ELIMINATING FALSE POSITIVE ICT RESPONSE THROUGH THE USE OF ORGANIC-METAL FINAL FINISH

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1 ELIMINATING FALSE POSITIVE ICT RESPONSE THROUGH THE USE OF ORGANIC-METAL FINAL FINISH Rita Mohanty, John Fudala, Sathiya Narayana MacDermid Enthone, West Haven, CT ABSTRACT In-Circuit testing (ICT) is one of the most effective ways a Printed Circuit Board (PCB) manufacturer ensures the quality of its products. As the name implies, ICT tests components that are part of an assembled PCB. In other words, the components under testing are in a circuit. The first and most critical part of an ICT testing is detection of shorts-and-opens. PCB final finish, which is the last step on the fabrication process, has the most significant impact on the ICT testing. Historically, there have been two major types of final finishes: organic or metallic. As the ICT test probes requires a conductive pad/surface to perform the test, metallic final finishes works well for ICT. Organic finishes, such as organic solderability preservatives (), faces challenges when it comes to ICT testing. Many fabricators prefer to use because of its low cost but end up losing the cost advantage by implementing additional steps on the fabrication process to add metallic finish to the test pads on a PCB, This paper will present results from a series of statistically designed experiments that includes different types of probes for in-circuit testing along with different types of final finishes to demonstrate pin testability of the various surface finishes. Through this exercise, we will be able to provide assemblers guideline in choosing the correct final finish for their end application. Key Words: Organic Metal () final finish,, ICT, probe design, metallic finish. must be electrically conductive for the ICT to yield the required measurement for defects identification. Figure 1. Percent of defects identified by ICT The test pad on a PCB is usually covered with either a metallic or an organic coating to prevent the copper pad from oxidizing. Figure shows a typical PCB [] with test pads. Metallic coatings such as HASL (Hot Air Solder Leveling), ENIG (Electroless Nickel Immersion Gold), ImAg (Immersion Silver) and ImSn (Immersion Tin) are naturally conductive and possess relatively less challenges for the ICT testing. However, (Organic Solderability Preservative) is an organic coating which is none conductive by nature and possesses some specific challenges regarding the ICT test. INTRODUCTION In circuit testing (ICT) is considered to be the most valuable tool in isolating defective printed circuit board (PCB) assemblies before it goes to more value added steps. Figure 1 [1] shows the cost associated with various assembly tests and its ability to identify defects. As the figure shows, ICT can catch over % of the defects at a relatively low cost as compared to other tests. ICT is a simple test method where a bed of nails is used to access the circuit nodes to measure the performance of the components. This testing checks resistance, capacitance and sometimes inductance between two points in the circuit board to identify opens, shorts and wrong placement of any components. The success of ICT primarily depends on the physics of intimate contact between the test probes and test pad on the board. Both, the test probe surface and the test pad surface Figure. Typical test pads covered with solder

2 The test probe must penetrate the thin coating in order to make contact with the copper pad for the ICT to work. As the coating is an organic coating it has the tendency to redeposit and accumulate onto the test probe tips, registering high resistivity. This causes the ICT to fail while the circuit is in fact is good. This is known as false positive and is one of the biggest drawbacks of finishes. Many fabricators prefers to use because of its low cost but end up losing the cost advantage by implementing additional steps on the fabrication process to add metallic finish to the test pads on a PCB. There is a distinctive, new class of final finish that delivers solderability and ICT capability of a metallic finish without the high cost of metallic finish. It is comparable to with the added benefit of ICT testability. This class of finish is known as Organic Metal (). It is based on nanotechnology and consists of a complex between organic metal and silver. This paper describes the results from a series of tests comparing with and presents an ICT test method particularly suitable for research environment. Figure. Probe types Flying Probe Tester The flying probe tester is a fixtureless device that uses a generic board holder and single to multiple probes which is controlled by a software program. It moves across the board at a high speed, hence the name flying probe, to access individual test point that are used to measure resistivity. This makes the flying probe tester cheaper to operate as no new fixtures are required to accommodate new board design. A custom test system was designed for this study which used a single probe that hits points on a test coupon with one stroke. The probe set up is shown in figure. IN CIRCUIT TESTING There are two primary types of ICT methods used in the industry today. They are known as bed-of-nail and flying probe. This study used both types of tester to compare testability of and the final finish. The bed-of-nail tester used n this study is a custom designed fixture by Alpha Metals, known as a Clamshell tester. While the flying probe designed was developed specially for this study. Both the tools are described below. Clamshell Tester On a clamshell tester the board is forced onto dozens of spring probes that are electrically connected to the test system. The test system can then make electrical resistivity measurements between these points. A clamshell tester requires dedicated fixtures for each PCB assembly which makes it expensive to run. Figure 3 shows Alpha Metals custom designed clamshell tester [3] which was used for the current study. This particular tester used four different types of probes which are shown in figure. Figure. Flying probe set up EXPERIMENTAL This experiment included both types of ICT test methods to compare testability of and surface finishes. Figure shows the test vehicle used for the clamhell ICT test which was developed by Alpha Metals to be used with an ICT bed-of-nails fixture as shown in figure 3. mil test pads 1 mil pitch Figure. Clamshell test vehicle Figure 3. Alpha Metals Pin-Test clamshell tester This test vehicl (TV) is a 7 x 7 FR board and mils thick. As seen in figure, this TV includes many test features. However for the current study only Surface Mount

3 Technology (SMT) pads were utilized. Each board was coated with the required final finish ( or ) following standard process as described in the Technical Data Sheet (TDS). After the coating process the thickness of the coating was measured and recorded. The thickness was maintained at.37 microns and the thickness was maintained at 9. nanometers. Two test conditions were used for the clamshell ICT testing. It consisted of freshly coated board (X) and after two times reflow (X) in a lead (Pb) free reflow cycle. This was done to simulate double sided boards and wave soldered boards which will experience multiple reflow cycles. Two replicates per test condition were run to increase statistical validity. As shown in figure, four different types of probes were used in this testing. In addition, four different forces were also used for each probe type. The force ranged from oz to oz as shown in table 1. Probe type Probe force (oz) Number of pins,,, 1,,,,,,,,, 1 Table 1. Clamshell test matrix Figure 7 shows the test vehicle used for the flying probe ICT testing. As it can be seen from figure 7, this test vehicle is a simple copper clad laminte measuring 3mm X mm. The board is coated with the desired final finish with the same thickness as described for clamshell ICT testing. The coated test vehicle was placed on a carrier as shown in figure. A custom designed fixture with a single probe is actuated through software control to hit the surface of the board at different points. As the probe hits the surface of the test board, the resistance is recorded. Two different types of probes with two different pressures were included in the flying probe ICT testing. This is shown in table. Three replicates per test condition were tested. Figure. Carrier for flying probe tester Probe type Probe force (oz) Number of hits,.,. Table. Flying probe test matrix RESULTS AND DISCUSSION Selected results from this study are presented here due to limitations of the paper. Interested readers may contact the authors for additional information. As the clamshell tester from Alpha Metals is an established test method, more variety of probe type and probe pressures were used to compare the two surface finishes. However, the flying probe tester (being developed for this specific study) used two, commonly used, probe types and probe pressures for comparison purpose. Results presented here were analyzed using MiniTab to validate differences between the two surface finishes using various statistical methods. Figures 9 and 1 show overall results for the two surface finishes at various test conditions for clamshell and flying probe testing respectively. It is clear from these figures that the surface finish shows a lot more variation (higher standard deviation) as compare to the, regardless of test conditions or test method. The large variation in resistance readings for the was expected and is one of the reasons for detecting false positive in a high volume manufacturing environment. Clamshell Boxplot of 1 1 x x Figure 7. Flying probe test vehicle Figure 9. values for all conditions using Clamshell ICT tester.

4 Flying Probe Boxplot of Flying Probe Boxplot of x x x x Figure 1. values for all conditions using flying probe ICT tester Figures 11 and 1 show the same data set as figures 9 and 1, with the added clean-up function of MiniTab. MiniTab has the ability to identify and remove outlier data points to allow us to take a closer look at the data set free of outliers. Even after the removal of outliers, the still shows much more variation as compared to. One important observation to be made here is the results from the crown probe type for the clamshell tester. We see the crown probe type produces higher degree of variation for both types of surface finishes. This is primarily contributed to the uneven landing (contact between the probe and test pad) of the probe on the surface. However, this phenomenon is not present in the flying probe tester due to the tester design. Hence, further statistical analyses presented here are based on flying probe tester only. To understand the statistical significance of the difference observed in the box plot, a two-sample T-test was conducted for and surface finishes. The Null hypothesis here is there is no difference between the resistance value for and. The test condition used in the analysis here is to favor testing. The conditions used for the Two- Sample T-Test are presented in table 3. Clamshell Boxplot of Figure 1. values for all conditions using flying probe ICT tester without the outliers Test Parameters Stetting Probe type Probe pressure. oz Reflow condition X Table 3. Test condition for Two-Sample T-Test Figures 13 and 1 shows the results obtained from MiniTab for Two-Sample T-Test. Both probe types show a p value of.. This tells us with 9% confidence, we can reject Null hypothesis. In other words, the resistance value is statistically different from the value. Furthermore, based on the results and analysis, we can say that the shows lower resistance and more stable ICT results as compare to the surface finish. Two-sample T for Surface Finish N Mean StDev SE Mean Difference = μ () - μ () Estimate for difference: % CI for difference: (-.197, -.13) T-Test of difference = (vs ): T-Value = P-Value =. DF = Figure 13. Two-Sample T-Test for flying probe tester with probe x x Figure 11. values for all conditions using Clamshell ICT tester without the outliers Two-sample T for Surface Finish N Mean StDev SE Mean

5 Figure 1. Two-Sample T-Test for flying probe tester with probe SUMMARY AND CONCLUSION A series of statistically designed of experiment was carried Figure 1. Two-Sample T-Test for flying probe tester with probe SUMMARY AND CONCLUSION A series of statistically design of experiments were carried out to compare the ICT testability of the Organic Metal surface finish and the surface finish. Two different types of ICT methods were employed with different probe types and probe pressures. This was done to simulate a real production scenarios where multiple probe types may be used based on availability and individual test protocol. The results and analysis presented hear clearly demonstrate the advantage of the Organic Metal surface finish when it comes to ICT testability. The type of surface finish used in each specific application is mainly dictated by the end user based on their requirements. However, if ICT testability is one of the top criteria then the Organic Metal has clear advantage over the. ACKNOWLEDGMENT The authors would like to express their sincere gratitude to Alpha Metals for making their lab and equipment available for testing. Without their support, this studies would not have been possible. REFERENCES [1] In Circuit Testing, Allen Buckroyd, Butterworth- Heinemann Ltd., 199 [] Printed Circuit Board, Wikipedia, [3] Pin-Testing