses and because our applications don’t have to withstand numerous heating and cooling cycles. And most of us check or even redo them each time we fly. In other words our sloppy practices (including the nasty practice of twisting wires together ) are usually good enough. But there’s no reason we cannot learn from professionals and improve what we do.

 

[techrat]

If you open the hood of your car, you'll notice all the wires are connected with locking molex connectors. It wouldn't trust screw terminals for long-term reliability, but I'm guessing most folks factor in the short flight time versus the price and flexibility. I think you're better with pins on a board with a plastic terminal block (like what RC cars use), but I'm not designing these things.

 

[techrat]

Thinking about it, I'm guessing that some manufacturers are trying to minimize the amount of soldering you have to do, which is why they use the screw terminals. Maybe they think that some people are afraid of soldering, and figure this way opens the market for them a little more.

 

[FredA]

Whenever possible, I try to buy my altimeters WITHOUT the terminals installed and then solder directly.
Having to remove them is sometimes difficult.

 

[techrat]

Whenever possible, I try to buy my altimeters WITHOUT the terminals installed and then solder directly.
Having to remove them is sometimes difficult.

Just flip the board upside down and solder to the underside, or is that not possible?

 

[Steve Shannon]

If you open the hood of your car, you'll notice all the wires are connected with locking molex connectors. It wouldn't trust screw terminals for long-term reliability, but I'm guessing most folks factor in the short flight time versus the price and flexibility. I think you're better with pins on a board with a plastic terminal block (like what RC cars use), but I'm not designing these things.

Cars are designed for assembly line manufacturing with plug together wiring harnesses that are pre-built by robots or in low paid off-shore factories. Molex connectors, Deans connectors, or any number of locking plug together connectors are great for long term use in challenging environments, but they are expensive to build up and require very expensive crimping tools which require a certain level of training. I think most of us don’t consider it worth the time unless we already have the tool and the training.
I’ve spent a lot of time in electrical substations where connections must be 100% or people’s power goes out. I don’t recall seeing any Molex connectors, but there are a buttload of screw terminals and miles and miles of point to point wiring, all done by substation electricians who are very proud of their work and very critical of sloppy practices.

Thinking about it, I'm guessing that some manufacturers are trying to minimize the amount of soldering you have to do, which is why they use the screw terminals. Maybe they think that some people are afraid of soldering, and figure this way opens the market for them a little more.

I haven’t met very many rocketeers who shy away from soldering, but there’s truly little need for it. Wires shouldn’t be tinned before being inserted in a terminal strip. A ferrule should be used instead.
A lot of people (maybe most?) replace the wires every flight. They buy electric matches with long leads and run them all the way to the avionics board. Battery wires and switch wires are reused. If terminal strips are used near the bulkplate, the wires between the avionics and the terminal strip are reused, but I know I check every connection before each flight.

 

[Steve Shannon]

Just flip the board upside down and solder to the underside, or is that not possible?

It’s a through-hole pad. Are you suggesting soldering directly to the pad on the bottom of the PCB instead of passing through the hole and then soldering to the pad? I don’t think anyone would do that. It would rip the pad off the board. The through-hole is an important part of the strain relief.

 

[Banzai88]

The average person solders WAY too hot and with insufficient flux or post soldering cleaning technique. This leads to through hole ferrule retention failure and copper run failure in short order.

Heck, even in military aviation components designed and constructed with the specific purpose of removal/replacement of high failure items where a socket is unsuitable (with qualified and certified micro/mini solder techs with the best of equipment and oversight), we rarely get more than a dozen or so solder cycles before board compromise begins to set in. The cycles at which we get trace or ferrule failure necessitating board repair is well south of 100 cycles for most items, even with the best care of our artisans.

For the average rocketry sport flyer, screw terminals are cheap insurance against board failure......even before we get into the realm of cold solder joints.

 

[MidOH]

Coming from the RC car racing side of the LGS, I'd prefer seeing solder posts. My altimeters don't get swapped too often.

My altimeters have screw terminals. My screw switch is hardwired. My 9v battery is hardwired, no connector. My aft chargewells use servo and JR connectors so that bulkhead can be removed. (My payload coupler is glued to the payload section. Sled has to be dragged out by the main shock cord)

 

[FredA]

Steve is right on - you'll rip the board up unless you use the thru-hole.

So yes, carefully use a solder-sucker to pull out the solder from each pin on the header then gently remove the header.
Then I solder a color-coded ribbon in place.
The ribbon goes to the 4PDT switch for pyro control - also soldered.
[Power goes to it's own 4PDT switch.]
Then the ribbon splits and continues to each bulkhead where it passes thru and terminates in a 4-wide terminal block.....yes, with screws.
The altimeter ribbon is carefully spread out and connected to the terminal blocks....each connection tightened and re-checked for accidental wire damage.
Then the altimeter side is coated in epoxy to make sure nothing moves as well as sealing up the hole in the bulkhead where the ribbon comes though.

E-matches get attached to the terminal blocks and covered in duct tape - but that is single-use.

 

[mikec]

It's common to use Euro terminals mounted to the outside of avbay bulkplates to attach initiators. This has worked fine for me, but beware that they can get gunked up and corroded with BP residue and are hard to clean. I've had one flight not deploy its main because the resistance at the Euro terminal was high but still passed continuity. Ohming them out periodically is a good idea.

 

[caveduck]

I've switched from using Euro terminals on the avbay caps to Phoenix Contact 4-pole PCB mount terminal blocks. Wires are soldered to the pins and I mill a recess and a 3/32" slot into the bulkhead. With heatshrink tube on the wires, they get retained very well when seated into the recess but can still be replaced when needed. They also take up a lot less space than the Euro blocks. You can epoxy them into place if you want but then they are really hard to remove. Here they are on Mouser: https://www.mouser.com/ProductDetail/Phoenix-Contact/1725672?qs=sGAEpiMZZMvPvGwLNS67143eXz6yLKIOf3OH3Qj5OVg=. There are less expensive equivalents but some of them look really bad. You can get push-in wire versions such as https://www.mouser.com/ProductDetai...=sGAEpiMZZMvPvGwLNS671967mXgdmx6ZmpLlqN3N49Q= that are less expensive but have lower current rating (2A vs 6A for the screw type) and more restrictive wire gauge limits (20-24 AWG).

Here's a 54mm sled set up with them. Note also the G10 sled ends fitting into milled slots on the inside of the caps. No threaded rod - passthru shock line instead, with the caps bolted into the ends of the FG coupler.

 

[Rocketclar]

It's common to use Euro terminals mounted to the outside of avbay bulkplates to attach initiators. This has worked fine for me, but beware that they can get gunked up and corroded with BP residue and are hard to clean. I've had one flight not deploy its main because the resistance at the Euro terminal was high but still passed continuity. Ohming them out periodically is a good idea.

What I have done with some ebays is to use a Euro terminal (for one altimeter) and then for the redundant altimeter's charge well, wire that initiator directly to that altimeter. Then I seal both sides of the hole that I pass the wire thru with hot glue. Yes, I had the Euros get dirty and they are hard to clean, but more recently after replacing some, I have been covering them with masking tape and that has worked ok.

 

[Hobie1dog]

Just as in troubleshooting, there's no substitute for a complete thorough visual inspection, and that includes checking every screw terminal before you launch a rocket.

 

[Steve Shannon]

Just as in troubleshooting, there's no substitute for a complete thorough visual inspection, and that includes checking every screw terminal before you launch a rocket.

Exactly, and every single time! I’m always befuddled by the people who want to leave their av-bats closed up and just change the charges and ematches.

 

[FredA]

I’m always befuddled by the people who want to leave their av-bats closed up and just change the charges and ematches.

I'm befuddled by lookie-loos who keep looking inside their ebay for no good reason.
Mine get a new battery at the beginning of the season and don't get opened again unless I want to unload the altimeter data.
To each his own...

 

[Steve Shannon]

I'm befuddled by lookie-loos who keep looking inside their ebay for no good reason.
Mine get a new battery at the beginning of the season and don't get opened again unless I want to unload the altimeter data.
To each his own...

Lookie-loos?
I’ve got to hand it to you, Fred, when it comes to name calling you really go all out.
But I agree, to each their own. That’s what’s great about this hobby.

 

[FredA]

Steve - just having some fun - look all you want.

 

[Sandy H.]

Steve: Does 'Bay Voyeur' seem more suitable?

I typically use screw terminals soldered to altimeters and also similar terminals attached to the bulkheads. I also use Deans RC connectors at times if I switch from one type of altimeter to another. I usually do a full disassembly of the av bay after each flight, not to be a 'Bay Voyeur', but because I'd rather have one or two expensive data logging altimeters than a whole bunch of simple altimeters (true, this started back in the early 2000's, where data-logging was a lot more expensive than today - I might want to re-evaluate).

Anyway, from my experience, if you pay attention, screw terminals are fine. Having said that, I fly way less aggressive then those dudes doing almost 300k ft on an O to an M (great job Kip!!!).

I think there are quite a few valid methods for the forces and vibrations us low-fliers experience, whereas there are probably better technical solutions for those who fly much more extreme (hobby, amateur, professional or commercial - the cost of failure changes quite a bit).

Good question, IMO and if there is a final 100% consensus, I will adopt it.

Sandy.

 

[Voyager1]

I'm befuddled by lookie-loos who keep looking inside their ebay for no good reason.
Mine get a new battery at the beginning of the season and don't get opened again unless I want to unload the altimeter data.
To each his own...

Indeed, to each his own! Frankly, I consider not checking my avbay prior to every launch is irresponsible if I’m relying on it for deployment. But that’s just because I’m a lookie-loo, and proud of it!

On the question of screw terminals, as a retired electronics engineer, I’ve rarely had a screw terminal failure, particularly on rocket altimeters. If good quality terminals are correctly installed, used and maintained, then they are very reliable. Mind you, to check their condition, you will need to open the avbay!

 

[cerving]

Plenty of things can happen during a flight, so you really need to check your AV bays after every flight. Yes, that's an inconvenience, but checking your connections, battery voltage, and the general health of your AV bay is a prudent thing to do from a safety viewpoint, just as you'd check your parachute harness or the glue joints on your fins.

 

[Former Squirrel]

Late to the party, but... I'm using copper foil tape, BT-20, and centering rings as electrical connectors. Starter wire and powder is sealed inside the tube. A "plug" of centering rings and small tube is glued to the bulkhead and is connected with more copper tape, solder and wire to the altimeter. Hardwired. Plug in your tube containing the charge, pull off the shorting tape and you are good for a continuity check. Remotely. With your hands and face elsewhere.

 

[SteveThatcher]

Another perspective... There is nothing wrong with the screw terminal blocks that are used in the altimeters. You do need to make sure the wire is properly clamped in the terminal. I always tin the stranded 22 awg wire I use before I wire a bay up. There are two problems with them. The first is operator error - to tight or to lose usually. The second is "the not obvious one". These terminals are not meant to be used over and over again. They are mechanical devices and are designed for a number of "cycles." A switch is designed to operate so many cycles. The terminal blocks have the same concept. The BIG difference is that the switch will last much longer than a terminal block with wires being taken off and put back on. At one point in time, I had found a manufacturer that actually specified the number of "cycles" that they design the terminal block for. Their number was 50... A switch is usually rated for 1000's of cycles. What you should do is always look for small parts falling out when you loosen or tighten one of the screws. When that happens, the block need to be replaced.

The terminal block lifetime is why I do not care for the CPR3000 design from PML. It is much better to have fixed wiring with good hardware connections to the outside of the bay.

98mm avbay