Measuring the Benefits of Nitrogen for Reflow U.S. Robotics, Morton Grove, IL

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1 Measuring the Benefits of for Reflow U.S. Robotics, Morton Grove, IL Chad Boeding, Mark Nowotarski Praxair, 777 Old Saw Mill River Road, Tarrytown, NY Julie Bradbury, Denis Jean U.S. Robotics, PCD Division, 6201 W. Oakton St., Morton Grove, IL Abstract The following report is a summation of the guidelines and benefits of nitrogen atmosphere reflow soldering at the Personal Communications Division of U.S. Robotics-- Morton Grove, IL. The study will review the equipment guidelines for nitrogen soldering, the cost, yield comparison, quality improvement and the effects of upcoming technology and changes. Introduction The benefits of nitrogen reflow soldering and the associated cost begins with proper equipment selection. In the reflow process, oxygen interacts with solder and flux and effects the formation of solder joints. This interaction impacts the surface tension of the solder, the wetting time of the solder and the surface damping of the solder 1. In a nitrogen environment, the surface tension and wetting forces are increased while surface damping is decreased. This directly contributes to defect reductions. To gain these benefits in the soldering process, the oxygen part per million (ppm) inside the reflow oven must be controlled. This can be an interesting task since the oven is continually open on both ends. A low oxygen ppm level is maintained by diffusing nitrogen inside the respective zones to displace oxygen. If the nitrogen flow is not accurately set or the reflow oven is not properly sealed, the nitrogen will not be able to displace the oxygen present. If this occurs, the benefits are reduced. Although there is no simple answer as to the necessary oxygen ppm level required inside the oven, with the use of low solids pastes, an oxygen ppm level of less than 20 has been found to provide the greatest benefit. At the oxygen level of 20 ppm or lower, the wetting force can be improved as much as 50% longer versus air. In addition, at the 20 ppm oxygen level, the surface tension is at its greatest and surface tension decay is minimized 2. When soldering in a 20 ppm oxygen atmosphere or less, oxide formation is minimal. As a result, surface tension and wetting is increased and surface damping is decreased. This allows for an improved joint formation which in turn, lowers the overall defects. The capability of the reflow oven to maintain an atmosphere of less than 20 ppm and of the operator to know the ppm level, dictates the ability to obtain the desired benefits To achieve the desired results for nitrogen soldering, it is important that the reflow oven is capable of maintaining the desired ppm atmosphere at a nitrogen flow rate of less

2 than 2,000 cubic feet per hour. A flow rate of 1,500 cubic feet per hour or less, while still maintaining the required oxygen ppm levels, is achievable by most high end reflow oven manufacturers. Because every reflow oven manufacturer produces ovens differently, it is important to weigh the ability of individual ovens to achieve the desired results. The importance of oven performance with regard to atmosphere stability, atmosphere purity and nitrogen consumption, cannot be understated. In fact, oven performance characteristics may be the single most important factor required to achieve the desired results and benefits that nitrogen reflow soldering can provide. Procedure All boards combined through-hole and surface mount assembly. The surface mount assembly included a mixture of 20 and 50 mil pitch components. The PCBs were rigid FR4 with a variety of 50/50, 60/40 and 63/37 HAL finish and a variety of solder mask. The reflow profiles were standardized across all lines and products using a fixed conveyor speed, set preheat temperature and time, and specified peak temperature and dwell above liquidous. Reflow profiles were verified using thermocoupled boards. The solder paste used was Sn62Pb36Ag02 with a no-clean flux. The wave soldering was carried out using a VOC free no-clean flux and there was no post solder cleaning process. To test the results of nitrogen soldering at U.S. Robotics, a total of three separate tests were performed. The goal of the tests were to compare soldering in air versus soldering in nitrogen. The tests were set up to analyze the effects of nitrogen versus air on: 1. Varying products, on varying production lines at varying ppm levels 2. The same product on varying production lines at varying ppm levels 3. The same product on varying production lines at ppm levels <20. Data from various production lines was collected, to normalized for equipment differences line to line Results: Test 1 In this study, both first pass and final test yields were analyzed to determine the benefit of a nitrogen atmosphere in PCD's reflow process. The first test analyzed the total yields of lines running in air versus nitrogen atmosphere. 2

3 ICT- 1st Pass Yields-for a Variety of Products 02 ppm Line Number Product Subgroup Sample Size Number Sample size ppm Line Number Final Functional Test Product Number Sample Size Subgroup Sample size

4 Note: = SP V34 FX Rush = SPv34Fax Matt = SP v34 Matt SGS = SP v34fx PC Win = SP 288 FX PC Win = SP v34 MAC Matt SGS Analysis: The lines running in nitrogen showed a clear ICT- yield improvement versus those running in air. The average yield improvement was 5.34% in nitrogen versus air. This equates to a 51% reduction in defects. In addition to the 51% decrease in ICTdefects, there were defect reductions in final functional test by 2.87%. This equates to an additional 38% reduction in defect on final functional test. Test 2 The second test focused only on the ICT-1st pass yields of one product. The product that was most readily available was the Sportster v34 Fax matt. A sample of larger than 10,000 was needed to minimize the possibility of random error. Those boards run in air were compared with boards run in nitrogen. Boards run in nitrogen demonstrated a yield improvement of 6.8%--a 59% reduction in defects. Test 3 In the third test, the ICT-1st pass yields of the Sportster 28.8 Fax Modem in air versus nitrogen were compared. Unlike tests 1 and 2 the oxygen levels in the ovens that utilized nitrogen were all set to maintain a level of 20 ppm or lower in the reflow zone. As stated earlier in this report, oxygen levels of 20 ppm or lower are required to maximize the molten solders' surface tension, and create a possible debridging effect. As with the previous tests, all the lines used utilized similar equipment and similar conditions. The following table shows the results of Test 3: ICT- 1st Pass Yields 02 ppm Product Subgroup Sample Size Number Sample size SP288FX SP288FX <20 SP288FX 8357 <20 SP288FX Analysis : Once again, the product that was soldered in a nitrogen reflow atmosphere demonstrated a substantial increase in yields. In this test, depending on the line, the nitrogen lines exhibited a 23-50% decrease in defects versus air. The results of the nitrogen on line 12 were artificially low. The yields for this line were skewed due to an abnormal rate of wave solder bridging which has since been resolved. 4

5 Quality/Joint Integrity: Increased surface tension, surface wetting; and decreased surface damping, effect the joints. The quality of the solder fillets was examined by microscope and X-ray, to see how the number of defects was reduced. Figure 1 showed the same component at varied oxygen levels. The board run in air demonstrated a large contact angle, dewetting, an abundance of flux residue and micro-solderballs. The board run at 150 ppm O2 showed an improved wetting angle, little to no dewetting, less visible flux residue and less microsolderballs. Figure 1 The wetting angle in air versus nitrogen with 15 ppm O2 is shown in Figure 2. Figure 2 5

6 A comparison of results in air versus nitrogen ( Figure 3) shows a lead reflowed in air with the upper section of the toe exposed due to lack of wetting. Micro-solder balls are scattered around the land. The lead reflowed in nitrogen shows good wetting and total encapsulation of the toe. Figure 3 Figure 4 shows X-ray photographs of J leads and resistors. X-ray was used to look for a voiding in air versus nitrogen. Those joints reflowed in air were observed to have more voids than those reflowed in nitrogen. In addition, the voids that were present in nitrogen, tended to be much smaller in size than those reflowed in air. The voids are seen as the lighter circles in the solder fillet at the ends of the pads. Figure 4.1 J-leads 6

7 Figure 4.2 Resistors Cost Analysis: Based on the results that U.S. Robotics has experienced, a cost benefit can be calculated. With the installation of the nitrogen generating facility, the costs breakdown is as follows: Conceptronics nitrogen consumption: 1500 cubic feet/ hour/line cost per hour: $1.65 /line/ hour Average repair cost: $.75 / defect Average defects reduced / hour/line (ICT-1st pass): 5.34 per 100 boards run / hour Average defects reduced / hour/line (Final functional test): 2.87 per 100 boards run / hour Savings per hour using nitrogen (ICT-1st pass): $4.01 / hour / line Savings per hour using nitrogen (Final functional test): $2.15 / hour / line Net Savings / hour / line (1st & final): $6.16-$1.65=$4.51 / hour / line Net Yearly Savings / line for running 14 lines: $38, Net Yearly Savings for 14 lines : $537,952.8 (a) 3 minutes of handling time / defective board. 24 hours / day 355 days per year (a) It should be noted that the ICT and Final test data used are the sum of all failures upstream of these test points i.e. bad printing leading to opens and bridges (although the effect of these can be minimized by use of nitrogen at reflow), bad placement of components both surface mount and through hole, component coplanarity, use of wrong surface mount or through hole components, bad wave soldering and failure of the test fixture itself. Many process improvements were implemented during the time that this data was collected. The production floor was also in the process of being ramped up from 6 lines to 14. The reader is encouraged to view the % reduction in defects needs with these factors in mind. 7

8 Summary All three production data sets, demonstrated an increase in yield for nitrogen atmosphere reflow soldering. % Decrease in Defects for Reflow in Mixed ppm & mixed product Mixed ppm & one product Fixed ppm<20 & one product % Reduction in Defects at ICT % Reduction in Defects at Final Functional Testing Figure 5 At a production rate of 100 boards per hour, U.S. Robotics, reflow soldering in nitrogen, can achieve a net savings of $4.51 per hour per line. The benefits of maintaining <20 ppm oxygen was not evident in the production data collected in Figure 5. The production data collected for this report was on boards that did not have any of the more advanced technology. It is anticipated that as the soldering becomes increasingly difficult, the impact of nitrogen soldering at <20ppm will become easier to recognize. For technologies that allow improved functionality in a smaller area i.e. fine pitch assembly 20 mil and below on OSP, nickel gold and tin polymer and ball grid arrays (BGA's), micro ball grid arrays (MBGA) and flip-chip, results of a preliminary DOE carried out by U.S.R corporate process suggest the cost savings will be even greater than seen in this report. References: 1. Nowotarski, M.S., DeWilde, R., The Effect of Oxygen on the Surface of Solder, Soldering and Surface Mount Technology, Wela Publications, Ltd., February Mead, M.J., Nowotarski, M.S., The Effects of on I R Reflow Soldering Defects, Proceedings of Nepconwest