A Flextronics-Vitronics Soltec collaborative investigation into techniques for reducing nitrogen consumption.

Soldering atmosphere influences final solder interconnect formation. Based on previous studies, SnPb and Pb-free solders perform differently under varying soldering atmospheres.^1,2,3,4,5,6 Pb-free pastes’ significant reductive influence on wetting and spreading may mandate an inerted atmosphere.^7,8 The question becomes, To what extent should nitrogen be used, if at all? Nitrogen adds cost to the assembly process, the amount dependent on geography.⁹ As a result, industry has shown wide interest in decreasing consumption.

This project identifies atmosphere’s influence on Pb-free solder joint formation by controlling the O2 ppm levels in the oven during assembly of specifically designed PCBs. The O2 ppm levels are quantified by measurements taken at defined locations in the reflow oven tunnel. Characterizing the atmosphere throughout the oven is critical to understanding the solder joint formation-soldering atmosphere relationship. Post-reflow, boards were subjected to secondary wave soldering where through-hole components were soldered. Through-hole penetration measurements were performed to compare the different process parameters.

Materials in this experiment exert significant influences on the outcome. The project studies Pb-free PTH components assembled on 0.093" and 0.125" copper OSP boards assembled with two fluxes. Wave profiles were optimized per flux type to minimize any adverse influences from flux performance. Characterizations included visual inspection and 5DX.

This project aims to identify relationships between various inerted soldering atmospheres and through-hole penetration. Surface mount defect data and through-hole penetration analyses were used to quantify effects of:

• Oxygen concentration during reflow.
• Nitrogen supply methods during reflow.
• Flux on wave soldering.
• Ambient on wave soldering.
• Board thickness.
• Through-hole design and component orientation.

Experiment

Materials. A 16-layer, FR-4 board with two thicknesses (0.093" and 0.125") was used in the experiment (Figure 1). The board material was TU 752 with a Tg of 170°C and Td 350°C. The solder mask was Probimer 65. The board dimensions were 5.5" by 7". The finish was Entek Plus HT copper OSP with a specified thickness of 20 µm. The surface finish was selected for its popularity and sensitivity to temperature and air environment.


OSP provides the copper surface a temporary layer of protection from oxidation. During soldering the organic coating is penetrated and dissolved by flux and temperature. Multiple exposure to thermal cycles results in coating cross-linking, making it less penetrable by weak organic acids used in actual flux formulations. Based on the OSP supplier’s specifications, this surface finish has the ability to withstand more than three Pb-free thermal cycles without significant wetting degradation.¹⁰

Most metals show a strong tendency to form compounds or oxides with O2. For Pb-free systems, which use alloys high in tin and copper surfaces, SnO (mainly), SnO2 and Cu2O oxides are formed. They form immediately under room temperature and grow thicker under reflow temperatures above 200°C. The surface tension of oxides is much lower than the values of their corresponding metals. This issue affects alloy and surface finish wetting behavior.¹¹

Flextronics performed the board design. It contains footprints for surface-mount and PTH components. Table 1 shows the design dimensions for the PTH components.


Components. Selected surface-mount and PTH components were used in this experiment. Because these components were intended for wave soldering, a glue dispensing process followed by curing was developed for the surface-mount components. The adhesive material was Loctite 3627. PTH components were manually placed immediately prior to wave soldering.

Resistors. Forty 0805 resistors were placed perpendicular to the board and 32 1206 resistors were placed on both orientations. Both sets of resistors had 100% tin finish over nickel.

SOCs. Twenty SOT-23s were placed parallel and perpendicular to the direction of the board. They had three pins and 100% tin finish. Thirteen SO-16s were also used (both orientations). They had 16 pins with gullwing leads. They were Pb-free with 100% matte tin finish.

PTH. All PTH components were rated for Pb-free processing and were Pb-free finished. The three 64-pin connectors were high-temperature terminal strips with gold finish and a pin diameter of 0.025". The three plastic dual inline package (PDIP-16) had 16 pins and 100% tin matte finish. They had a pin pitch of 0.100" and pin diameter of 0.015". The 25 axial resistors had two leads with 100% tin finish with a pin diameter of 0.022”. For all PTH components except DIPs, leads were mechanically trimmed to achieve lead protrusions of more than 0.140". DIPs had a visible lead protrusion on the assembled 0.093"-thick boards only. IPC-A-610D defines a minimum lead protrusion where the end is discernible in solder for Classes 1, 2 and 3, with exception of pre-established lead length components (DIPs) where the lead protrusion may not be discernible.¹²

Flux. Flux 1 was a VOC-free no-clean with 4.5% solids content. Flux 2 was a resin alcohol base no-clean with 7% solids content.

Alloy. The alloy was SnAg3Cu0.5 (SAC 305).

Design of experiments. A full factorial experiment was designed (Table 2). A total of two repetitions were performed for each condition. For reflow, five oxygen atmospheres and two methods of supply nitrogen were used. For wave, two fluxes and two wave ambient were used on two board thicknesses.


Assembly process and equipment. A total of 144 boards were assembled under various atmospheric conditions, using two fluxes and two board thicknesses.

Printing process. A DEK Galaxy printer was used to screen-print the adhesive. The printing parameters were:

• Print speed: 20 mm/s.
• Print deposits: 2.
• Squeege: metal with 60° angle.
• Stencil thickness: 0.006".
• Stencil type: laser stencil and electropolished.

Proper volumes of adhesive were printed for all components except the SO-16, which had a higher standoff than the actual deposit. A thicker stencil (more than 0.010") should be used for this application. Additional adhesive material was dispensed using a semiautomatic dispenser to the SO-16 pads.

Pick-and-place. A Universal Instruments GSM was used to place surface mount components on the bottom side of the board.

Reflow. A 13-zone (nine heating, four cooling) forced convection oven (model XPM3 940) was used to reflow the assemblies. Five atmospheres were evaluated. The atmospheres are denoted by the oxygen content in parts per million (ppm): 25, 500, 1200, 2500, and 210,000. The four lowest O2 levels were used for both nitrogen supply methods. These methods will be referred to as “full tunnel” and “reflow only.” Note: The air atmosphere (210,000 ppm O2, or 21%) can be applied only as “full tunnel.”

The full tunnel method supplies nitrogen to every zone of the oven, resulting in almost-consistent oxygen concentration throughout the tunnel inclusive of the cooling zones (Figure 2). The reflow-only method selectively supplies nitrogen to specific zones. In this case, the oven was equipped to supply nitrogen only in heating zone 9, which influenced the behavior of zone 8 and cooling zone 1 where the alloy reflow takes place (Figure 3). Ppm measurements were taken using three oxygen analyzers located in heating zones 1, 3, 5, 7, 8 and 9, and cooling zones 10, 11 and 13.



Reflow profiling was performed using scrapped test vehicles with attached type K thermocouples. The thermocouples were placed on four locations of a bare board: topside near moveable rail trailing edge, topside center of the board, topside near fixed rail leading edge, and bottom-side center of the board.

The reflow profile was a direct ramp-to-peak style profile, which produced a maximum solder joint temperature of approximately 250oC and a total time of above liquidus (TAL) between 65 and 75 sec. Two reflow settings were developed for the two board thicknesses. Reflow profiles were used instead of curing profiles to simulate double-sided assemblies, while simultaneously curing the adhesive. Table 3 contains a summary of the reflow parameters.


Wave soldering. A Delta Wave 6622 with a dual head spray fluxer and three heating zones (calrod system, top and bottom heatings with forced convection on the bottom and IR lamps on the top; and double waves – chip and main with smart wave) were used. Wave profiles were developed and optimized for the two board thicknesses and two fluxes based on manufacturer recommendations and observations from initial runs.

Flux amount was optimized by weighing a standard test board. Flux specification or golden boards defined the appropriate amount of flux. Fax paper or pH paper was placed on the topside of the boards, which were then run over the fluxer to ensure good through-hole wetting.

For the VOC-free flux, the maximum specified amount of flux was 1500 µg/in. This amount resulted in poor overall soldering on the 0.125"-thick setup boards. The addition of flux showed improvement in through-hole penetration. However, the excessive amount of flux resulted in top- and bottom-side board contamination. Therefore, results for this flux and board thickness were discarded.

For the alcohol flux, the amount was based on previous experiments performed in collaboration between Vitronics and Flextronics because the MSD for the flux does not specify the optimal amount.

For all assemblies, SAC 305 was used at 265°C solderpot temperature. During this process, the topside temperature was below 217°C in all cases. Pallets were used to wave solder all boards. (Settings and observed values for the wave process are available upon request from the authors.)

Results and Discussion

Visual inspection. Bottom-side visual inspection of the wave-soldered surface-mount components and through-hole components was performed on all boards. The most prominent defect was solder bridging or shorts. The formation of solder bridging is affected mainly (Figure 4) by board thickness, flux type and wave-soldering atmosphere. At 95% confidence interval one way, ANOVA favors the use of the alcohol-based, no-clean flux, nitrogen over the wave, and 0.093”-thick boards. The reflow process did not influence bridging.


During visual inspection, a 34% reduction on bridging was observed when using nitrogen on the wave soldering process, 27% reduction when using 0.093”-thick boards, and 26% when using low-rosin, no-clean flux. Reporting the number of bridging entails counting the number of pins involved in each bridge. Figure 5 illustrates a bridge that involves three pins.


5DX inspection. Data acquired from x-ray inspection were used only to inspect bridging of surface-mount components. All bridging defects were found on the SO-16 components (Figure 6). The most affected location was U3, for which the orientation was perpendicular to the direction of the board and located on a second row (Figure 7).



Placing the components parallel to the direction of the board showed a major improvement. The bridging DPMO (defect per million opportunities) averages for parallel versus perpendicular locations were 6,727 and 70,409 for the 0.093"-thick boards, respectively. For the 0.125"-thick boards, the DPMO averages were 111,545 and 65,008 for parallel and perpendicular, respectively. This opposite trend was observed mainly when using alcohol-based flux. It seems the high preheat temperatures (topside temperature 128°C) burn off the flux on the bottom-side before wave soldering.

5DX for PTH parts. 5DX was used to inspect through-hole penetration. Solder penetration analysis in each via was accomplished by first taking x-ray measurements at 0, 10, 20, 30, 40, 50, 60, 70, 80, 90 and 100% intervals of through-hole. This technique provides quantitative data by characterizing grayscale intensity. A threshold value was determined based on experimentation and calibration performed prior to this experiment. Based on this threshold value, grayscale values below the threshold value are recorded as no solder, whereas grayscale values above the threshold value are recorded as the presence of solder. Through-hole penetration was reported as the maximum height or interval where solder was observed.

Wave parameter effects. The purpose of this investigation is to identify the impact of reflow soldering atmosphere on joint formation. Consequently, the wave soldering process was optimized to the extent that it does not exert a significant influence on the joint formation. Attempts to make the wave soldering process neutral were moderately successful. The data indicate board thickness significantly affects through-hole penetration. Therefore, the data were separated and analyzed based on board thickness and flux types. Flux exerted a definitive effect, but this was minimized by applying increased amounts of flux as required to optimize results on baseline runs (as described in the Experiment section).

Board thickness effects. The 0.125"-thick boards show instances of poor hole penetration when air profiles were used during reflow (Figure 8). IPC-A-610D standard indicates a minimum of 75% fill for Classes 1, 2 and 3 is acceptable.¹² Other significant factors were nitrogen supply methods and oxygen ppm levels (Figure 9). For boards reflowed with nitrogen, the data show applying nitrogen in the reflow areas results in only slightly better through-hole penetration. For boards reflowed in ambient atmosphere, the data provide guidance in respect to nitrogen use during wave soldering. However, on average, a through-hole penetration of 62% was achieved in this case, versus 59% when using air in wave.


For 0.093"-thick boards, acceptable solder joint penetration for all joints was observed (Figure 10). The reflow atmosphere did influence through-hole penetration, but not to the extent to make the joints unacceptable. Major factors found to affect through-hole penetration were flux, reflow atmosphere and nitrogen supply method. Boards reflowed in air showed a decrease in solder penetration. At a 95% confidence interval, the data favor use of the alcohol-based, no-clean flux and reflow-only supply method.


It can be concluded that for 0.125"-thick boards, acceptable through-hole penetration can be achieved when nitrogen environment is used in the reflow process as high as 2500 ppm O2, and nitrogen can be applied only on the reflow areas. For the 0.093"-thick boards, nitrogen’s benefits in reflow were not observed unless 100% through-hole penetration is required.

Board designs and component types effects. Board layout included through-hole designs attached to different layers (one to eight layers), pad diameters (0.060" and 0.070"), and hole diameters (from 0.035" to 0.047"). In addition, three types of through-hole components were hand-placed and assembled as described. Each component had varying pin counts and pin diameter.

Pin connector. The pin connectors were placed on various locations on the board in two orientations (parallel and perpendicular to the board direction), with four drill sizes (0.035", 0.039", 0.043" and 0.047"), two pad sizes (0.060" and 0.070"), and connected to 2, 4, 6, or 8 layers. The 0.125"-thick boards reflowed in nitrogen showed that drill sizes and component orientation have a significant effect on through-hole penetration for the pin connector (Figure 11). Nitrogen supply method or oxygen ppm levels were not significantly different. Note all joints showed acceptable through-hole penetration per IPC guidelines.¹³


The 0.125"-thick boards reflowed in air were characterized by poor through-hole penetration, as observed in (Figure 12). To obtain an acceptable through-hole penetration (>75%), a drill hole of 0.035" on a 0.070" pad attached to maximum two layers is required (based on 12 data points). The component needs to be located parallel to the direction of the board and can be waved soldered under ambient conditions.


For the VOC-free based, no-clean flux and 0.093”-thick boards, the factors that affect through-hole penetration were layers, pad sizes, drill sizes, oxygen ppm levels, and orientation, while for the 0.093"-thick boards assembled with the alcohol-based low rosin, no-clean flux, the factors were drill sizes, oxygen ppm levels, nitrogen supply methods, and orientation. Note all through-hole joints are acceptable. Table 4 shows a summary of the best settings for each flux.


In summary, a nitrogen environment with as high as 2500 ppm of oxygen supplied only in the reflow zone provides acceptable through-hole penetration on 0.125"-thick boards. A minimum and maximum pin-to-hole ratio of 0.64 (0.025"/0.039") and 0.71 (0.025"/0.035"), respectively, are recommended. This type of component should be located parallel to the direction of the board for better results. For 0.093"-thick boards, nitrogen in reflow benefits were not observed unless 100% through-hole penetration is required.

Axial resistors. The resistors were located at various positions on the test vehicle and oriented in two directions, with two drill sizes (0.035" and 0.039"), one pad size (0.060") and connected to one, two and four layers. Good solder joint penetration was observed on through-hole penetration for 0.125"-thick boards when reflowed in nitrogen.

When boards were reflowed in air, data analysis suggested use of a 0.039" drill size, connected to four layers, and ambient air in the wave in order to obtain good through-hole penetration (Figure 13). For 0.093"-thick boards, acceptable through-hole penetration was observed for all joints. The best settings for each flux can be found in Table 5.


For this type of component and for both board thicknesses, it is recommended the components be oriented perpendicular to the board direction; and that nitrogen in reflow using the reflow-only supply method and up to 2500 oxygen ppm level be used.

DIPs. The DIPs were connected to up to six layers, while the drill and pad sizes were kept constant at 0.035" and 0.060", respectively. It has to be noted that no lead protrusion was observed when components were placed on 0.125”-thick boards. Data analysis indicates unacceptable through-hole penetration on the 0.125”-thick boards assembled with the alcohol-based low rosin flux and reflowed in air.

To improve results when assembling under ambient reflow condition, analysis of the data indicated the use of nitrogen in wave soldering. This is applicable only for the pins attached to six layers (based on four data points) (Figure 14).


Assemblies soldered using VOC-free flux on the 0.093"-thick board resulted in unacceptable through-hole penetration when connected to six layers (Figure 15) at all atmosphere levels. In contrast, those assemblies soldered using the alcohol-based low-rosin flux resulted in acceptable through-hole penetration.


DIPs showed a major challenge in comparison with other PTH components. On the 0.125"-thick boards, the lack of lead protrusion produced unacceptable through-hole penetration. During wave soldering, the component leads assist with the capillary action of solder wicking up the via.¹¹

As mentioned for the other components, nitrogen use during reflow shows major benefits on 0.125"-thick board assembly. Nitrogen can be supplied to the reflow areas only and up to 2500 ppm of oxygen. For the 0.093"-thick board, the flux type was the major influence, and the analysis favors the low-rosin alcohol flux.

Conclusions

The purpose of this collaborative investigation was to provide evidence on the impact of atmosphere on solder joint formation. Materials selection provided data specific to board complexity, flux type on a CuOSP board finish. Investigation of the influence of various process parameters and materials resulted in the following conclusions:

• The use of nitrogen in the reflow oven process is necessary when thick boards (0.125"-thick boards) are assembled to achieve more than 75% through-hole penetration.
  
  
• Nitrogen consumption can be reduced by applying it only on the reflow areas and using higher oxygen ppm levels (up to 2,500 ppm).
  
  
• Flux chemistry is a very important factor that can change results dramatically. In general, alcohol-based flux selected in this study was more robust than VOC-free flux. This should be considered when optimizing for through-hole penetration.
  
  
• The use of nitrogen in the wave soldering process is beneficial for reduction of bridging and when nitrogen has been used in the reflow process.
  
  
• The orientation of the component with respect to the direction of the board is important and depends on component type.
  
  
• When components have been reflowed in nitrogen, it is possible to obtain acceptable hole penetration using a variety of through-hole designs, while limited designs when reflow in air.

Acknowledgments
The authors would like to thank George Oxx, Aaron Unterborn and Doug Watson for their support on the planning stage of this project; Eric Cruz for performing 5DX analysis; Roy Palhof, Norm Faucher and Jon Silin for their support on board assembly; and Linlin Yang for data collection support. The authors would also like to thank the management of Flextronics and Vitronics Soltec.

Ed.: This article was first published in the SMTA Pan Pacific Symposium Proceedings and is reprinted here with permission.

References

1. C. Boeding, et al., “Measuring the Benefits of Nitrogen for Reflow,” Nepcon West, 1997.
   
   
2. F. Klein, “Advantages of Protective Atmosphere Control for No-Clean Solder Paste,” Soldering and Surface Mount Technology, vol. 8, no. 2, 1996.
   
   
3. Y. Wu, Y. Chan and J. Lai, “Reliability Studies of Plastic Ball Grid Array Assemblies Reflowed in Nitrogen Ambient,” Electronic Components and Technology Conference, 1998.
   
   
4. V. Mohan and K. Srihari, “Wetting Ability of Lead Free Solders in Nitrogen and Air Atmospheres,” Technical Report, Area Array Consortium, January 2002.
   
   
5. C. Hunt, D. Lea and S. Adams, “Evaluation of the Comparative Solderability of Lead-Free Solders in Nitrogen – Part II,” SMTA International, September 2002.
   
   
6. S. Aravamudhan et al., “Effect of Oxygen Concentration During Reflow on Tombstoning for Passive Resistors for Lead Free Assemblies,” SMTA International, September 2006.
   
   
7. A. Teredesai, Self-Centering of Offset Chip in a Lead Free Assembly, Binghamton University, 2003.
   
   
8. T. Gentry, “Organic Coatings: OSPs and the Assembly Process,” Circuits Assembly, August 1996.
   
   
9. C. Harper, Electronic Packaging and Interconnect Handbook, McGraw-Hill, 2000.
   
   
10. F. Van Der Pas et al., “Selecting the Right Final Finish for RoHS Compliant PCBs,” Printed Circuit Design and Manufacture, March 2007.
    
    
11. K. Wassink, Soldering in Electronics, second edition, 1989.
    
    
12. IPC-A-610D, Acceptability of Electronic Assemblies, February 2005.

Ursula Marquez de Tino is a process and research engineer and Dr. Denis Barbini is manager of advanced technology at Vitronics Soltec (vitronics-soltec.com); umarquez@vsww.com. Wesley Enroth is with Flextronics.