crimping and splicing

Crimping and Splicing Tutorial

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As homebuilt aircraft owner/builders how are we to know if the critical wiring we’re manufacturing and installing in our aircraft is of the highest quality that we can reasonably create? Of course one way is to stick to reputable suppliers when we purchase our wire and cable, but that alone doesn’t guarantee that the finished product will be reliable. We still need to terminate the ends, usually by crimping or soldering. Of the two, crimping is by far the most reliable… if you do it right. In this article I’ll try to explain why. Industry and government agencies deal with quality and reliability issues by requiring some type of standards to be met when selecting components and also during the manufacturing processes. I’ll be referring to two or three of them as I go along. Let’s start by looking at AC 43.13-1B, Acceptable Methods, Techniques, and Practices – Aircraft Inspection and Repair. It was created by the FAA and it calls for the use of a particular type of terminal lug, one that meets a standard called MIL-T-7928. Although we’re not bound by AC 43.13-1B when construction our kits, it won’t hurt to see why the FAA and the military insist on this particular type of terminal. Maybe they know something…

Figure 1 – Three Typical Ring-Tongue Terminals In figure 1 you see three ring-tongue terminals that all look similar. There appear to be workmanship issues with the two on the left – The insulation on the left terminal exposes part of the interior of the terminal called the wire barrel and the stamping of the wire size on the middle one partially missed the terminal. But the real differences are inside…


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Figure 2 – Rear View of Terminals The two terminals on the right both have a metal section at the back to capture the insulation and act as a strain relief. That will prevent the wire from always flexing right at the exit of the terminal where it might eventually break. The terminal on the right also has a funnel to feed all the wire strands into the terminal barrel – Those are both MIL-T-7928 requirements. Do you see how smooth that entrance is? The other two don’t have a funnel and the wire’s strands can get hung up and bent back on those sharp interior edges you see inside. This could create a hidden defect – the last thing you want on your aircraft! Notice that the two terminals on the left have their wire barrel area rolled into shape with the ends just pushed together. The edges of the wire barrel area on the right hand terminal have been brazed to form one solid tube. Only the terminal on the right meets the MIL-T-7928 requirements. Think of MIL-T-7928 as the Good Housekeeping seal of approval for crimp terminals. It spells out the type and quality of materials that are to be used in the manufacturing process as well as tests that must be successfully completed to quality. My favorite MIL-T-7928 qualified terminals are manufactured by Tyco/AMP1. They’re called PIDG (Pre-Insulated Diamond Grip) ring-tongue terminals. They come in various wire and lug sizes and they’re color coded using an industry standard color code (Table 1.) You can find them at Mouser, Digikey, and other major electronics suppliers.

1 For ring-tongue terminals, wire size 22-16, I use Tyco/AMP part no. 51863 for the #6 stud size and part no. 31890 for the #8 stud size. There are other similar versions that have slightly different dimensions. The important thing is that they comply with Military Specification MIL-T-7928. AMP makes all kinds of ring-tongue crimp terminals. Don’t assume its PIDG just because it has “AMP” stamped on it. They also make cheap terminals just like everyone else. PIDG terminals should cost you about twenty five cents in small quantities.


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For the right hand terminal to gain that coveted MIL-T-7928 certification AMP had to pull random samples off of their assembly line, hand crimp wires to each one, and then run them through the following battery of tests:

50 current cycles at 20 Amps Vibration for 18 hours in 2 planes with 16 Amp test current After the vibration test, pull the wires to destruction to test tensile strength2 Immersion in lube oil and hydraulic oil at 105°C for 20 hours3 Flammability test4 Salt spray for 48 hours and then test for voltage drop and tensile strength Heat age at 121°C for 120 hours2 Low temperature crimp: Terminals were soaked at -5°C for one hour, crimped,

and then soaked at -65°C for one hour before testing. Axial Load at 90% to 95% relative humidity at 40°C for 96 hours then apply an 8-

pound load to the insulation to try to pull it away from the wire barrel. For certification all of the samples have to pass. Any terminal that can survive all of that without a failure is good enough for me, especially if it’s readily available for about twenty five cents a piece. Now let’s look at crimpers…

Figure 3 – AMP 59250 PIDG Crimper

2 The requirement was 38 pounds. The weakest sample failed at 60 pounds. 3 The insulation was then tested at 1500 volts. There can be no rupture, cracking, breakdown or flashover. 4 After the terminals are lit on fire they must all extinguish within 30 seconds. The samples all extinguished in less than one second.


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Figure 4 – Daniels HX4 I picked the AMP crimper up for five dollars at a surplus store about 20 years ago. New, these will set you back about $1300 but you can find them used for $100 or less. The Daniels HX4 is also a great tool. I purchased mine on eBay for about $75. As currently configured they are both referred to as “T”-Head crimpers since they have a “T” shaped head and can crimp two sizes of terminals. Most terminals conform to the industry standard color code that you see in Table 1.

COLOR WIRE SIZE (AWG)

AMP 59250

Daniels M22520/5-01

DANIELS DIE # (Single Terminal)

Yellow 26-24 No No Y673

Red 22-18 Yes No Y672 Blue 16-14 Yes Yes Y671

Yellow 12-10 No Yes Y670 Table 1 – Standard terminal Color Code Both of these manufacturers make excellent crimpers in a variety of sizes and shapes. The two that I mention here are very common. The AMP 59250 crimps the red and blue terminals (Table 1) so that will get you from 22 to 14 gauge wire. It can’t be changed since the head is part of the crimper. It has a 4-step adjustment for the depth of the insulation crimp and it does a superb job. The Daniels HX4 crimper uses removable dies. You can find the bare HX4 tool for a very reasonable price but the dies are very expensive. There is a common military configuration that you will see labeled as M22520/5-01. It comes with a die labeled M22520/5-100. Although the markings on the die would lead you to believe that you can crimp all three of the small terminals with one side and the large yellow with the other, this is not true. This die will crimp the blue terminals on one side and the large yellow ones on the other, for a range of 16 to 10. You could also keep your eye out for the Daniels single terminal dies in table 1 to cover other sizes. Of all the terminal sizes Red is the most common. On a light aircraft 22, 20, and 18 gauge wire covers just about everything except battery and starter size cables.


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One feature that makes this class of crimper acceptable is that it is self-completing, meaning that the ratchet mechanism between the handles will not let you make an incomplete crimp. They also have precision machined dies.

Figure 5 – Crimper Head of AMP 59250 In Figure 5 you can see that there are two sides and the terminal sizes are color coded, so I can deal with wire sizes from #22 up to #14 with only two types of terminals and this one tool. Notice also that each side has two sections. This is very important. The front section crimps the terminal insulation portion, forming a strain relief, and the back section crimps the wire portion of the terminal. OK, we’ve got the terminals and the crimper selected, now I want to introduce another document -- IPC/WHMA-A-6205, Requirements and Acceptance for Cable and Wire Harnesses and Assemblies. It sets standards for crimping, splicing, hand soldering, and wire harness fabrication. This particular standard was created by a working group of industry leaders including Boeing, Lockheed Martin, Northrop Grumman, Raytheon, and NASA, to name but a few.

5This document sells for $90 but you can find a sample at: http://www.ipc.org/TOC/IPC-A-620.pdf. You can also find a training and reference guide sample at: http://training.ipc.org/demos/pdf/drm-wha-a.pdf. Both are worth looking at.


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To make IPC/WHMA-A-620 as universal as possible it defines three categories of acceptance criteria, depending on what you’re doing. The classes are defined as follows: Class 1: General Electronic Products

Includes consumer products, some computers and computer peripherals, and hardware suitable for applications where the major requirement is the function of the completed assembly. Class 2: Dedicated Service Equipment Includes communications equipment, sophisticated business machines, and instruments where high performance and extended life is required, and for which uninterrupted service is desired but not critical. Typically the end-use environment would not cause failures. Class 3: High Performance Electronic Products Includes equipment for commercial and military service where continued performance is critical. Equipment downtime cannot be tolerated, end-use environment may be uncommonly harsh, and the equipment must function when required, such as life support systems and critical weapon systems. Table 2 – IPC/WHMA-A-620 Classification Descriptions The difference between these classes is not in the manufacturing procedures, but in the acceptance/reject criteria and the type and quality of components and tooling that must be used to meet the requirements. Because of this it can be used as the quality standard to manufacture anything from a clock radio to a heart defibrillator. And now that the criteria are defined to select my terminals and crimper I’m ready to get to work. Once I start working on that crimp I’ll fall under the acceptance criteria of IPC/WHMA-A-620. And since my life might depend on this crimp I’ll be working to Class 3 acceptance criteria. I’ll start by stripping back the insulation on my wire. First of all, never use solid conductor wire on an aircraft! Use only tin or silver plated stranded copper wire. Of course there are also specs for aircraft wiring but that’s an entire interesting subject on its own. If you stick with reputable aircraft suppliers you’ll be ok. I’ll find out how much insulation to strip by consulting the manufacture’s specs6 and gave it a try. Let’s see how I did...

6 The manufactures specifications and requirements are always up to date. Often MIL and industry specs lag behind. The trend is to accept the manufacturer as the final word on their products. You’ll seldom go wrong following their documentation.


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Figure 6 –Stripped #18 Gauge Wire Here are the Class 3 acceptance criteria for a stripped wire…

Strand Damage -- Strands are not scraped, nicked, severed, or otherwise damaged.

Conductor Deformation – Strands are not flattened, untwisted, buckled, kinked, or otherwise deformed.

Wire Separation – No birdcaging. (This is when the strands are separated from each other in the shape of a bird cage.)

Damaged Insulation – Insulation has been trimmed neatly with no signs of pinching, pulling, fraying, discoloration, charring or burning.

This particular #18 wire has twenty strands. If even one strand is nicked or cut that is a reject under class-3 criteria. If the insulation has any cuts or breaks, or if the insulation has been reduced in thickness by more than 20% these are also rejects. There are many kinds of wire strippers and I’ve tried them all. I have yet to find one I really like7. I’d be keelhauled at work for suggesting this but this particular wire was carefully stripped using a razor blade. It’s completely unacceptable in manufacturing, and the only time I’ll deviate from industry accepted practices, but with practice and some care you can do a very nice job. Always inspect your work very carefully. If it doesn’t meet the criteria for class-3 then cut off the end and try again. Nicks or cut strands are absolutely unacceptable. Over time those damaged strands will break from

7 The strippers that are approved are all similar to the crimpers in that they don’t require a surgeon’s touch to properly strip the wire. You just stick the wire into the correct slot and squeeze down on the handles. The idea is to take operator skill out of the equation and leave it to the intrinsic design of the tool to get it right every time.


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fatigue. Correctly stripping the wire is actually the only part of this job that takes much skill or practice. For this reason crimping is far superior to hand soldering.

Figure 7 – Terminal and Wire in Die Prior To Crimping (Daniels Crimper)

Once the wire is stripped to the length recommended by the terminal manufacturer we’re ready to proceed. Figure 7 shows why this type of crimper and die is nothing like a hardware store special. There are three parts that we need to look at. At the far right is a spring-loaded clamp that holds the terminal – its part of the crimper. It’s the part clamped over the ring-tongue lug. By pushing the terminal all the way into the crimper until it bumps up against the clamp we have properly positioned the terminal and prepared it for the crimp8. Next carefully insert the wire into the terminal until it bumps into the clamp and protrudes out the front slightly. You can just see the wire in that little indentation on the bottom side of the clamp. This is very important. We’ll want to see that wire flush with the end or protruding slightly through the terminal when we’re done. Now, notice the two die sections I mentioned earlier. The section at the right will compress the metal barrel inside the terminal and make a tight bond with the wire. There are serrations inside the barrel that provide maximum contact area and tensile strength after the crimp. The section at the left of the picture will compress the portion of the terminal that captures the insulation to act as a strain relief. Remember that left hand terminal? It didn’t even have this feature and I wouldn’t use it on my lawn mower!

8 You can just see the two metal portions of the terminal that are going to be crimped through the insulation.


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The top and bottom portions of the die will come together during the crimp operation and determine the exact amount of compression that is applied to each section of the terminal. The ratchet will insure that the die sections meet completely and firmly before it releases the handles. The precision machined die and the ratchet mechanism insure that every crimp is exactly the same – no skill is required since the tool does it all. This perfect repeatability is why crimping (using the correct equipment) will beat hand soldering every time.

Figure 8 shows what a completed crimp should look like. We know for an absolute fact that the wire is in there all the way because we can see it protruding just slightly out the other side. That area is called the “brush inspection window.” Here’s how IPC/WHMA-A-620 defines an acceptable Class 3 crimp:

No insulation in the conductor crimp area

The conductor end is visible within the brush inspection window

No conductor strands broken, folded back into crimp area, or not captured by the conductor crimp area

Crimp indentations uniform and meet contact/tooling manufacturer’s requirements9

Minor deformation of the contact is acceptable if it does not alter its form, fit, or function10

Figure 8 Now let’s take a look at the back of the terminal…

9 Tyco Electronics Application Specification 114-2157, 09 Sep 08, Rev E, Pre-Insulated Diamond Grip (PIDG) Terminals, Splices and End Caps 10 Minor deformation is considered acceptable in Class 1 and 2 but a “Process Indicator” for Class 3. That means that it passes but something is not quite right and needs to be fixed in your process.


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Figure 9 – Terminal Rear Detail Oh, that’s sweet! The IPC standards go on to describe how the insulation portion of the crimp should look. It should look just like this, with good compression to the insulation. An irregular shaped insulation crimp is normal and acceptable as long as the crimp provides support to the wire without damaging or crushing the insulation. That’s all there is to it. Think about how this terminal will hold up in your aircraft over the years compared to that auto parts terminal we were looking at with no insulation crimp at all. PIDG terminals have been on commercial aircraft for over 20 years and they’ve accumulated hundreds of millions of flight hours without a failure. This is very comforting but let’s make sure… It’s a good idea to do a destructive pull-test of a crimp at least once with the terminals, crimper, die, and wire you’ll be using for each wire and terminal size. This is your chance to catch any problems before you start. To do this test all you have to do is strip the wire and crimp it in the usual way and then clamp the tongue (not the body of the terminal) into a vise and then slowly pull on the wire until something fails11.

11 Tyco Spec 114-2157 calls for mounting the terminal vertically and then slowly adding weight over a one minute span. For my #18 wire the commercial requirement is a minimum of 20 pounds prior to failure and the military requirement is 38 pounds. That’s the difference between class-2 and class-3 acceptance criteria. In the MIL-T-7928 acceptance test the weakest terminal actually failed at 60 pounds, but you can’t get any better than the wire failing outside the crimp (Figure 9) so that is a perfectly valid test and much easier to do.


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Figure 10 shows the result of my tensile strength pull test. Notice that the crimp did not fail. The wire broke just outside the crimp after stretching from 6.0 inches to 7.3 inches! With a good crimp the wire at the crimp site is work hardened by the crimping process so that portion of the wire should be slightly stronger than the rest of the wire12. In a good crimp the wire will probably break somewhere other than the crimp. If the wire pulls out you have a major serious problem and the crimp is a non-conforming defect. Stop and fix that problem before you proceed. If your destructive test passes as this one did, you’re good to go! See how easy that was – way easier than soldering!

Figure 10 AMP PIDG splices are crimped in the exact same way as a terminal, except there are two sides and you do two crimps. It’s that simple. Do a destructive test in the same way, except this time you’ll clamp one of the wires in the vice. You may be tempted to solder the wire into the terminal after you crimp it for good luck. Resist that impulse as it could reduce reliability. Any flux that wicks up into the crimp could corrode the terminal over time and the heat from soldering might damage the plastic insulation.

12 AMP is a bit more caucus and says that the tensile strength will approach that of the wire itself.


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AC 43.13-1B has a few remaining things to say on the subject of terminal lugs:

11-174c – No more than four terminal lugs or three terminal lugs and a bus bar should be connected to any one stud. Terminal lugs should be so positioned that bending of the terminal is not required to remove the fastening screw or nut and movement of the terminal lug will tend to tighten the connection.

11-174d – Spacers or washers should not be used between the tongues of terminal lugs.

A few last thoughts… Did I mention that hand soldering was completely eliminated in all commercial and military aircraft at least thirty years ago because it’s much less reliable than crimping? Folks who hand solder in the electronics industry must receive specialized training and recurrent yearly testing and recertification. Hand soldering is a difficult skill to master, but if you must solder here are a few things to consider:

Never under any circumstance should you use acid core solder. It’s highly corrosive. There are no exceptions. Don’t use it.

Use only SN63PB37 or eutectic rosin core solder13. You’ll never find this at any hardware store. Again, Mouser or Digikey are your best sources.

Always use a temperature-controlled soldering iron14. Solder is used to make an electrical connection, not a mechanical one. Don’t

depend on the solder to hold things together. It has no mechanical strength. Never move the wires while the solder is cooling. You’ll get a bad joint every

time - guaranteed. Use only enough solder to make the joint. You should be able to see the outline

of the wire strands through the solder. Never carry globs of molten solder to the connection. Solder starts to degrade the

moment it melts and so does the flux. Place the iron’s tip against the work and then run the solder in from the other side and let the work melt it – not the tip. As soon as the joint is complete get out of there.

A good solder joint should be shiny. Grainy is bad. Solder has a typical shelf life of two to three years. Use fresh stock. Solder is hazardous. Read the material safety data sheet (MSDS).

Well, that’s what you’d be exposed to in the first 30 minutes of a week-long hand soldering course. I think you’ll agree that crimping is the best choice in most cases. By the way, in 13 years of working in, on, and around commercial and military aircraft I’ve never seen a spade, flange, hook, knife, or Faston terminal – only ring-tongue terminals. If things start to go all pear-shaped on you in flight that ring-tongue will stay in place as long as the mounting screw hasn’t come completely unscrewed. It might arc and spark a bit but at least it won’t slip completely off the mounting screw. That might just save your life some day. Stick with ring-tongue terminals.

13 Kester 44 activated rosin core solder is a very good choice. 14 Set the tip temperature to between 600° and 700°F. I usually settle on 650°F.

crimping and splicing

Documents

brush inspection window

insulated diamond grip

hand soldering

splicing tutorial

tensile strength

juan rivera

large yellow

strain relief