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Flexible Test Strategy Improves Efficiency
December 31, 1969 |Estimated reading time: 6 minutes
In the contract manufacturing sector, a pay-per-use test strategy is a proven testing solution for generating high-quality manufacturing services at low cost.
Steven Schmidt
The contract manufacturing (CM) business is a highly competitive industry. CMs often face unpredictable and variable workloads from customers that require innovative equipment and expense strategies. They must have the manufacturing flexibility for handling different types of assemblies, volume and delivery schedules without passing prohibitive costs to customers.
Pay-per-use (PPU) systems reduce the financial risk of board test because they offer a range of test capabilities that are paid for only when they are used. For example, a full combinational tester can be purchased for roughly the same cost as an entry-level unpowered tester and used as is. If in-circuit test (ICT) or combinational testing are needed, these additional capabilities are unlocked using test credit buttons (the user only pays for the capabilities that are used). As a result, PPU testers enable CMs to answer specific customer needs with highly flexible and cost-efficient electronic manufacturing solutions.
The Need for Flexible ICT Capability
One company* discovered PPU systems after upgrading its ICT capability. At that time, the company was only using a manufacturing defect analyzer (MDA) for defect inspection test, which resulted in a low yield percentage into the functional test step. Although ICT was initially justified based on savings expected by improving first-pass functional test yields, the company found important collateral benefits from being able to offer customers more flexible test options.
The initial cost study looked at five areas where savings could be realized by adding ICT capability. To get concrete comparison numbers, 20 printed circuit board assembly (PCBA) types were evaluated - all of medium complexity with annual volumes averaging 20,000 pieces. The five cost-saving areas looked at were:
Immediate feedback: the ability to rapidly uncover process problems involving defective or incorrect components. Savings would be realized by reducing or eliminating rework.
Improved first-pass yield: reducing time spent on troubleshooting and repairing defective PCBAs.
Reduced functional test time: two minutes per assembly were saved on half of the board types.
Elimination of functional testing: on 10 of the board types.
Increased outgoing quality level: by capturing out-of-tolerance devices that would not have been seen by functional testing.
It is estimated that adding ICT capability to the test floor could save the cost of 2,230 work hours per month.
When the company had only MDA and functional test in place for these assemblies, the typical first-pass process yield for medium-complexity, medium-volume boards was about 93 percent. After ICT was introduced ahead of functional test for those boards, first-pass yield at functional test went to roughly 99.5 percent. In addition, first-pass yield at ICT has increased to 96 percent, indicating that ICT and rapid problem identification has had a positive impact in the production process.
PPU Reduces Financial Risk of Board Test
After the need for ICT was identified, the company surveyed the market for ICT equipment and realized that many were too expensive. The company was able to break this impasse by purchasing two PPU board testers set up as MDAs. The units were capable of operating as high-end ICTs and even had limited functional test capabilities. These capabilities can be unlocked as needed and purchased at an additional cost only for the time the extra capabilities are used.
When acting as an MDA, the tester incurs no additional costs. When the tester acts as an ICT, an additional cost per test minute is paid. The rate depends on the additional capabilities unlocked. An example of one of the features that the company is able to unlock is TestJet** technology. This test technique employs an unpowered, vectorless test technology for testing digital integrated circuits (IC) that uses an external plate, suspended above the digital part, and separated from the lead frame by the plastic or ceramic material of the device housing. The lead frame and the external plate form a small capacitor that can be measured by stimulation with an AC source. Each pin consists of a part of the lead frame so that each can be detected as a separate capacitance. The big advantage of this technology is that no knowledge of the core functionality of the device is needed; it depends only on physical properties of the packaging, and requires test-probe access to all device pins.
PPU allowed the company to make a capital investment equal to what it would pay for an MDA, while making it possible to expense the additional ICT costs and tie them to specific PCBA types that require the additional capabilities.
Customer Requests Satisfied
This PPU strategy plays perfectly with the demands from the customers. Different PCBA types follow different test-flow patterns depending on the board`s complexity, volume and customer demands.
There are four basic patterns shown in Table 1. ICT is not used for boards that undergo visual inspection only or for those just using functional test. Roughly 5 percent of the company`s boards - those that are smallest and made in the lowest volumes - get visual inspection only. Those assemblies that are less complex and have a robust functional test (about 25 percent of the assemblies produced) only have functional test performed.
Of the remaining 70 percent of board production, only 20 percent (14 percent of the total) never see functional test. The rest, some 56 percent of the company`s total production, go through ICT followed by functional test. Having most of the process defects captured first by ICT drastically reduces functional test time for most boards.
PPU testers are not used for functional testing. The company builds boards used in a variety of applications, from office automation to test and measurement, medical, and industrial automation. Testing may include CPU boards operating at 5 to 15 V and drawing a couple of amperes one day, and electric vehicle controllers running at 100 V and 800 A the next. The needs are just too broad to allow the company to standardize on one functional tester. It is much more cost-effective for it to have custom functional testers dedicated to specific boards.
While the testers are capable of controlling a suite of GPIB or VXIbus instruments to perform these functional tests, it is not cost-effective for this particular company to use them this way. If there were extra capacity in these two testers beyond what is needed for ICT, it might be cost-effective. As it is, however, the company needs them full time for ICT. Adding functional test burdens would bog them down unnecessarily.
How PPU Works
PPU testers include the capability to generate ICT routines automatically. Computer-aided design (CAD) information is downloaded from a customer for the target PCBA to the tester. It automatically generates the test software needed for a complete test, including a design for the fixturing. A software engineer modifies the test software to eliminate unwanted resources.
A coin-shaped electronic button that fits into a socket at the bottom of the tester unlocks the beyond-MDA resources needed for this particular test. The "coin," which is purchased separately, begins with a certain number of test "credits." Each time the tester calls on the extended resources, it automatically debits the coin for the amount of time used at a rate that depends on the resources used.
When the "coin" runs out, another one is inserted. Coins can be purchased as needed. The tester helps keep track of the additional cost and relates it to the customer order that required it.
The company is able to satisfy the customers` different board requirements because it can offer them enhanced test capabilities beyond simple MDA testing without making all customers pay for it. If the testers had been purchased outright, the company would have had to amortize them completely as a capital cost spread out over all production runs for all customers. While the customers appreciate the company being able to offer enhanced capabilities, none of them want to pay for capabilities they do not use themselves.
*Apsco International
**Agilent Technologies
STEVEN SCHMIDT is the vice president of quality and engineering for Apsco International, 3700 Lane Road, Perry, OH 44081; (440) 352-8961, ext. 325; Fax: (440) 354-7333; E-mail: sschmidt@ apscoinc.com.
Figure 1. Pay-per-use on in-circuit testers allows companies to tailor test costs to the individual customer`s test needs.