|Cirris Pin-Sight. |
A key problem with electrical wire harness assemblies constructed with crimp & poke style terminations is contact retention or terminal "push back." Many connectors in use today utilize crimp on, insertable contacts for termination.
Most common problems with wiring harnesses are electrical issues — crossed, or inverted wires, open contacts or shorts between wires. Another very troubling problem however is mechanical in nature — unseated contact pins or terminals. A harness's construction might be electrically perfect, all wires in the correct cavities, no opens or shorts present, yet if there is a single contact not fully locked in, that harness could be a recipe for disaster.
Depending on the application of the harness, planes trains or automobiles vs. washing machines vending machines or computers, the resultant effect of a pushed back contact could range from mere annoyance to catastrophe.
Contact Retention Failures
Contacts that are inserted into the correct cavity but "push back" later, although a mechanical issue, can cause a number of electrical problems; opens, high-resistance connections which can cause heating and increase the risk of fire, and probably the most frustrating of all, intermittent connections. One of the biggest challenges to finding unseated contacts is that, in most cases, they pass electrical testing. This is because electrical testing is generally done with test fixtures built from the actual mating connectors of the harness to be tested, or standard spring probes. When the harness is plugged into the tester, non seated contacts "touch" the test mating pin, often sufficiently to cause the connector to pass the test.
Verifying Contact Retention
There are several possible methods to confirm contact retention. Probably every assembler ever trained in building harnesses has been taught the "Push-Click-Pull" method for verifying that contacts are appropriately locked in.
The IPC/WHMA A-620 Standard, section 19.7.5, reads: ". . .if test requirements are not otherwise established the "push-click-pull" method of pushing a contact into the insert until the retaining mechanism clicks and then pulling on the attached lead until it is taut shall be used. While "taut" is a subjective measure, the force is expected to be well above the force required to insert the contact (pull harder than you pushed for contact insertion). Yet unseated terminals remain a problem. Why? Obviously, humans make mistakes. Even if you have the most caring, conscientious people, without some form of verification, unseated contacts will occur.
How to Verify
Push-click-pull is easy to implement; no tools or fixtures are required, and it complies with IPC/WHMA A-620 standard for all classes of assemblies when using crimp and insert style contacts. The disadvantages are that there is no concrete specification on pull force, and the methodology is highly susceptible to human error.
Hand testing of each contact using contact retention hand tools to push on contacts from the mating side to verify that all pins are locked in. Tools from companies like Daniels Mfg. (www.dmctools.com) and Russtech Engineering (www.russtechengineering.com) are inexpensive and easy to use. They allow the user to meet specifications that require push back testing from the front of the contact, such as NASA MSFC-STD-781 REV A 4.2.1.
These tools are relatively inexpensive, easy to use, and provide measurable feedback on force used to verify contact retention. This method meets specifications where push tests are required, and no fixtures needed. However, this method adds labor cost (time required to check every pin by hand after assembly) and the discovery of errors occurs after point of assembly, rather than at time of assembly.
Electrical testing using spring probes for pushback. We would like to think that electrical testing finds all problems that can occur on wiring harnesses. As mentioned earlier, electrical testing can miss unseated terminals when actual mating connectors are used in the test fixturing. To solve this issue test fixtures can be constructed using heavy pressure spring-loaded pogo pins. When the correct force pogo pin is used it can push back the contact when the harness is mated for testing. However a 2-stage fixture must be employed that engages fully, then backs off slightly to prevent the pushed out pin still being in contact with the pogo pin during the electrical test.
|Push test fixture (photo courtesy ECC). |
This type of testing meets specification for push test from the mating (engaging) end of the contact. It is fast, all pins are tested at once with automatic (usually pneumatic) engagement of the test head, and it allows the use of lower-cost spring probes. The disadvantages include its requirement for a specific test fixture for each connector. Insertion force increases with pin count (3 to 5 lbs. x number of pins in connector, depending on force of spring probes) requires mechanical levers or pneumatics to engage test head. It calls for a more sophisticated 2-stage fixture to fully engage then back off, and it is difficult to keep probes accurately positioned.
Another option is to use switch probes in the test fixture. This works basically the same as described above, only the switch probe pogo pins don't require the 2-stage fixture. Instead they are designed so that current only flows through the pogo pin when the plunger is depressed enough to close the internal switch. If a pin pushes out it will fail the test as an open circuit, even though the pogo pin is still in contact with the pushed out pin.
This test meets the specification for push test from the mating (engaging) end of the contact. It's fast, since all pins are tested at once with automatic (usually pneumatic) engagement of the test head. Does not require as sophisticated a test fixture such as 2-stage.
On the con side, this test requires a specific test fixture for each connector. The insertion force increases with pin count (3 to 5 lbs. x number of pins in connector, depending on force of spring probes) requires mechanical levers or pneumatics to engage test head. Also, it is difficult to keep the probes accurately positioned.
|Contact Retention Test Tool (photo courtesy Russtech Engineering). |
Several companies provide pogo pins and switch probes that are suitable for use in these types of fixtures. Among them are: Lonestar Industrial ( http://www.lonestarindustrial.com); IDI ( http://www.idinet.com); The Peak Group ( http://www.thepeakgroup.com); Everett Charles Technologies, ( http://www.ectinfo.com); Connect2it ( http://www.connect2it.com.
Companies that provide pushback fixtures include: Electrical Continuity and Components (ECC) Web: ( http://www.eccco.com); Hale Manufacturing ( http://www.haleman.com); and Desert Engineering ( http://www.desertengrg.com).
Pull Testing and Pin-Sight
Cirris Systems Corporation recently introduced a guided insertion tool for error-proofing the process of inserting wires into electrical connectors. It features a built-in force sensor gauge that allows the operator to use the pull back method to perform a test to verify contacts are locked in, right at the point of insertion. The contact retention force can be programmed from 1 to 6 lbs. Once the wire is inserted into the correct cavity the operator must pull back until a good tone sounds, indicating that the proper pull back force has been achieved. If the contact is not locked in it will pull out of the connector before the pass indicator sounds. Marlin Shelley, President of Cirris Systems explained, "In developing Pin-Sight we talked with many contract manufacturers building harnesses for the military/aerospace industry. Second only to their desire to solve their issues of crossed, or misplaced wires, was the desire to solve the problem of unseated terminals. We were able to integrate a highly accurate force gauge into the base of Pin-Sight to accomplish this."
|How Pin-Sight zeroes in on missing positions. |
The new technique is easy to use, is integrated with the assembly process, and opertes in real time, providing immediate feedback for unseated contacts. It allows the assembler to correct the problem the moment it occurs. The technique complies with IPC/WHMA A-620 standard for all classes of assemblies when using crimp and insert style contacts, and no fixtures required.
The disadvantages are that it is more costly than Contact Retention Hand Tools, and it does not meet specific requirements of the NASA spec of testing from mating side of the connector.
Preventing Push-Back Issues
Where possible it is worth looking into connector designs to reduce, or eliminate, the possibility of terminal pushback. Terminal Position Assurance (TPA) devices are plastic inserts that keep contacts in place after insertion into the connector body. Pull-to-Seat Terminals require crimping after the wire is inserted through the connector body. They are 100 percent effective in eliminating pushback, however they require significant changes to the assembly process. Both of these solutions are more common in low pin count automotive and appliance connectors. Overmolded or potted connectors can also help prevent pushback, however they do not assure that pins were fully inserted before molding, so it's very possible to have pin height problems.
Unseated terminals in electrical harnesses that can push back and cause electrical disruption are a significant concern in the wire harness industry. The potential magnitude of a problem increases with the importance of the application — spacecraft, aircraft, vehicles, weapons, and othermilitary systems. Consideration should be given to the magnitude of potential problems when choosing the best solution for solving this very important quality issue.
Contact: Cirris Systems Corp., 1991 Parkway Blvd., Salt Lake City, UT 84119 800-441-9910 or 801-973-4600 fax: 801-973-4609 E-mail: firstname.lastname@example.org Web: http://www.cirris.com