Fixture and Fixture Test

What is a Fixture Test?

Fixture test uses a ‘Jig’ or better called ‘Fixture’ to transfer the electrical current from the machine to the SMT or holes on the printed wiring board (PCB). Hence, performing the electrical test as defined (link to ET).  The fixture uses various sizes and types of conductive wires called ‘test pins’ to transfer the current from the machine to the PCB.  The initial cost to produce a fixture is high but this cost is offset by the speed of performing the test on each PCB.  Due to the cost of tooling each part number a fixture test is most economical from medium to high volume batches.

About the Fixture

Fixtures are produced in a couple different ways one of the most common ways is the leaning pin fixture use a CNC drilled fixture plate material is mounted in a stack up with some sort of spacer and the pins are inserted on angles to deflect from the PCB board to the base of the machine this type of fixture is called the leaning pin fixture and typically used on a universal grid machine. Another common fixture uses some sort of plate material to hold a pin or a socket to match the PCB test location.  The socket may be wired to some connection point that is cable to the machine and connected to the machines scanning cards. 

Fixtures are made in various shapes, sizes and layers the most economical way to produce a fixture is to standardize the raw plate material into a couple common sizes.  Low-cost fixtures may be made from polycarbonate or paper phenolic material, higher technology fixtures are made from FR4, G10 or proprietary material.

Learn more about the universal leaning pin fixture
Learn more about the dedicated fixture.
Learn more about the hybrid fixture.
Learn more about the tension probe fixture.
Learn more about the custom/special fixture.

Test probes/pins have various diameters, lengths, and ends all designed to have a solid connection between the PCB and interphase to the Electrical tester.  The primary function of the test pin is to transfer the electrical current from the Electrical tester to the unpopulated PCB board.

POSITION OF TEST CONTACT POINT AND SELECTION OF TESTING PROBE TYPE

The PCB design is made as close as the CAD design as possible. However, the real PCB is not 100% same as CAD design, since there are the production related errors such as the material shrink/expansion, the pattern shift/distortion, the error by drilling accuracy, etc. The testing machines also could be the source of the position error. For the case of the fixture, the limitation of positioning accuracy by the drill machine could cause the probe position error. The position of test contact point in the pad is important to improve the test productivity for the electrical test.

In general, the probe position for test is better to be the centre of test pad on a PCB to avoid the errors from the PCB, the test equipment and fixture, since it gives the largest deviation for such positioning errors between the probe and pad. And it reduces the chance of retry due to the less probe contact error, thus higher productivity of test.

Probe position for the fixture type tester such as the dedicated tester or/and the grid tester

Now, for the case of fixture type testing, the positioning logic becomes more complicated as described in below.

Ideally, it is better to position the probe at the centre of the test pad, but as both the probe cost and drill cost increases by decreasing the probe diameter, the fixture manufacturing team prefers to use the probe diameter as large as possible. However if we use larger diameter probe, it become difficult to maintain the necessary clearance among the probe pins, especially for the design like quad flat package (QFP) pad. If there is not enough clearance even off centre the contact point from the pad, then the CAM engineer in the fixture manufacturing team selects the smaller diameter probe pins to gain the enough clearance among the pins.

In the example of Figure 1, where we consider X axis as the horizontal direction and Y axis as the vertical direction, each test pad size is X=2.0mm, Y=0.2mm and multiple pads are located Y direction by the same centre-centre distance of 0.5mm. Assume 0.3mm as the clearance min limit of pin surface – surface distance for the 0.6mm diameter probe pin for a fixture. If we position the test contact point at the centre of the pad (refer Fig.1-a), there are not enough Y direction clearance (i.e. 0.6mm pins could physically touch each other and cause the electrical shorts). So we cannot finish the CAM job by this design.

In contrast, if we position the probe contact point by stagger, such as the bottom point right, 2nd bottom point left, 3rd bottom point right, and so on (refer Fig.1-b), then the clearance distance is now between the probe pins of odd pad or even pad, respectively. And thus the clearance distance become 1mm and enough to pass the clearance limit in the example, so that we can use this 0.6mm probe pin, which gives lower cost than the 0.4mm or smaller pin.

In general, this stagger process is automatically done by CAM software or/and the fixture design software. Now, many of the commercial software move the position at the pre-defined constant distance from the pad edge (refer Fig.1-b) for the entire PCB design. The purpose to set the constant distance from edge is to give the positioning margins from the pad edge, even using the larger diameter pins (note: larger diameter pins generally requires larger positioning margin). And those software select the smaller diameter pins, in case there is not enough clearance at the position which is set by such constant distance. So the constant distance cannot be large, but needs to be as small as possible. Or the cost of pin and drill job increases.

However, in the real design, many of the test points set by such constant distance still has the clearance margin with the neighbour pins. If a test point still has the excess distance from the clearance limit, it is better to move the position of that test point toward the centre of the pad to enlarge the distance from the pad edge, since the positioning error is the statistical thing in the practical world and larger margin from the pad edge gives lower probability of the contact failure of probe pin to the pad. As the test step cannot accept even 1 pin contact failure of thousands of (or 10s of thousands of) pins in a fixture by the nature, it gives large difference to the pass rate of fixture pin contact by having more test points with the larger positioning margin than the pre-defined constant distance for all from the edge.

Fig.1-a)

Position the contact point at the center of the pads

Fig.1-b)

Position the contact point at the constant distance from the pad edges

Fig-1.c)

Position the contact point with more margins from the pad edges

For the fixture of the conventional dedicated tester, we can move the probe contact position in the pad as close toward the clearance limit as possible. However, for the grid tester (or universal tester), the positioning logic needs to have the further consideration.

Since the pin positioning by certain grid design is required at the machine side, and no such grid design exists at the PCB test pad side for the fixture, the fixture designer needs to consider the probe pin deflection to connect each points by the fixture. And in general, it is easier to deflect the pin by positioning the pin-pad contact position as far as possible among the pins.

Thus the optimum position of probe pin in the pad is defined by “Large enough distance from pad edge, but largest distance from other pins”. By this optimization, it gives easier pin deflection. And we can produce the fixture which gives the lowest cost with high pass rate of pin contact to pad (e.g. Fig.1-c), thus higher productivity of test process.

OPTIMUM PROBE SELECTION BY THE RESIST DESIGN

As described above, “Position of test contact point and Selection of testing probe type”, the fixture production team try to use as large diameter pin as possible for the fixture type testing to reduce the fixture cost, especially for the dedicated fixture. However, it is not necessary yes to the question if we can use unlimitedly larger diameter pin as long as it does not violate the pin-pin clearance limit. The typical example for the question is the probe selection for an isolated test pad, of which the neighbour pin is located a couple of 10mm far away. It is because, there could be the physical interference, which prevents the normal pin-pad contact, by the PCB design and probe type as described below

These days, PCB design tends to have the smaller test pad and smaller resist open area by the finer pitch design. If we place the large diameter probe for such pad with the small resist open area without thinking the physical interference, there could be the pin-pad contact failure due to the size of resist open area, resist thickness and the probe head shape, as described in Fig.2-a.

Furthermore, there is stronger interference by the size of the resist open area and resist height for the universal fixture, since the probe pin comes with the angle (non 90°) to the contact pad surface due to the deflection, where the probe pin of the conventional dedicated fixture comes vertical (90°) to the pad surface like Figure 2.

As the result, when we select the probe pin, we need to consider the PCB design, fixture type and probe type, such as the diameter size and the head shape, as the parameters for the fixture design. Or there will be a fixture of low pass rate of pin-pad contact, and then the team in the test floor will suffer with the low productivity of the test job.

2-b) shows the solution to avoid the interference between the probe and resist by selecting the different shape of probe head. Fig.2-c) shows the solution by selecting the smaller diameter probe pin.

Fig.2-a)

The pin hits the resist pattern, so the test is unstable

Fig.2-b)

Selecting 60°angle pin, no interference against the resist pattern, higher pass rate

Fig.2-c)

Selecting smaller diameter pin, no interference against the resist pattern, higher pass rate

@Gardien, we are using the in-house custom software to automatically select the optimum probe position and the probe type with the consideration of the physical interference between the probe pin and the resist material.

@Gardien we are using the multiple custom developed software to respond for the variety of user needs, and providing the test services by the various tester types, and the fixtures production services for the various fixture type testers with the reasonable cost and the short lead time.

Test Probe Pins

Test probe pins have various diameters, lengths, and ends all designed to have a solid connection between the PCB and interphase to the Electrical tester.  The primary function of the test pin is to transfer the electrical current from the Electrical tester to the unpopulated PCB board.

Fixture Mounting Parts

The mounting parts play a key role in separating the cnc drilled plates to ensure that the test pins do not short.  If the parts are not correct length the test pin could be shifted from the nominal position causing the fixture to have poor performance.

The Fixture Tester

Typically, a fixture tester will perform some sort of pattern scan once for continuity and once for the isolation test. This scan and test will allow for a full parametric test.  This means that every net will be checked against each other for isolation.

Example: The scan would start at the blue point and go up and down sensing all active tester points.  This scan creates a full parametric test because all points are tested against each other during the isolation test.

Connection resistance plays a major role in the fixture test because there are multiple connection points, and the shelf life of a fixture can change the resistance of each test pin.  When the fixture is not in use it should be stored covered and in environment-controlled location.  To compensate for connection resistance manufactures, perform a calibration to ensure the resistance at the point where the pin meets the PCB is as close as possible to 0 ohms for a 2-wire system.  Fixtures that are measuring 4-wire resistance ultra low values do not require this.  The lowest resistance that currently can be reliably measured on a fixture tester is 5 ohms.  The maximum test voltage that can be used on a solid-state universal fixture tester is 300V.  The maximum isolation resistance that can be measured reliably is 100 M ohms

2-wire Test (standard)

2-wire testing uses one test pin on each test point on the PCB, connection resistance plays a major role when testing at 5 ohms.  OEM’s have developed functionality to calculate this connection resistance and offset this resistance when testing at lower resistance ranges

4-wire Test (low resistance test)

Please see 4-wire kelvin testing for more details, 4-wire testing requires two test pins on the same pad or hole to negate the connection resistance.  The fixture must be designed in this way in order to perform this test.

Universal Testers- Bed of Naila

Universal Tester have a fixed spacing in the active test field of the machine, this base is programmed in the fixturing software to assign the test pin from the PCB to a point in the active test field in the tester. There are complex algorithms that calculate the minimum spacing between the pins in each plate of the fixture.  This spacing has a direct relation to what test field point is used in the tester. If the spacing is not maintained or the CNC process is off the holes will break out causing a pin to pin short (false pin to pin short inside the fixture). The netlist controls the information of what PCB test point is connected to what test field point on the tester.

Originally the industry started with a test machine pitch of 100 mils called Single Density.  The distance in X to the next point was 100 mils and the distance in Y to the next point is 100 mils.

Density Progression

Test pin assignment PCB test point to Testers field

Automation

The market has various types and manufactures of automated fixture testers.  Automated testers have a pile for PCB’s that pass electrical test and a pile for the not good PCB’s.

Dedicated Tester

Dedicated fixture tester has a cable connection that used to interface the fixture to the tester.  During the setup process each tester cable must be plugged into a fixture connector.  A dedicated fixture does not have leaning test points from plate to plate, this allows for smaller pins and fine pitches can be performed on this style of a fixture.

PCB to Fixture Alignment

One of the main limitations of any fixture is the registration of the PCB to the fixture.  The PCB registration to each test pin becomes a critical challenge as the pitch of the PCB is lower.  The industry has designed several different schemes to counter act this alignment.  Mechanical devices shift the board tooling in the direction of the shift as well as the top fixture to bottom fixture alignment. Some Electrical Testers have a mechanical adjustment to adjust the top field of the tester to the bottom test field.

The other option is to have CCD cameras that sense targets on the PCB and automatically shift how the PCB is places onto the fixture.

Why Gardien’s Solution?

Your Gardien Local Service Centre will offer different types of hipot services.  Each hipot is specifically tooled/fixtured to test a part using a combination of UCAM and Fixgen Software.  The Fixture production process is controlled to ensure an on-time fixture delivery in Gardien’s proprietary job flow system called Ontrack. This creates a seamless and error free flow of information from the customer supplied data to the Service Floors at Gardien.

Gardien’s team is trained and qualified on all internal process as documented Quality Management System.  The internal process has specific inputs for incoming, certified, and not good boards as well as descriptive educational programs on various board types, and surface finishes. 

Gardien certifies each order processed with a Certificate of Compliance with details about how the order was processed, what specifications were used to certify the product, equipment used with calibration expiry date, team member who processed the order, quantity, and failure analysis. 

Gardien strongly recommends that the test sequence and parameters are clearly stated on the manufacturing drawing or at a minimum agreed upon during the quoting or contract realization phase. In addition, any PCB sent to us for processing should have a unique identifier on each peace for electronic traceability.

KEVIN SYVRET

FINANCE DIRECTOR, NORTH AMERICA

Mr. Syvret joined Gardien Services Canada as Financial Controller in 2001 and assumed the role of Finance Director North America in 2007. Over the past 20 years, Mr. Syvret has held senior finance and accounting roles, including with Canadian Pacific Ltd and Sprint Inc.

NIRAJ PATEL

VICE PRESIDENT, GARDIEN NORTH AMERICA

Mr. Niraj Patel joined the company on a part time basis in 1993 while attending college full time. He worked in various positions until he graduated from the Centennial College Computer Systems Technology program, whereupon he was hired full time. He progressed through several testing and CAM department positions and was appointed North American CAM Manager in 1998. After 18 years with the company, Mr. Patel was promoted to Senior Vice President of Operations, Gardien Services Canada Inc. in 2011. In January, 2015 Mr. Patel became Vice President NA of the newly combined organizational unit North America which includes the Canada and USA.

TODD KOLMODIN

VICE PRESIDENT QUALITY, NORTH AMERICA

Mr. Todd Kolmodin, a native to the Pacific Northwest was born in Seattle Washington. At a young age he moved to the Portland Oregon area. In March of 1986 he graduated from ITT Technical Institute with a degree in Electronics Engineering Technology. Todd began his tenure in the Electrical Test field that same year.  Todd spent eight years working in an independent testing facility in Wilsonville Oregon where he built skills in all aspects of Electrical Test and Quality Assurance. 

Todd then spent a short term with Yamamoto Corporation on their Engineering staff covering PCB drill, finishing and Electrical Test before joining Probe Test Fixtures in 1995. His charge was CAM and Fixuring Manager.  Todd joined the Gardien Group in September 1998 continuing the same roll and taking on the responsibilities of Quality Manager for Oregon operations. In 2008 Mr. Kolmodin was promoted to Vice President Quality USA. In this position Mr. Kolmodin oversees and maintains the Gardien ITAR registration, ISO Multi-Site registration and multiple site Lab Suitability with the Defense Logistics Agency Land and Maritime. Now also maintaining the Gardien QMS system Todd now serves as Vice President Quality North America.

Mr. Kolmodin is also a published columnist and Technical Presenter with multiple appearances at the IPC Apex Expo Conferences in both San Diego and Las Vegas. 

ROLAND VALENTINI

CHIEF OPERATING OFFICER

Mr. Valentini was appointed COO in July 2010 after successfully serving as head of North American operations since 2005.

Roland has over ten years’ experience in the PCB industry, including sales, service, and project management at Atotech GmbH, Germany mainly relating to PCB plating equipment.
He has significant sales experience from his tenure at Mack Rides GmbH and headed up worldwide installations for Lurgi Bischoff GmbH.

Mr. Valentini received a Master’s Degree in Mechanical Engineering from the University of Mannheim, Germany.

RICHARD HORSMAN

EXECUTIVE CHAIRMAN

Prior to Richard joining Gardien in September 2014, he was Chairman/CEO of tool chain vendor Atego Group Systems Limited which was sold to PTC Inc.

Prior to joining Atego, Richard held a number of Executive and Non-Executive roles including Senior Independent Non-Executive Director of Plethora Solutions, an AIM listed Pharmaceutical Company and CEO of AIM listed Cybit Holdings plc. where he grew the company from inception to revenues of over 25 million and took the company through multiple acquisitions before managing a sale of the business to Francisco Partners.

Richard previously held a number of senior roles in the Software industry.

RICK MERAW

GROUP VICE PRESIDENT QUALITY

Mr. Meraw was appointed Group Vice President of Quality in Jan 2015 after successfully  serving as Senior Vice President of USA operations since 2011.  Prior to being responsible for USA operations Mr Meraw was responsible for Canadian operations since 2001.

Rick has over twenty years’ experience with Gardien Group and has various roles including, service technician, customer liaison, Quality manager, technical support, project management, and software design/ implementation.

In June 1992 he graduated with an Electronic Engineering Technologist degree, with honors from a local community college.

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+65 6225 2238

info.group@gardien.com

ROLAND VALENTINI

CHIEF OPERATING OFFICER

Mr. Valentini was appointed COO in July 2010 after successfully serving as head of North American operations since 2005.

Roland has over ten years’ experience in the PCB industry, including sales, service, and project management at Atotech GmbH, Germany mainly relating to PCB plating equipment.
He has significant sales experience from his tenure at Mack Rides GmbH and headed up worldwide installations for Lurgi Bischoff GmbH.

Mr. Valentini received a Master’s Degree in Mechanical Engineering from the University of Mannheim, Germany.