A caller into the Helpline requested assistance and recommendations for performing quality testing of his bare ceramic hybrid printed circuit boards (PCBs). The caller wanted to verify the integrity of the bare ceramic substrates for shorts, opens, and node continuity before they were populated with the expensive components. The caller was considering a “bed of nails” type test fixture, but the quantity of boards for each particular design was low; therefore, it would be ineffective to repeatedly modify the fixture for each new prototype ceramic board layout.
Available options / evaluation steps
To determine the available options and formulate a recommended solution for this Helpline caller, a number of specific, rigorous steps were undertaken:

1. Reviewed the customer’s detailed requirements for his hybrid modules and the characteristics of the ceramic circuit boards that require continuity testing.
2. Discussed concerns regarding costs, flexibility, quality, and reliability.
3. Determined applicability of PCB testing standards; both commercial and military.
4. Researched additional relevant information from EMPF’s industry and equipment partners regarding the fundamentals of PCB / substrate testing.
5. Analyzed the technical performance specifications of “flying probe” test machines, reflecting the current state of technology for PCB bare-board testing.

The EMPF considered several potential options initially, including a “bed of nails” test fixture, a “flying probe” style tester, an optical vision inspection system, X-ray machines, and even manual, point to point verification.

The “bed of nails” tester is more applicable for high-production volume, stable designs, with all components located on the spacing grid. It also requires reworking the test fixtures for each new board. However, this method does have the advantage of rapid test cycles, since the test grid is basically a parallel operation.

The vision, X-ray, and manual test methods were deemed insufficient to produce the level of reliability desired.

Customer application
The caller’s company produces high power electronic hybrid modules offering excellent thermal handling characteristics. Ceramic substrate material is superior for high frequency microwave circuit applications due to its low loss and low dielectric constant. A direct-bond copper process, along with a thick-film printing technology to form passive, conductive, resistive, and insulating circuit elements onto ceramic, alumina, and beryllia substrates, permits circuit component operation at much higher power and temperature levels than a fiberglass mounting medium (FR4).

Approximately 60% of the caller’s modules are directed to the military sector (low volume but harsh operating environment) for use in high reliability products such as microwave and radar applications.

Customer-specific requirements:

• Ability to test for shorts, opens, and circuit continuity.
• Flexibility of test to handle both new prototype units and revisions to existing PCB circuit designs without incurring excessive re-fixturing costs or response-time delays.
• Physical size dimensions range up to a maximum of 6.0" x 6.0" with typical thicknesses between .020" and .120".
• Test screen-printed resistors which are laser-trimmed from 10 ohms to 10 Megohms at .1% tolerances.
• Fragile handling requirements of the ceramic material compared with FR4 boards, particularly for forces applied in the “Z-axis”.
• Test cycle of less than 15 minutes per board.
• Test node point spacing of .020" minimum.
• Typical total circuit test nodes per board < 200.
• Quality and reliability. Both commercial and military specifications were discussed.

Technical information
Within the past decade, the trend in the test industry has been toward flying probe test equipment to accommodate an even tighter pitch and feature definition, while permitting testing of both board surfaces and even “Z-height” test probe programmability.

The EMPF has recently partnered with two manufacturers of these flying probe test machines which seemed applicable to addressing the caller’s needs. Both Hioki of Japan and SPEA America have placed evaluation versions of their flying probe test equipment on the EMPF demonstration factory floor. They have also provided operator- and technician-level training.

Test time-shortening schemes have been developed to eliminate many of these “shorts” tests, based on the assumption that only adjacent (within .050" or so) networks on the PCB trace layout need to be tested against each other. These schemes can significantly decrease test time but must be evaluated to justify risk / cost tradeoffs. The key to implementing these reduced test times is the CAM software-generated “adjacency” file derived from the PCB Gerber file database. IPC-D-356A design interface standard offers guidance in this area.

Other desirable flying probe machine features and specifications include single- or double-sided board probing; multiple probes (each side); positioning accuracy of probes (.010"); speed of motion (50 ms/test); Z-axis probing placement (test points on components); Z-axis soft landing (less stress for the ceramic substrate); board size capability; highest speed and productivity (no fixtures); program generation interface to CAM system; and in-circuit testing capability.

Proposed solution
The EMPF technical staff proposed a solution of using a flying probe style tester. The programmability and versatility of these machines is a logical solution to the low volume mix and frequent product changes and revisions encountered. Being fixtureless and grid-less, no constraints are placed on the layout of the design to keep all component leads, connector spacing, or any test node “on-grid”. The flying probe’s placement accuracy produces test point spacing within .010" (due to the accuracy of the linear motor positioning system). This flexibility is not possible using existing “bed of nails” technology.

Summary
The Helpline caller was provided with the information to contact the flying probe equipment manufacturers directly. As an alternative, they were offered the board testing services of the EMPF demonstration factory, pending detailed definition of board design documentation and equipment availability on the EMPF demonstration factory floor.

PCB test equipment is constantly advancing to keep pace with the latest board design and component developments. Please contact the EMPF Helpline for advice / recommendations concerning the latest electronics PCB manufacturing test processes and machines.