Case History/Utilization of Nitrogen in IR Reflow Soldering. S. Jacobs Air Products and Chemicals, Inc. U.S.A.

Transcription

1 Case History/Utilization of Nitrogen in IR Reflow Soldering S. Jacobs Air Products and Chemicals, Inc. U.S.A. 1

2 Abstract This paper presents the results of a two month test program which evaluated the effects of nitrogen atmosphere on the infrared solder reflow process. Data was collected by alternating between air and nitrogen atmospheres on two week intervals during the two month test. The data taken, solder joint defect type and quantity of defects, was from actual production SMT boards that were being produced during the interval. These test conditions yielded the following result: roughly a 75% reduction in solder joint defects in the nitrogen atmosphere as compared to the air atmosphere. Although this data is subject to many other manufacturing variables, it is argued that the sample data is random and is indicative of yield improvements capable in a "true" manufacturing environment. The yield improvements are achieved mainly by increasing the wettability of the component terminals and PWB pads. Component terminals and PWB pads exhibit some degree of oxidation due to shelf age, environmental conditions and oxidation during the pre-reflow stages of a normal IR reflow process. The economic feasibility of nitrogen atmosphere reflow is also presented based on this data and its effect of reduced rework and inspection. Introduction The author was delegated with the obligation of finding a way to reduce the overall solder defect levels after IR reflow operations and thus reduce inspection and rework cost at this point. Infrared reflow has gained popularity for the attachment of surface mounted printed wiring board assemblies (PWBA) in the past several years. The quality of solder joints depends heavily on the tight controls of temperature parameters during this process, however, the author found that even with tight controls over the infrared reflow process, optimal results (defined by the lowest defect levels) could not be achieved. Soldering defects in the manufacture of surface mounted circuit assemblies are defined by a set of Manufacturing or Quality Standards. There are those that critically have an effect on the electrical function of the assembly, and the rest (for all practical purposes) are cosmetic in nature. Table 1 lists a number of defects generally found by visual inspection techniques. Critical defects are nonsubjective. Either they are present or they are not. Opens and solder bridges are easy to detect with unaided vision and/or ring light magnification. They require no judgement by the inspector. In contrast, the non-critical defects require even the most experienced operator to make a judgement based on the interpretations from sketches and photographs available in the Manufacturing or Quality Standards. Often, a solder joint is rejected and consequently repaired based solely on the fear that a quality control technician may disagree with a pass call. The end result is that the cost of manufacture cannot be controlled and minimized as one would wish. Critical Open Solder Joints Solder Bridges Tombstoning/Drawbridging Table 1 Non Critical Non Wetting Insufficients Excess Solder Cold Solder Joints Observations of the products, components and visual rejected defects indicated that the vast majority of the rework and repair was for non-critical defects. Of the noncritical defects, non-wetting was the largest defect at roughly 75%. Reduction and control of this type defect is what the author concentrated on removing. The rootcause of non-wetting is generally one of the following: 1) contaminated plating of component terminals or PWB pads, too thin of plating which an allow intermetallics to form on the terminals or pads, or 3) oxidation due to storage and/ or other environmental conditions. All three of these conditions can typically be found on a manufacturing floor to some degree. Furthermore, soldering fluxes generally used in many of the solder pastes today are relatively mild in oxide removing capability. This mild character makes it easier to remove flux residues during cleaning, however, it can limit their ability to overcome resistance to wetting. It should be noted the preheating stages of the IR oven also advances oxidation. Based on earlier limited work with nitrogen in a reflow process at a previous employer, and work reported in the literature (1) (2), a very preliminary test was done with the assistance from Air Products & Chemicals. A small experimental procedure was performed on a Vitronics seven zone IR furnace. This furnace was originally purchased with nitrogen capability. A schematic of the nitrogen system is provided in Figure 1. As can be seen, all nitrogen lines were piped in 1/4 inch plexiflow tubing with flow meters sized to provide a gas flowrate of only 450 scfh. This flowrate was estimated to be inadequate for defect test purposes, resulting in an atmosphere containing 2 to 3% oxygen as measured on a Thermox oxygen analyzer. 2

3 Figure 1 Table 2 Flowmeter Flow Rates Resulting Oxygen Profile Distance (ft) Oxygen (ppm) 1 11, , , , , , , , , , ,000 In this experiment, strips of bare FR4 epoxy laminate material were run through the furnace in air and in the 97% nitrogen atmosphere. The pieces fired in air exhibited a significant brown discoloration, while those fired in the nitrogen atmosphere were slightly discolored. It was our feeling that lower oxygen levels would remedy this problem. Also, a limited quantity of products were processed in the nitrogen filled oven. The solder connections of the small sampling appeared to be distinctively shinier than the ones from standard production, and exhibited better flow characteristics. At this point in time, we felt that the results provided a strong case for a longer experiment with nitrogen atmospheres. The main objective of the extensive testing was to quantify the ability of nitrogen to eliminate non-wetting solder joint defects. Production Test Program Air Versus Nitrogen in IR A test program was designed in which normal two week production quantities would be alternatively IR reflowed in air and in nitrogen over a two month time frame. The first two weeks were allocated for running the production in air, second two weeks in nitrogen, third two weeks in air, and, the last two weeks in nitrogen. The production lots were taken randomly as the orders for products were received. Some products were single side SMD assemblies and others were double side BUD populated. Fine pitch products were part of the mix and component population varied over a wide range. Some products were single boards while others were panel arrayed configured. The products used almost the entire spectrum of standard surface mounted components types; PLCCs, SOICs, SOTS, Xtals, POTs, ceramic R and Cs, QFPs, MELFs, ect. No special controls were placed upon the solder paste printing operation or pick and place. The inspection criteria used was the normal standard already in place and was applied by visual inspection screening operators. The production data, consisting of number of boards handled, the types of defects and the quantity for each were input by the operators into an in-house designed Defect Counting 3

4 Figure 2 The solder paste used during the entire test run was RMA type with a 88% metal load, -200 mesh particle size and a viscosity range between 700 kcps to 950 kcps. Three solder paste suppliers products represented this material. The paste was applied to the substrates using a metal stencil to a thickness between 10 mils thick and 11.5 mils for standard pitch components. On fine pitch boards, a 6 mil step-down stencil was used to deposit 5 to 6 mils on QFP pads and 10 mils on the standard pitch component pads. Figure 3 represents the board surface temperature profile used through-out the test. From ambient, the temperature is ramped to approximately 150 degrees C at a rate not to exceed 2 degrees C/second. After that point, the temperature drops about 5 degrees as the product transfers from the ramp up zone into the preheat and reflow zone. Between these two zones, there are no IR heat panels, therefore, the experienced drop in temperature. After that point, the temperature elevates at a slow.5 to.6 degrees per second up to about 17? degrees for the final reflow temperature spike. The temperature for the reflow is indicated at 215 degrees plus or minus 5. The IR oven provide. 60% convected heat and 40% radiated IR energy from panel heaters. Figure 3 A number of preparations had to be made before the test runs could begin. IR Reflow Equipment Figure 4 illustrates the piping modification made to the Vitronics to improve the flow of nitrogen in the furnace as well as the nitrogen supply to the machine. Plexiflow tubing diameter on the gas inlet, preheat and the hot zone lines were increased from 1/4 inch to 1/2, while the entrance and exit curtains were left unchanged. New flow meters were installed with the capacity to supply a maximum of 1000 scfh to the hot zone, 500 scfh to the preheat zone, and 200 scfh to each curtain. With better flow rates, a reduction in oxygen level in the furnace was expected. 4

5 Figure 4 Oxygen profiles were taken in the furnace with optimized nitrogen flow rates as follows as shown in Table 3. Table 3 vflowmeter Flow Rates scfh Resulting Profile Distance (ft) Oxygen (ppm) As can be seen from the data, the furnace modifications resulted in a 10-fold oxygen reduction from the original test. Given the fact that promising results were obtained in the preliminary test, it was expected that this profile would be acceptable for the defect study. However, it was felt that oxygen levels could be reduced even further if a gasket were installed to prevent air from seeping in through the hinged frame design. A silicon rubber material was cut into 1" wide strips and placed between the furnace base and the lid (hinged design without a muffle) to act as a gasket. Before any production boards could be run, the oxygen profile had to be optimized against the temperature profile. The previous optimized flow settings in Table 3 were used as a starting point. The last two flowmeters had a significant cooling effect on the temperature profile, causing a 5.2 degree C drop per second immediately following the reflow stage (see Figure 5A). Also, the exit curtain flow was so extreme that some components were shifting on the molten solder pads. The flow rates settings were optimized to yield the lowest oxygen concentration with the same temperature profile as with air (see Figures 5B and 5C). With the gasket and the flow optimization, the oxygen level in the hot zone was reduced to ppm. The new flow settings are given in Table 4. 5

6 During the course of the test run, one other experiment was conducted. A total of 16 panels, each containing 4 small pc boards, were used for three purposes; 1) to compare the solderability of brand new components and aged oxidized components when both types are soldered at the same time on a common substrate, 2) to determine the effects of atmosphere on void formation in the solder joint when soldered in air versus nitrogen, and 3) to determine the effects of air and nitrogen upon misaligned components. Figure 6 shows the relative position of the different component populations on the test panel. The test boards were solder pasted using the standard RMA paste, applied to a thickness of 10.5 mils. The components were hand placed. On the misalignment test, the parts were positioned so that at least one or both terminated ends extended outside the solder pad by.020". All of the components were the 1206 package (.120" X.060"). Figure 5 Table 4 Flowmeter Flow Rates Figure 6 Experiment Procedure As previously stated, the experiment consisted of running standard production boards in the two different atmospheres. Eight weeks worth of production was alternated between air and nitrogen atmospheres every second week. A total of 6,778 boards were soldered in air while 12,729 boards were soldered in the nitrogen atmosphere. From a solder joint population standpoint, this equates to approximately 1,488,419 and 2,518,448 joints, respectively. Defect data was kept real-time on the Defect Counting System. 6

7 Results/Discussion Test Boards Figure 7 Illustrates The Results Obtained On The Three Different Tests Performed On The Test Boards Figure 7 1. The solderability of new parts was the same using either air or nitrogen atmospheres. Since the new parts were relatively free of oxidation, their soldering performance was similar under the two different conditions. However, when aged parts were used (2 to 3 year date codes), the nitrogen results show a 79.3% improvement in solder joint defects than when soldered in the air atmosphere. All of the defects that were identified by the experienced operator were for non-wetting. The, improvement obtained in the nitrogen atmosphere is attributable to the better wetting performance provided by the inerted atmosphere. nitrogen. However, a marked difference exists on soldering of aged (oxidised) parts. In this case the data indicates a 20% improvement in self alignment when soldering in nitrogen versus in air. This improvement is due to the greater wetting force that nitrogen allows in the soldering process. It has been previously demonstrated that the wetting force is 50% greater in nitrogen than in air (1). 2. Xray examinations of 80 joints made in air and 80 joints made in nitrogen shows a 55.5% reduction in voids when the solder joints were made in the nitrogen atmosphere. This is important because voids provide a point of fracture for applied stresses (3) & (4). Therefore, solder joints containing a large number of voids will have a higher potential for failure and typically will fail sooner than joints without. 3. On the self alignment of the new parts, the data shows a negligible difference between soldering in air or in 7

8 Production Test Figure 8 illustrates, the differences in soldering performance between an atmosphere of air and another that has been inerted with nitrogen. The data shows significant reductions in defects in four out of seven types. In an overall comparison, there were 75% less defects made in nitrogen than in the air environment. Each of the comparisons is discussed below. Figure 8 Non-wetting defects were reduced by 87.7% when reflow soldering occurred in a nitrogen atmosphere. This relationship was demonstrated in the previous Solderability Test where a similar result was experienced. As mentioned earlier, and as reported in the references, a nitrogen controlled environment has a positive effect on the soldering performance of an IR reflow process and other mass soldering processes as well. Insufficient solder joint defects were reduced by 77% when reflow soldering occurred in a nitrogen atmosphere. This improvement is due to the better wetting characteristics of the molten solder in an inert atmosphere. The forces of the solder in nitrogen tend to drive the liquid solder to creep towards the lead significantly better than in air. Also, from an inspection operator point of view, the much shinier solder joint made in nitrogen, makes it easier to see and to judge than the one made in air having a less visually detectable surface luster. Solder bridging defects were reduced by 77.6% when reflow soldering occurred in nitrogen. This was unexpected since there is no basic data from other sources that indicate similar results. There may be some improvement when using nitrogen due to the better coalescence of the solder particles, but, it is not expected to be as significant as the data implies. Solder bridging is normally attributable to: a) too much paste on the solder pads, b) solder paste slumping, c) misregistration of the paste on the pad when applied in the screening process, d) misalignment of fine pitch QFPs, and, a) to a certain extent, coarse size particle solder paste on fine pitch QFPs footprints. Any of these variables or a combination thereof will create solder bridging. The defect data collected during the experiment was not finite enough to allow an accurate identification of the reason(s) for the better performance of reflow soldering in nitrogen as related to solder bridging defects. This aspect would have to be exclusively tested with a dedicated controlled experiment in order to make a determination of the root-cause. Opens were reduced by 6% when reflow soldering occurred in nitrogen versus when soldered in the air atmosphere. The nitrogen atmosphere did not promote a significant reduction in opens because opens are primarily caused by a) insufficient solder paste deposit and/or b) non coplanar devise leads. The wetting improvement of a nitrogen atmosphere can only marginally correct for lack of paste or for non coplanar leads. Cold solder connection defects were reduced by 50% when reflow soldering occurred in the nitrogen atmosphere. These defects, whether occurring in air or in nitro- 8

9 gen atmospheres, were low magnitude problems in the total population of solder joints. Excess solder defects were reduced by 70% when reflow soldered in a nitrogen atmosphere. However, the number of defects for air and nitrogen were small percentages from the population universe. Very often, a solder joint that exhibits less than ideal wetting has a slightly convex solder fillet. A solder joint that has ideal wetting, always exhibits a concave solder fillet. Operators look to the concave solder fillet as an overriding criteria for good soldering. As a result, defective calls are made in confused identity. Solder joints made in nitrogen will be less prone to get rejected because of the better wetting performance it allows. Tombstoning/Drawbridging defects were reduced by 66.6% when reflow soldered in a nitrogen atmosphere. Tombstoning is a condition when a ceramic capacitor or resistor will be standing vertically on one of its ends after reflow. Drawbridging is when the part is in the act of tombstoning but the solder solidifies before it has a chance to reach a vertical axis. This phenomena is most common to cylindrical shaped diodes and resistors (MELFs). The point contact of the round shape is considerably more sensitive to the surface tension and cohesive strength of the molten solder as related to the wetting of the component terminal. Especially on this component type, solderability (wettability) is curtailed by the normal oxidation that occurs to plated surfaces, resulting in less cohesive attraction and the creation of tombstoning. In the IR, the nitrogen helps to increase the cohesive and wetting strength forces of the molten solder as it was demonstrated in the Self Alignment test. Direct Cost Savings Economic Evaluation A direct cost savings is realized by reduced inspection, touch up and repair immediately after IR reflow soldering. Visual inspection using 10x magnification is made after IR reflow. Based on the two month test results, it was shown that a 75% reduction in soldering defects could be achieved when nitrogen was used. Conservatively, nitrogen reduced the average defect level from 1200 ppm to 300 ppm, a reduction of 900 ppm. Using 6,000,000 component placements (PMT) per month, the use of nitrogen reduces the number of defects by 46,980 defects per month. It requires approximately 1.33 minutes to find, identify and repair each defect. Using a loaded labor rate of $21.00 per hour a net savings of $21,869 is realized. Other Cost Savings Other cost savings and,cost gains are less tangible and are harder to measure and/or estimate. However, they probably will surpass the above direct savings at IR. These other savings are listed as follow: a) line down due to component or PWB non wetting b) in-circuit test failures c) functional test failures d) returned material from customers e) customer satisfaction and repeat business Line Down Savings A line has to be shut down when a component or PWB has a non wetting problem and an alternate part is not available. The time lost is the change over time to start the next job in queue. The incidence of this problem is low, however, the cost is very expensive due to loss of business revenue. A few hours of line down savings is estimated to be $6,000 per month. In-Circuit And Functional Test Savings The largest percentage of the defects (60%) found at IR reflow are non-critical defects that do not meet Workmanship Standards (i.e., non wetting, excess, insufficient, ect.). The remaining 40% are critical defects (i.e., open and solder bridging) that will cause electrical failures. Visual inspection finds most but not all of the critical defects. It is estimated that 1 out of 40 of the critical defects are passed on to test. The use of nitrogen then reduces the number of defects by 470 per month. 46,980 defects/mo.) * (.4 critical) * (.025 missed) = 470 per month savings The average cost to diagnose, find and repair the defect at test is $5.00 per defect or a saving of $2, per month. (900 ppm) * (6,000,000 PMT) * (8.7 solder joints/pmt) = 46,980 defects (1,000,000) saved per month Returned Material Savings The quality of a solder joint is subjective. In questionable 9

10 cases where our Quality Control Department says "use as is" with the best of judgement, there is an opportunity for a customer to reject a board for dull looking solder connections, insufficients, ect. Again, this may not happen very often, however, cost avoidance here could add to $8,000 per month. planned for the new IR furnace to be fitted for nitrogen use as well. Insofar as soldering quality and defect reductions are concerned, there are examples that attest to the benefits of using nitrogen: Customer Satisfaction Customer satisfaction is invaluable! How does one measure customer satisfaction as a result of consistent high quality solder joints? It is difficult to put a dollar figure down as a measure of savings or cost avoidance, but it is a real asset. Nitrogen will buy customer satisfaction which translates to increased business which would otherwise be lost. Summary Table 9 below lists the cost gains and cost avoidances when nitrogen is used. The cost of nitrogen to achieve the savings shown was $11, per month in 3 IR furnaces operating 3 shifts. Rework savings after IR of $21,869 pays for the nitrogen. 1. The Production Manager would not use the third IR oven until it was fitted with nitrogen capability. He opted to run the product through the in-line pick and place process and then cart the assemblies to one of the other two IRs where nitrogen was in use. This was done rather than to add more screening inspectors and rework operators to handle abnormally higher amounts of defects. 2. Figure 9 shows an assembly with an unevenly distributed component layout with heat sinking PLCCs contained in a relatively small fixed area. As a result, the rest of the board toward the edges discolored considerably due to uneven heat dissipation. The problem was solved by flowing a greater amount of nitrogen in the furnace. Figure 9 Table 9 It is difficult to quantitatively measure the total cost savings of improved quality. Although the labor savings at IR are Rework after IR $21,869 Line down 6,000 Electrical test 2,350 Returned material 8,500 Customer satisfaction??? Gains & Avoidances/mo. $38,219 easy to determine it is our belief that the true pay back is in the sum of the other costs. The above savings reflect one case at Betel Corporation at a given point in time. PPM defect levels have been reduced since the experiment was performed in the July, August, and September time frame in Therefore, a different set of savings would be expected if the experiment were repeated at another point in time at Betel or repeated at another company. History Of Nitrogen After The Experiment Since the experiment was performed, the volume of business has increased to an extent that a third additional SMT line has been added to handle additional production. A fourth line is planned for the first quarter The current three systems are being operated using nitrogen. It is 3. Figure 10 illustrates a Pareto distribution of the different types of defects related as a percent of the total and the ppm levels for each. This data represents 5 months of current production, approximately one year after the Production Test was performed. The figure also indicates the current average defect level compared against an estimated prediction of defects if nitrogen was not in use for the current products. A direct comparison cannot be made since many of the current products were not being produced before the test was conducted and many of the products then are not being produced now. 10

11 Figure 10 Figure 11 is a breakdown of the defect levels based in solder joint population of the current products in three categories of board complexity (light, moderate and heavy). This graph is also an arbitrary representation designed to illustrate potential results and defect reductions attainable with a nitrogen system compared to air. The category segregation allows the readers to estimate savings based upon their production make-up. As can be seen from the graph, complex boards can be soldered with significantly less defects and low complexity boards can be soldered in some cases with no defects. As above, an estimate of defect levels is made for boards reflowed in air atmosphere was used since the Defect Counting System was not in use prior to the nitrogen experiment. However, long term experience with air atmosphere soldering provides a solid basis for estimates used in each board complexity category. Figure 11 11

12 Conclusion The production test showed that a significant reduction in subjective, non critical type defects can be achieved when nitrogen is used in IR reflow soldering. Almost universally, these defect types are the ones that undergo rework to make them conform to the ideal joint. It is difficult, if not impossible, to have a work force of several inspectors, rework operators and quality control technicians that can have identical, consistent judgments when it concerns soldering inspection quality. As shown in the test, the uncontrolled air atmosphere of an IR reflow process produces a significant amount of these defects. In so doing, the product is exposed to more rework than needed and to the risks of critical damage from carelessly handled soldering irons. Nitrogen in the IR is a tool for controlling the environment inside the machine and consequently controlling the quality output. The economic feasibility in using nitrogen for IR reflow soldering depends on the defect levels being experienced and the dominant types of defects produced. A barometer for this would be, that if the levels exceed 700 parts per million and 60% or higher are for subjective type defects, there may be sufficient grounds to investigate the implementation of nitrogen. This of course, would also depend on the existing production volumes and future projections. The test also showed other relevant benefits when using nitrogen that were not included in the economic justification. Prevention of charring/discoloration of substrates, self alignment improvements of parts and the reduction of voids which implies better solder joint reliability. References 1. M. S. Nowotarsky and M.J. Mead, "The Effects of Nitrogen for IR Reflow Soldering", Proceedings of SMART IV Conference; January, 1988; Los Angeles, CA. 2. M. S. Nowotarsky and B. Down, S. Davoudi, "The Effect of Nitrogen Atmosphere on the Wave Tinning of Component Leads", Proceedings of Nepcon West 1986; Anaheim, CA. 3. Dr. Jennie S. Wang, "Solder Joint Integrity - An SMART IV Conference; January, 1988; Los Angeles, CA. 4. Dr. Jennie S. Wang, "Solder Paste in Electronics Packaging", Van Nostrand Reinhold, 1989, pp

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