Technology | Pcb Test

Fixture Design

The days of randomly assembling push rods and support around the fixture are over. Today not only the clearance between fixture parts and board components needs to carefully considered and measured, but Design For Test and Fixture Design Rules need to be part of the design process in order to make the testing process reliable and the fixture handling user-friendly and safe.

We have developed a Fixture Design Process that incorporates the Design for Test and Fixture Design Rules into a CAD Standard Platform. Our Fixture Design Process offers the advantages of a free and flexible CAD drawing tool that enable us to design fixtures with a high level of flexibility and precision while ensuring that Design for Test and Fixture Design Rules are automatically incorporated into the design.

T-1 Photo of a CAD design close up showing small distances of board’s components to test probes and board’s components to push fingers

Our Fixture Design Process integrates the fixture components, the PCB assembly CAD drawing, the fixture files and all required test devices into one CAD drawing. This process enables us to recognize any displacement between PCB assembly CAD file and fixture files as well as to measure any distance between fixtures and PCB assembly components. It also permits us to address any infringement of the Design for Test Rules and to communicate with our customers and test software developers using accurate visual information.

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Fixture Drilling and Milling

Fixture Offset is the sum of all the tolerances and deviations resulting from the PCB assembly and Fixture manufacturing processes. This group includes offsets in the PCB assembly print and tooling holes location as well as offsets in the position and straightness of the tolling pins and socket in the fixture.

Fixture Offset is mainly 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 contacted. Using high precision Drilling and Milling Machines supported by regularly calibration process and the appropriate Drilling and Milling Process are absolutely essential for achieving a low Fixture Offset.

T-5 Photo close up showing few test (2-3) pads with several probes hit marks

Temperature and Delamination are the main factors that affect the quality of the drilling and milling process. Controlling temperature in the drilling and milling tools as well as on the plate during the entire drilling and milling process is indispensable to achieve straight drills and minimal tolerances when drilling for the tooling pins and sockets. Delamination is a specific problem associated with the drilling of fiber composite material typically used as a Fixture plate material. It reduces the structural integrity of the fixture plate and results in drills with high tolerance and poor assembly properties Measures to control temperature and avoid delamination is a muss in any fixture drilling and milling process. Based on more than 10 years of experience, we have developed a Drilling and Milling Process specially designed for manufacturing Fixtures that includes several techniques to prevent delamination and control temperature during the entire drilling and milling process.

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Fixture Wiring

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 the fixture is done using the wire wrap technique. The Wire Wrap technique makes strong and reliable joints quickly and it is easy to use in plates densely packed with test probes.

T-9 Photo of a Genrad full bank with the top side open and showing the wiring

Wire wrap technique uses pneumatic, electrical or hand-operated tools. All of them use the same principle of wrapping the wire around the pin by a rotation movement. As the wire is wrapped, the corners of the pin penetrate the surface of the wire producing a reliable electrical contact.

T-10 Photo showing Probes wired

A solid, round wire usually between 0,25 mm (AWG30) to 1 mm (AWG18) is used for wire wrapping. Due to the tension raised during the wrapping process, the wire must have a good elongation before reaching the breaking point. They should also have plastic insulation elastic enough to support stretching out and of a material that can be easily piled out and torn off with knives blades.

T-11 Photo showing an Agilent test fixture being wired

A good wire wrap wiring has the following attributes:

5 or 6 tightly spaced turns around the pin. Wire turn on top of each other or wide-spaced is a sign of wrong wrapping technique. By overturning too much back force have been applied to the wrapping tool; by spiraled turns, the wrapping tool has been pulled out to fast.

T-12 Close-up photo showing wired probes

- 1 1/2 to 2-1/2 turns of insulated wire wrapped around the post. Insulated wire wrapped around the terminal greatly increases the ability to withstand vibration.

T-13 Close-up photo showing a double wired probe

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. This is a sign of using a wrong size wrapping bit or ware out or broken wrapping bit.

T-14 Photo showing an Agilent probe plate with wired probes

Wires with different colors are used to “visually” separate probes groups such as the wires of the single board in a multi-board panel or electrical signals such a power and GND signals. Using wire with different colors helps to identify probes or signal during a fixture check and the test program debug process.

T-15 Photo showing a long wire interface with groups of cable with different colors

Well distributed and pull relief wiring layout. Wires must me bundle, guided and fixed in a tidy manner. Wires must be long and pull relief fixed to avoid stress on the wired terminals

T-16 Photo showing a fixture bundle, guided and fixed wiring

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Fixture Testing

All too often PCB assemblies are damaged or degraded or the test process is disrupted due to a malfunctioning test fixture. Misplaced pushdown rods or PCBA supports, wrong probe height or force, an insufficient pressing force due to vacuum leakages or mechanic defects can damage the PCB assembly or generate pseudo failures that disrupt and disqualify the testing process. This problem may occur on all types of vacuum, pneumatic and manual fixtures. A thorough test on any fixture is and absolutely necessary step in any fixture development.

Our Fixture Testing Process is divided into several areas, each of them focusing on a different aspect of quality or functionality:

Visual Check: In this area, we look for failures or missing assembly concerning the overall fixture look and feel as well as the required labeling, identification, and safety features. During this test, we check for scratching or damaged surfaces, unfinished assemblies, proper customer labeling and component identification, correct safety indications and spare parts.

T-19 Photo of a fixture place on the test table focusing on the interface and or VTEP labels

Design Check: In this area, we make sure that the fixture’s components and assemblies correspond with the designed model. We check and measure the height profile of the fixture, the probe set height and probe stroke distance as well as the correct dimension and set high of any fixture component such as capacity probes, optical or magnetic sensor, and light emitters. The main part in this area is the Pointing Accuracy Test that checks and measures the pattern of the contact marks on the test pad.

T-20 Photo of several Test Pad with few contact marks

Mechanic and Vacuum Check: In this area, we test the correct function of different mechanics and pneumatic parts as well as the correct values of pneumatic and vacuum pressure. One important part of this area is the Vacuum and Pneumatic Long-Time Test. We operate the fixture under standard working vacuum and pneumatic pressure for several cycles to check for leakages and variation in the operating pressure levels.

T-21 Photo of a fixture showing pneumatic open/close pneumatic cylinder

ESD Test: In this are we measure the ESD values and electric insulation on the fixture plates and fixture components that are in contact with the PCB assembly during the test.

T-22 Photo of the ESD measurement device performing a measurement on a moving plate

Wiring and Electrical Check: This is the longest and most intensive part of our Fixture Testing Process. In this area, we check the correct function of the electrical components such as switches, counter, capacity probe, sensors, etc as well as the complete fixture wiring. All probes wiring are tested with our Wiring Test Machine that automatic test each electrical wire between probes and the interface pins. All power and external device wiring are then manually tested.

T-23 Video of an automatic probe wiring test with the Wiring test Machine

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Test Probes

Under Construction

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Finite Element Analysis (FEA)

FEA works by dividing the real object into a mesh consisting of millions of smaller, simple parts called the finite elements. The method approximates the real object unknown reaction to the conditions by generating simple mathematical equations that model each finite element. A computer then assembles each finite element into a large system of mathematical equations to predict the behavior of the real object. There are several FEA tools in the market, as dedicated software or as a part of the CAD software

FEA is used as the basis for simulating how the PCB assembly will behave under the forces generated inside the test fixture by showing areas of unbalances forces on the PCB assembly. These areas of tension are the product of unbalanced forces resulting from probes generating pressure while contacting the test points and push down rods and stoppers counterbalancing these forces.

T-25 Screen shoot or Photo of a computer monitor showing the preparation to run an FEA Analysis

In the process of designing a fixture, the distribution and position off probes are given by the PCB assembly test point layout. Push Down rods and stoppers are then distributed and position to counterbalance the forces generated by the probes. In principle, the pressure forces generated by the test probes are counterbalanced by locating one push down rod or stopper in the same position of each test probe, but on the opposite side. Bottom test probes are counterbalance with push down rods and top test probes with stoppers. This, however, is seldom the case due to the limited space available to set stoppers and push down rods. This limitation results in some areas of the PCB assembly, usually those with a high probe density, where the forces cannot be fully balanced. The unbalanced forces produce flexion on the PCBA board that may cause the component to break or solder joint to fracture. This situation is very critical when the PCB assemblies have BGAs or large ICs since these devices are easily damaged when board flex and they are generally accompanied by large amounts of small SMD components.

PCB-Test incorporates the FEA in our Fixture Design Process implementing an FEA tool as part of our CAD design platform. The outcome of the simulation is depicted via a color scale that shows the pressure distribution or flexion areas along with the PCB assembly. The scale goes from blue color to red color; red indicating areas with large flexion and blue indicating areas with very low or no flexion at all.

T-26 Photo of an FEA analysis run showing areas of stress

The information produced by the FEA permit us to:

Remove, add or relocate stoppers, push down rods and reinforcing element to improve force balance on the PCBA
Reduce to pressure on the PCBA by reducing probe force
Coordinate with board designer possible changes in the probe layout to reduce the number of probes in high-density areas and/or reduce component dimension to make space available for additional stoppers and push down rods.

The following picture sequence shows the addition and relocation of stoppers after running the FEA to improve force balance and reduce board flexion on a PCBA

First design: 6 Stopper, max flexion of 0,25 mm

T-27 Photo of the first run of an FEA Analysis showing the PCB with a stress area

Step 1: 1 pc Stopper added, max flexion 0,12 mm

T-28 Photo of a second run of an FEA Analysis after changes showing the PCB with a new stress area

Step 2: Stoppers added and shifted, max flex 0,06 mm

T-29 Imagine a desfășurării celei de-a treia analize FEA după modificările apărute, care evidențiază placa de circuit imprimat fără o zonă de tensiune

We recommend running FEA analyses in cases were:

PCB assemblies with high density on probes, especially in boards with BGAs and large ICs
Dense packed PCB assemblies with little space available for stoppers and push down rods
PCB assemblies with board thickness less than 1,5mm and a probes distribution that indicate possible flexion

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