Pcb Test

Criteria to evaluate when selecting quality test fixture vendors

Criteria to evaluate when selecting quality test fixture vendors
adaugat 24 Oct 2020
Every ICT or Functional Test System requires an interface, known as test jig or test fixture, that enables the system to access and test the PCB  that is under test.
The main function of the test fixture is to provides support for the PCB and to connect it to the test system. PCB Manufacturers will invest a considerable amount of money in test fixtures during the test system lifetime. When low-quality test fixtures are purchase resources and above all time is wasted in reworking on the fixture or retesting the PCB assemblies.  As a consequence, the overall performance of the test process suffers, the manufacturing cost increase and the planned delivery time fails.
A quality test fixture is a key factor to maximize the return on any test system investment and a guaranty for a low cost and on-time production.

In selecting quality test fixtures vendors, test system owners need to carefully evaluate  several areas:
  • Vendor business qualification
  • Vendors test fixture components
  • Vendor manufacturing processes

Vendor business qualification
Buying quality test fixture starts with the selection of the right vendor. Selecting a vendor with a proven history of supplying quality test fixtures is the first step. The business qualification evaluation process should focus on the vendor’s machine park and staff qualification as well as on its quality process and guaranty programs.
Machine Park and Staff Qualification: The best way for evaluating a prospective fixture vendor is by visiting the manufacturing facilities. A guided tour through the production facilities will offer plenty of opportunities to assess vendor’s machines and get information about their service and calibration plans. Having the right equipment in excellent conditions is essential for manufacturing quality test fixtures. Similar, talking to machine operators and technical staff is the best way of evaluating their qualifications and experience as well as the vendor’s continuous qualification and education programs.

Quality/Service/Guaranty Programs: 
One important aspect of vendors' business qualification is the process and plans that are in place for securing a faultless manufacturing process.  Test fixture production requires a precise process with little room for error or deviation; therefore it is important that the fixture vendor is ISO 9000 qualified or has clear, standardized and documented manufacturing processes. To guaranty short delivery times, the fixture vendor also requires a fast and sound response to last minutes changes in specifications or to discrepancies among the information or material received by the customers.  A qualified and responsive staff that address customers requirements and concern on time and in deep and a process that solves any issue fast and without delay is a key factor for delivering quality fixtures.
Customers' testimonials are the best objective source for information about vendor business stability, customer responsiveness, and staff expertise. Asking for referrals and following them up is the best way to ensure the vendor has sound business qualifications.

Quality and Robustness of the test fixture components
Material components used to build test fixtures will have a great impact on the test quality as well as fixtures’ life expectancy and life cycle cost and because they are such a critical factor, they must be evaluated carefully during the fixture vendor selection process. 
Fixture Kit: PCBs life cycle will require that test fixture withstand different stress situations during its lifetime. At high production pick, test fixtures in high volume production need to withstand regular usage and long periods of continuous operation. In the long run, the fixture needs to withstand several years with occasional use and long storage periods. Due to this requirement fixture kits should be constructed robustly with high quality and durable material that will withstand heavy usage and at the same time have a long life expectancy. Reinforced aluminum housing with a shock resistance coating is preferred due to its robustness, lightweight, and long-lasting quality. Housings made by composite material are heavy and unpractical; moreover, composite material tends to decay with time, especially in unprotected  and humid storage locations. Strong hinges and spring-loaded lid supports manufacture to withstand heavy usage and to have a long life expectancy are particularly necessary for protecting operating personal from injuries. This is especially important in test fixtures with Top Side Contacting Vacuum Gates and Hold Down Gates since the fixture’s top part is most likely to be heavy.
Probe Plate, moving plate, and top plate material
Plate material used to build the test fixture plates should be made of a material that can withstand the forces acting on them during the complete life expectancy. Pressure forces generated by the test probes and pushrods are directly translated into the plates creating flexing forces that will tend to bend the plates. Plates will withstand these forces as long as their flexing strength is higher than the flexing forces. However, plates' flexing strength gets significantly weekend through the milled areas and drilled holes required for assembling probes, pushrods, and other fixture components. In high-density probe areas or in high count probes fixtures the combination of high pressures forces and weakened flexing strength can result in plates being bent when probes reach the final contact position. Epoxy paper plate material as CEM1, sometimes used for its reduced weight, has a strength coefficient lower than glass fabric laminate material such as G10 and will easily bend as flexing forces appear; moreover epoxy paper plates decay with time since they are prone to absorb humidity weakening their strength and deforming the surface of the plate. Similar, top plates need to have the required thickness to withstand the pressure forces build on the gate. Remedies such as crossbars built to reinforce their strength are not always suitable since probes and other fixture components prevent their correct positioning and they significantly increase the weight of the fixture. For high probe count fixtures or fixtures with high-density probes area, G10 material with sufficient thickness is a key requirement to build quality fixtures.
ESD conform Material
Electrostatic Discharge is the rapid transfer of an electrostatic charge between two objects. It happens when two objects loaded with different electrical potentials come into direct contact with each other. Electrostatic Discharge is an ongoing challenge in the manufacturing of electronics in that, as circuits get smaller and more compact, they become more susceptible to electrostatic discharge. Electrostatic discharge during the ICT or Functional Test may happen when a fixture component is loaded and then discharged via the PCB under test. Poor fixture design or inappropriate material can cause the test fixture to accumulate significant charges that will eventually discharge through the PCB. The safe dissipation of any Electrostatic charge occurrences through proper design and manufacturing precautions is critical. 
In preventing or reducing Electrostatic Discharge damage, it is critical that the test fixture is properly grounded and all equipment parts that come into contact with the static-sensitive devices must have a sufficient grounding path to dissipate accumulated charge. Grounding alone, however, will not prevent all Electrostatic Discharge events from occurring.  As most of the fixture components that come in contact with the PCB such as the stoppers and pushrods must be not conductive, vendors tend to use insulating material for these parts. However, insulating materials naturally accumulate an electrostatic charge and grounding the materials alone will not remove or reduce it. All material and fixture components that come in contact with the PBC should be manufacture with material that has static dissipative properties so that any accumulated charge can be dissipated to the ground. Not only the moving and pressure plates need to be ESD conform, but most important, pushrods and stopper have to be manufacture with ESD conform materials. Proper grounding and the use of conductive or dissipative materials is a key issue in the design and manufacturing of quality test fixtures. Asking for the fixture components ESD certification is an important step during the fixture vendor selection process.
Gate/Gate hinges
A critical element in a test fixture is the gate. Gates are designed for easy use and to secure operator safety, but equally important, they should be built to withstand the forces that act upon them during the test process. Push down fingers and top contacting probes will transmit the pressure forces, not only to the top plate but also to the gate mechanical structure. Axial forces acting on the gate mechanical structure may deform its geometry and do irreparable damage to the gate structure and the fixture.  Gates build with plastic composite material are inadequate to withstand these forces and its geometry will be damage after some use.  In Top Side Contacting Vacuum Gates and Hold Down Gates the weight of the gate, as well as the axial forces, plays a crucial role in the gate’s hinges selection. The lever-action and the axial forces acting on the hinge define the load to which the hinge can support and requires that the entire gate hinge system is especially stable. Weak or inappropriate hinges not designed to support the required load will wear out or get damaged very fast.
The end effect of using weak or inappropriate gates and hinges is a misaligned gate that results in probes missing the target, pushrod colliding with components and irreparable fixture damage. Quality test fixtures build for high volume production and a long life cycle requires strong a gate’s mechanic structure builds with an aluminum profile of sufficient strength and heavy load metal hinges.
Vendor manufacturing process
With the current PCB board complexity and electronics component miniaturization, developing test fixtures without a thorough design process is not possible. The days of randomly assembling push rods and PCB’s support around the fixture are over. Today not only the clearance between fixture parts and board components needs to carefully measured, but Design for Test and Fixture Design Rules need to be integrated into the design process in order to make the testing process reliable and the fixture user-friendly and safe. Fixture vendors design process should incorporate the Design for Test and Fixture Design Rules into a flexible CAD Platform that integrates all fixture components, the PCB CAD drawing, fixture files, and all required tests devices into one CAD drawing. This process permits to recognize any discrepancy between PCB samples, CAD file, and fixture files as well as to address any collision or infringement of the Design for Test Rules. This tool offers a great advantage since it enables the communication between fixture vendors, test owners and test software developers using accurate visual information as they can receive the fixture CAD Drawing for review and eventually validation before the proper fixture manufacturing start.
The ability of a probe to hit the target is affected by two main factors: Fixture Offset and Probe Pointing Accuracy. Fixture Offset is determined by the quality of the drilling and milling process. The higher the standard in this process the higher the accuracy and the smaller the test pad that can be securely contacted. Fixture Vendors must have in place high precision drilling and milling machines supported by regular maintenance and calibration plans, but more important an appropriate drilling and milling process as it is absolutely essential for achieving a low Fixture Offset. Temperature and delamination are the main factors that affect the quality of the drilling and milling process. Drilling and milling processes must be specially designed for manufacturing test fixtures and must include several techniques to prevent material fracture and control temperatures during the entire drilling and milling process. Controlling these two factors is indispensable to achieve straight drills with minimal tolerances and a crucial factor in manufacturing quality test fixtures
The typical number of electrical joints in a fixture varies between few wires in a Function Test Fixture up to a few thousand in an ICT Fixture. Most of the wiring work in a test fixture is done using the wire wrap technique as this method makes strong and reliable joints quickly and it is easy to use in plates densely packed with test probes. 
As with all technical methods, wire wrap has to be correctly implemented to achieve good results. Test owners need to look carefully into fixture vendor wiring process and look for the attributes that define a quality wire-wrap technique. 
  • 5 or 6 tightly spaced turns around the pin. Wire turn on top of each other or wide-spaced is a sign of wrong implemented wrapping technique
  • 1-1/2 to 2-1/2 turns of insulated wire wrapped around the post as insulated wire wrapped around the terminal greatly increases the ability to withstand vibration
  • No bare wire extending away from the post (pigtail). Pigtails occur when the last turn of the wire is not completely wrapped around the terminal
  • Wires with different colors to easily “visually” separate probes groups such as the wires of a single board in a multi-board panel or electrical signals such a power and GND signals
  • Well distributed and pull relief wiring layout. Wires must me bundle, guided and fixed in a tidy manner.
Fixture wiring is a very critical work in the manufacturing process and checking its quality is an important step during the fixture vendor selection process.
Testing Process
Test fixtures are developed to detect failures occurring on the production line, however this is only possible if the test fixture itself is free of failure. All too often PCB boards are damaged or the test process is disrupted due to a malfunctioning test fixture. Misplaced pushrods or PCB supports, the wrong probe set height or force, an insufficient pressing force due to vacuum leakages or mechanic defects can damage the PCB board or generate pseudo failures that disrupt and disqualify the testing process.
Fixture vendors testing process should cover several areas, each of them focusing on a different aspect of the fixture quality or functionality
Visual Check: In this area, failures or missing assembly concerning the overall fixture look and feel as well as the required labeling, identification, and safety features are checked
  • Design Check: This area makes sure that the manufactured fixture fully corresponds with the designed model.  The design position and height profile of each fixture component, as well as the probe set-height and probe stroke-distance,  must be checked and documented and equally important a Pointing Accuracy Test on the test fixture must be performed and documented.
  • Mechanic and Vacuum Check: In this area, the fixture should be tested for a correct functioning of its mechanics and pneumatic parts as well as the correct values of pneumatic and vacuum pressure
  • ESD Test: In this ESD values on the fixture plates and other fixture components that are in contact with the PCB assembly must be tested and documented
  • Wiring and Electrical Check: This is the longest and most intensive part of the fixture testing process. In this area, each electrical component such as switches, counter, capacity probe, sensors, etc assembled in the fixture should be tested for correct functioning, and more important the complete fixture wiring must be tested for wrong connections, opens and shorts wires. 
Finite Element Analysis and Strain Gage Test
Finite Element Analysis (FEA) is a numerical method used for the prediction of how a part or assembly behaves under given static or dynamic stress conditions. FEA is used to simulate how the PCB board will mechanically behave during the test process by showing areas of unbalances forces acting on the PCB assembly. The information produced by the FEA permit fixture vendors to improve force balance on the fixture or coordinate with board designer layout changes in the PCB to reduce the number of high probe density areas or make space available for placing additional stoppers and pushrods.
Strain Gage Test (SGT) is a methodology to measure flexion on specific areas of a body while it is under static or dynamic stress. SGT is used to measure flexion when the PCB board is compressed inside the test fixture. PCBs are set under stress due to pressure forces generated by the test probes and acting on them during the test process. If not correctly supported, the PCB board may bend causing damage on components, welding junctions or on the PCB board itself. Strain Gauge Test is the practical assessment that the fixture was mechanical properly designed and duly manufactured.

Both methodologies are usefully when designing and testing complex test fixtures and the equipment required to implement them must be available at vendors manufacturing sites.
Fixture Shipment
The process for shipping the fixture is as critical as any other fixture building process. Considering the significant cost of a fixture, vendors need to have a packing and delivery processes in place that securely protect the fixture during the transport. Unfortunately, many vendors overlook this process, potentially leading to delivering a damaged fixture.  Fixtures need to be packaged in shock-protective boxes, with sufficient cushioning material and wrapped in cushioning plastic foil to protect the fixture in case the box crashed or is dumped during transport.
The above criteria for selecting a fixture vendor is intended to help test system owners to protect their test system investment and the overall performance of their test processes. A  thorough and systematic evaluation of potential fixture vendors will greatly reduce the risk of frustration, unexpected manufacturing cost and delayed delivery time caused by unqualified vendors, incompetently managed fixture projects or by a malfunctioning or poorly constructed test fixtures. 
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