Minding the Business of Contract Testing
Test strategy, business decisions and a cost/benefit analysis are vital to outsourced contract testing for electronics and other products.
Lower cost, higher quality and reliability are longstanding tenets in the manufacturing and assembly world of medical products. In the business of outsourcing contract testing of electronic products, these tenets are especially true and involve a number of variables, ranging from the types and quality of test gear used to the smallest test jig or fixture necessary to conduct proper testing. Those variables must align properly to a medical product’s required reliability standards. By doing so, both the OEM and its electronic manufacturing service (EMS) provider can achieve the most effective use of the outsourced electronic test dollar.
A printed circuit board is being functionally tested using a special test fixture and software-based analysis tool. Photo courtesy of NextLogic.
Devising a Test Strategy With Your EMS Provider
When setting out to determine testing needs for an electronic product, it is a good idea to work in tandem with an EMS provider that is an expert in a variety of testing technologies. By developing such a business relationship, the OEM has an accessible database crucial for analyzing cost and performance trade-offs in various scenarios without worrying that the partner may have a vested interest in any one solution. An association such as this also can assist in forging an effective test strategy.
There is no single inspection or testing system that would meet the needs of every manufacturing environment. Therefore, a number of factors must be considered in developing any given strategy. However, an effective one takes into account several common considerations, including:
• Product design and testability requirements
• Availability of test equipment
• The manufacturing process deployed
• Selecting the right test for a given product
• Product maturity and lifecycle
• Product complexity
• Specific product design and types of components used
• Tailoring test coverage for specific product modules
From a dollars and cents—as well as reliability—perspective, it’s important to target the proper test procedure for a particular product. Otherwise, both profit and reliability are jeopardized. If a product is sufficiently mature in its lifecycle—say, at least two to three years—there is a high probability that most, if not all, of the defects and faults already have been debugged and eliminated. Thus, the printed circuit boards (PCB) should be fully functional. In these instances, if necessary, only minimal debug and test time may need to be applied, and a straightforward testing strategy would suffice.
The testing of complex PCBs, on the other hand, whether for a new or mature product, incurs considerably more detail, especially if the PCBs use surface mount technology (SMT), which now is commonly used for medical devices, especially handheld products. There is a gamut of issues with which to contend, including top and bottom SMT component mounting, whether quad flat packs and/or ball-grid arrays are used and the types of ICs required, such as FPGAs, DSPs and memory chips.
In this regard, design-for-testing (DFT) or tailoring the test coverage to specific product modules is essential in a test strategy. This particular aspect assures sufficient test point coverage in a PCB’s digital and analog modules, for example, because each requires different testing methodologies.
A test strategy’s foundation is to first get a good understanding of the types of testing available and where they are most appropriate from a product and cost point of view. The three main test types are in-circuit test (ICT), flying probe and functional test:
• The ICT traditionally is used on mature products, especially in subcontract manufacturing. It uses a bed-of-nails test fixture to access multiple test points on the PCB’s bottom side. With sufficient access points, the ICT can transmit test signals into and out of PCBs at high speed, evaluating components and circuits to quickly identify opens, shorts and defective components for subsequent re-work or repair, if needed. In addition, the ICT collects data that can be used for in-line process control, providing real-time feedback to enable fast error correction in the upstream processes.
• Flying probe, meanwhile, is applied to prototype and low-volume production PCBs. This test is performed using a defect analyzer for detecting shorts or opens. It performs electrical process testing without using a bed-of-nails fixture interface between the tester and the PCB under test. In short, flying probe can be performed without an expensive fixture. Four to eight test heads move across the PCB under test at high speed. Electrical probes located on each head make contact and test device vias and SMT pads on the board to provide sequential access to the test points. Flying probe often is used to validate line setup without the cost and cycle time related to designing and building traditional bed-of-nails fixtures. Hence, flying probe can provide fast turnaround and high-fault coverage benefits—but without the test fixture cost, which sometimes can run into outrageously high numbers.
• A functional test is used as a final manufacturing step. It provides a pass/fail determination on finished PCBs before they are shipped. Functional testers typically interface to the PCB under test via its edge connector or a test-probe point. This testing simulates the final electrical environment in which the PCB will be used. The most common form of functional test, known as “hot mock-up,” simply verifies that the PCB is functioning properly. More sophisticated functional tests involve cycling the PCB through an exhaustive range of operational tests.
Business Decisions Impacting Your Strategy
Increasingly, OEMs are working with testing laboratories and EMS providers to satisfy their needs pertaining to electronic products. These partnerships involve everything from achieving additional certifications and capabilities to considerably more tracking of component information. The emphasis is on component traceability, lot levels and procurement sources to determine root cause in the event a medical product experiences problems and failures.
From a business perspective, the consequence for the OEM leveraging outside partnerships often is extra cost associated with outsourcing work to testing houses and contract manufacturers. In some cases, however, cost can be mitigated. For example, if an OEM’s PCBs aren’t required to undergo FDA-approved contract testing, the FDA may conduct sample testing at an EMS provider’s or contract test house’s laboratory. This approach is acceptable to the FDA and prevents OEMs from incurring the extra costs of being limited to FDA-approved contract testing. But this sample testing depends on the particular medical product and its applications.
In addition, it is sometimes still conceivable for OEMs to economically perform testing in-house without having to outsource. For example, a functional test with one or two fixtures or jigs can be performed by EMS test engineers or experienced technicians who are trained in reading schematics and debugging.
On a larger scale, however, the main question associated with outsourced contract testing focuses on the product quantity shipped annually and how much of the test cost can be amortized. Getting a good handle on the different tests and their associated costs can help to get a better gauge on amortization.
To cite an example, the manufacture of ICT fixtures alone can cost from a few thousand dollars to more than $50,000—and lead times for making these fixtures are four to six weeks. These particular fixtures are created for mature and complex products with long lifecycles. Products under ICT must be powered to make sure all component values, footprints, solder joints and polarities are correct. Also, since a PCB undergoing ICT is populated with more than 1,000 components, it makes sense to power the board to check the integrity of active and passive components.
In this example, the cost of manufacturing ICT fixtures is so high that justification is demanded for making these dollar outlays. That’s when the number of product shipments factors in. For illustrative purposes, if an EMS provider is shipping 50 to 100 boards a month, and it is determined that the product’s lifecycle is expected to be at least two years or longer, then the cost of this expensive ICT fixture could be amortized over that period. Keeping all other incidental costs in mind, the incremental cost of the fixture per board supplied is relatively small.
An AOI operator reviews first article PCB before it moves to production. Photo courtesy of NextLogic.
As far as functional and flying probe amortization, an OEM decision may be relatively simple because these tests normally do not require expensive test fixtures, a hallmark of ICT testing. Although their test speed is very slow compared to ICT testing, cost also is relatively low—from a few hundred dollars to a few thousand dollars.
The level of reliability a medical product requires also plays into the business decision. To achieve high reliability goals, a sound product testing methodology must be created at the prototype stage. That is when both OEM and EMS engineering staffs initially come together to discuss design and manufacturing objectives. In effect, the PCB prototype serves as a product R&D tool, allowing the EMS provider to increase reliability by defining and building in design-for-manufacturability and DFT procedures within the different stages of the product development.
Those include testing different environmental cycles that a circuit board undergoes to check voltage checks, current levels, temperature changes and humidity. Then, there is environmental stress above and beyond regular in-circuit and functional testing. Failure rate analysis can be performed even at the prototype levels to increase product yield and reliability, for production runs later.
Generally speaking, an OEM’s first prototype of a medical product, in most instances, operates at 60% to 80% of designed specifications. It rarely operates at 100%, unless large dollar amounts and extremely talented and knowledgeable engineering resources are deployed along with considerable effort. In most cases, a few OEM engineers work on a prototype and, generally speaking, their main focus is on testing a few initial critical features that comprise the core functionality of the product. After that is achieved, they work on ancillary functions and refine these specifications and improve the DFA and DFT issues.
The many variables associated with electronics testing often make it challenging to come to the right conclusions and justify certain cost levels. One way to arrive at the right level of cost justification is to do a cost-and-benefit evaluation based on each type of test. When quantity is limited from a few boards to maybe 50 or so, a good testing solution could be flying probe; conversely, if the quantity is higher, ICT generally is a good choice.
A flying probe test is being conducted on the bottom side of an SMT board with the use of two flying probes. Photo courtesy of NextLogic.
Automated optical inspection (AOI) is used to complement ICT to provide an inspection spectrum wider than either process alone. An AOI system locates such defects as missing components, incorrect component values, polarity errors, placement mistakes and solder defects. When used alone, the cost of AOI can run from $50 to $100 per hour of usage. When used in conjunction with ICT, though, costs can range upwards of a few hundred dollars. However, the benefits are that if AOI has been a part of the manufacturing process, initial defect detection, if any, can be caught at AOI levels and the testing at ICT level goes smoothly and fast.
At the other end of the cost spectrum, functional test runs around $25 to $50 per hour for testing time. It is specific and relatively slow. This test sometimes requires a special jig or fixture and can increase the cost from a few hundred to few thousand dollars.
Flying probe is between $100 to $200 an hour, including tester time and test engineering personnel time. The overall cost generally is based on time with both direct and indirect costs incurred. The direct cost is for programming the flying probe tester, which can take up to few hours at most. After the tester is programmed, indirect costs start for time devoted to debugging the program by placing the board on the tester. It can take a few hours to sometimes a day or so to debug and tweak the program to perfection. After the program is debugged, the correct criteria are entered into the flying probe tester and, from this point forward, it takes from five to 20 minutes for each board to be tested.
Flying probe may be an ideal test for a PCB populated with only 300 to 500 components, top and bottom. It takes only a few minutes to test each side of the board, and there’s no extra fixture cost. So, it is highly probable that first-pass yield will be 90% or better using a flying probe tester. The other benefit for using this test is that the programming and debugging times are very small compared to ICT test.
As is often said, outsourcing allows an OEM to concentrate on its product and business development. Beyond this, outsourcing product testing alleviates the OEM from major capital expenditures for equipment and buildings, as well as maintaining the high cost of a cadre of test engineers, technicians and supporting personnel.
Most often, OEMs rely on an EMS provider to include testing as part of its product design and assembly. An EMS company adds considerable flexibility to an OEM’s technology storehouse. Based on the fact that an EMS provider designs and assembles a broad range of PCB products for an equally varied group of customers, it accumulates a much wider arsenal of knowledge, experience and expertise than its OEM customers.
For example, an EMS provider working with military or aerospace customers can deploy the same high reliability principles in DFT for medical products. A highly knowledgeable EMS provider can draw from its experience base and make valuable suggestions relating to different reliability techniques and standards. These proven techniques and methods, such as higher reliability solder joints and more robust PCB fabrication, have been successfully implemented in those other industries.
Ultimately, an EMS provider is perhaps in the best position to help an OEM evaluate its test options and suggest the best test methods to improve product performance, manufacturability, quality, reliability and, most crucial, cost.