DVD-60C The Seven Sins of Wire Harness Assembly

Transcription

1 DVD-60C The Seven Sins of Wire Harness Assembly Below is a copy of the narration for DVD-60C. The contents for this script were developed by a review group of industry experts and were based on the best available knowledge at the time of development. The narration may be helpful for translation and technical reference. Copyright IPC Association Connecting Electronics Industries. All Rights Reserved. Introduction Having a little problem with your wire harness assembly??? In the beginning we re taught the correct way to assemble wire harnesses. But when we do this work every day it s easy to start doing things without thinking about them and pretty soon we can develop bad habits. We ve identified these bad habits as the seven sins of wire harness assembly. Let s look at the list. There s incorrect wire preparation, improper harness layout, improper labeling, crimping defects, soldering defects, missing or incorrect hardware and improper tying. Problems in any one of these areas can ultimately cause an entire electronic system to fail. In this program we ll be examining each of these sins explaining the potential problems, how they can cause an undesirable result and what needs to be changed to atone for the transgressions. Sin number one Incorrect Wire Preparation Let s begin by reviewing the wire preparation process. It involves selecting the right wire, cutting it to the correct length and properly stripping each end. Wire preparation is typically done on automated machines for large lots, or manually for small lots and prototypes. Let s look at what could go wrong on the automated machines. First of all, it s important to verify the proper wire and to make sure you enter the correct wire and strip lengths. This can be found on the assembly documentation. 1

2 Once the machine is running and doing its job, it s easy to let your mind wander and be thinking of other things. But it s important to monitor the machine and periodically check to make sure that the wire strips are done properly. If there are nicks or cut strands on the conductor, it usually means the cutting depth is too deep and you should adjust the machine accordingly. Your company will let you know the maximum allowable strand damage for the type of wire you are stripping. There can also be unacceptable insulation damage from stripping. When the insulation is compressed, it usually indicates that the rollers are too tight. Compressed insulation can possibly damage the conductor underneath and affect the proper operation of the wire. Frayed insulation usually means that the cutting blades are probably dull. If this is the case, blades should be changed. Now let s take a look at manual wire stripping. The biggest cause of conductor and insulation damage occurs when using an inappropriate tool. One of the safest manual wire stripping methods is the thermal stripper which melts or softens the insulation. Use your fingers to twist the insulation slug off in the direction of the wire lay. Do not use the stripper to pull off the insulation slug as it can potentially damage the wire. Mechanical strippers can also be used to safely strip the insulation from wires. The wire is placed in the correct diameter hole for the correct strip length. The handles of the tool are then closed. As the wire and tool are pulled apart, the insulation is stripped off. Sin number 2 Improper Harness Layout The proper layout of the wires is critical for correctly connecting the harness inside the chassis of the final product. That s because specific lengths of wires need to break out of the bundle in different locations in the chassis. A lot can go wrong while forming the harness. If you re not paying close attention, it s easy to route a wire in an incorrect location. It s important that the correct wire is in the right position. For example, if you were to route a smaller AWG wire in place of the correct larger wire, and it gets connected inside the system, there may be damage to the wire and possibly the system. Another big problem occurs when a wire is missing. That s why it s essential that you follow the exact layout specified in the assembly drawing. For complex wire harnesses, a template or form board is usually created to make the job easier. The form board is the physical duplication of the assembly drawing made to the exact same scale as the finished wire harness. In addition to following the layout shown in the assembly drawing, it s important to follow the correct sequence for laying out and routing the individual wires. 2

3 Sin number 3 Improper Labeling Believe it or not, incorrect labeling is the most common error in the wire harness assembly process. This is one error that can cause major headaches for the customer especially when connecting the harness inside the chassis. Labels are used to identify individual wires and connector designations, as well as the part number, revision number and date code that allows for manufacturing traceability. Problems with the labeling include incorrect information on the labels, illegible marking -- or bar codes that can t be read or placed in the wrong location. One method to verify the information printed on the labels is to have a system to check for accuracy before all the labels are printed. A simple way to do this is to print ONE of each label required for the harness/assembly. Attach the sample labels to your traveler or work order and send it to your QA department for inspection. It is important that QA check the labels against the drawing to ensure all information required is correct. Once QA approves the sample, the balance of the labels can be printed. After the information is confirmed, the size of the labels must be correct. If they are too large, they will wrap around the wires and overlap. The placement of each label must match the drawing and must be easily visible -- and not upside down. Incorrect wire ID labels can cause even bigger problems. That s because these labels show the to and from designations for the wires. When the markings are incorrect, or when a correctly marked label is attached to the wrong wire, there will be incorrect terminations when the wire harness is connected to the chassis. This heinous sin can be avoided by checking and double checking the labels against the assembly documentation. Sin number 4 Crimping Defects As you know, crimping is the most common method of terminating the wires in a harness. If the crimp is not made properly, there can be open or short circuits or worse, there can be a marginal crimp that passes a functional test, but later fails. This usually happens at the most inconvenient time, creating unhappy, frustrated customers. Crimping tools are either powered or manual. Powered crimpers are typically used for large production lots. The termination hardware comes on a reel that is loaded onto the machine. To make acceptable connections, it s critical that you set up the machine properly. Then it s simply a question of paying attention to the job of feeding the wire and making the crimp. When a small number of crimps need to be made, it s often easier to use a manual crimping tool. Based on the wire size and the termination hardware, you ll need to select the proper crimping tool and die for the job. In order to maintain uniform crimp terminations, most hand crimpers have a ratcheting mechanism that prevents the tool from releasing until the crimp has been completed. During calibration, this ratcheting mechanism is also checked for reliability and is adjusted when necessary. It s important to verify that the crimping tool has a current calibration sticker. A tool that is not calibrated properly will not produce acceptable crimps. 3

4 Whether you re using powered or manual crimping tools, problems can be avoided when you understand the anatomy of a crimp. Remember, on open barrel terminations there are two U- shaped areas one that terminates the conductor and one that terminates the insulation. The conductor crimp is the mechanical compression of the metal terminal around the wire conductor. This creates a continuous conductive electrical path. Pull tests are used to verify the conductor crimp during initial set-up. Another method of checking the conductor crimp is to examine the conductor crimp height. The conductor crimp height is measured from the top surface of the formed crimp to the bottom most radial surface. The bellmouth is the flare that s formed on the edge of the conductor crimp which acts as a funnel for the wire strands. This funnel reduces the possibility that a sharp edge on the crimp will cut or nick the wire strands. The conductor brush is made up of the wire strands that extend past the conductor crimp on the contact side of the termination. This is done so that mechanical compression occurs over the full length of the conductor crimp. The conductor brush should not extend into the contact area. Now, let s look at the insulation portion of the crimp. The insulation crimp, or strain relief, is the part of the termination that provides support for insertion into the connector housing, and allows the termination to better withstand shock and vibration. This crimp needs to hold the insulation as firmly as possible without cutting through to the conductor strands. A bend test is typically used to verify the insulation crimp. An ideal, or target open barrel crimp should look like this: The wire strands should be visible at the contact end of the wire barrel. All wire strands should be inside the crimped wire barrel with the seam closed. A bellmouth should be visible at the wire entry end of the wire barrel. Both wire strands and insulation should be visible between the conductor crimp and the insulation crimp. And the insulation crimp should fully wrap and support the insulation without cutting or breaking it. Now let s look at what can go wrong. Crimping defects are either caused by an incorrectly set up crimping tool or by an improperly prepared wire. For example, when crimp height does not meet your company s workmanship standards, it indicates that the crimping tool is not set up correctly. A crimp height that s too small may cut strands of wire, or fracture the metal of the conductor crimp section. In addition, a crimp height that s too small reduces the current carrying capability of the crimp. A crimp height that s too large won t compress the wire strands properly. The result will be excessive voids in the crimp section providing a lack of contact between the wire strands and the metal of the terminal. In addition, a loose crimp will oxidize over time and eventually cause an weak or intermittent electrical failure. The solution to crimp height problems is easy. Simply adjust the conductor crimp height on the crimp press or hand tool, make new samples and re-check the crimp height. Loose wire strands are an example of a wire problem. If all the wire strands aren t fully enclosed in the conductor crimp section, both the strength of the crimp and its electrical current carrying capability may be greatly reduced. The loose strands may also cause a short circuit, or arc to a 4

5 nearby contact. To solve this problem, gather the wires back into a bunch before inserting them into the terminal. Another example of improper wire preparation occurs when the strip length is too short. If the strip length is too short, or if the wire isn t fully inserted into the conductor crimp section, the termination may not meet the specified pull force because the metal-to-metal contact between the wire and the terminal is reduced. To solve the problem, simply increase the strip length. Another crimping problem caused by short strip length occurs when the wire is inserted too far forward into the crimp sections. When the insulation is too far forward in the insulation crimp section the conductors will protrude into the transition section. Since there is reduced metal-tometal contact, there may be a reduced current carrying capacity and/or wire pull out force. To solve this problem, the wire must not be inserted into the press with so much force that it overcomes the wire stop on the press. The wire stop may also need to be adjusted so that it places the stripped wire into the correct position. Your company will inform you of other workmanship standards related to crimp acceptability. During the crimping operation It s important that you remain alert to potential problems and to not let your mind wander, thereby allowing defects to be created. Sin number 5 Soldering Defects Another method of connecting the wires of a harness is by soldering. The different types of terminals include turrets, bifurcated, pierced, cups and hooks. A lot can go wrong when you re soldering wires to terminals. Potential problems include incorrect soldering temperature; improper soldering techniques; and wrong soldering iron tip size. Let s review what we know about soldering iron tips. As you know, the amount of heat that is transferred to the parts being soldered depends not only on the temperature of the tip, but also on how much of the tip actually touches the parts to be soldered. When selecting a tip, always select one that has the greatest contact area a without overhanging the joint area. A conical, or pointed tip is used to transfer heat into a small area. A chisel, or screwdriver tip has a much greater contact area than a conical tip and will therefore transfer more heat. It s important to realize that these solder connections should be completed in the shortest possible time to prevent excess heat from spreading to nearby areas of the board causing thermal damage. The first step in the hand soldering process is tinning the tip of the soldering iron. We do this because oxidation can form on the iron tip. Oxidation acts as a barrier or an insulator meaning it can slow down the transfer of heat. It s important that the tip be cleaned and tinned both before and after you solder. Now let s take a look at the soldering operation -- using a pierced terminal. The first step is to tin the stripped wire. This is done so when the wire is bent it won't be damaged, and to improve solderability. To tin the wire, you can move it across a solder coated iron tip, move a tinned tip over a stationary wire, or dip it in a solder pot. 5

6 Pierced terminals have a hole in the middle. Wire can enter from either side, or from the top. The wire should be pushed through the hole, then wrapped and positioned to contact both the flat sides of the terminal. Make sure you set the soldering iron temperature based on the guidelines set by the solder manufacturer. Now, let's watch the proper soldering operation for this particular terminal. There are four basic steps. First -- the iron is placed at the point of greatest thermal mass in contact with the terminal and the wire. Next a solder heat bridge is made to increase the thermal linkage between the tip and the work. Additional solder is then applied on the opposite side of the point of contact to form the necessary solder fillet. The solder wire is removed when a sufficient amount of solder has been added to the joint. Finally, the iron is withdrawn from the same angle it was introduced to the joint. Let s watch this correct soldering technique once more in real time. It s very important to clean the joint after the soldering is complete -- using an appropriate cleaning solution. Flux should not remain on the mating surfaces. A preferred or target connection is where all of the metals are covered by solder and the outline of the lead remains visible within the solder connection. On a proper solder joint, the solder fillet is slightly concave, or curved inward. Sin number 6 Missing or Incorrect Hardware When you re working quickly trying to meet production deadlines it s easy to overlook a wire that needs termination hardware, or to attach the incorrect hardware to a wire. That s why it s important to take the time to double check the information in the assembly documentation. Verifying your work will save time in the long run since there ll be fewer rework situations. Some assemblies require the addition of jackscrews, stand offs, clamps, washers or screws. If the wrong hardware is installed, it may prevent the installation of the harness. For instance, if the wrong screw thread size is used, it will not mate with the intended mating hardware. Sin number 7 Improper Tying Tying the wires is typically the last step in the wire harness assembly process. The two basic parts of tying are the application of the tie wrap and the breakout of the wires. The use of plastic tie wraps is a common method for bundling the wires. The ties maintain the form of the harness, along with defining the location where the individual wires can be broken out of the harness at their designated connection points. Plastic tie wraps can be installed manually, or with a handgun. The ties are tightened around the wires and the tail is cut off. The tie wrap gun should be set to the proper tightness. If the tie wrap is too tight, the wires can be damaged. If it s too loose, the tie won't hold the bundle securely. Tie wrap locations are called out in the assembly documentation. There are a number of alternative methods used to separate and secure the cable bundles. Among them are spiral wrap, heat shrink tubing, conduit, loom and expandable sleeving. Adhesive heat 6

7 shrink should always be used to close both ends of expandable sleeving to prevent the sleeving from sliding. Summary You ve just taken a refresher course in how to avoid the most common pitfalls in wire harness assembly. These seven sins include incorrect wire preparation, improper harness layout, improper labeling, crimping defects, soldering defects, missing or incorrect hardware and improper tying. Your excellent skills and focused attention are what s needed to produce quality wire harnesses. You re now armed with the information to defeat the seven sins of wire harness assembly. Fighting the good fight will result in more reliable products and a contented feeling of accomplishment. 7