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Fluxless Soldering in Wave Soldering Equipment Using Forming Gas G. L. Arslanian Air Products and Chemicals, Inc. U.S.A.
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Abstract A fluxless method has been developed using forming gas to reduce metal oxides and enhance wetting of solder on printed circuit board metal interconnects. Dilute mix-tures of hydrogen in an inert nitrogen carrier gas were used to solder components to printed circuit boards with Sn/Pb 63-37 tin/lead coating and copper metallizations. The form-ing gas, a reactive gas, reduced the surface metal oxides sufficiently to allow for wetting and to form metallurgical solder joints.
Introduction Solder joints are routinely used to form the interconnec-tions between the components and printed circuit boards (PCB). These joints are made between solderable metal-lized surfaces such as Cu, Ni, or tin/lead. The metallized layers are often exposed to an oxidizing environment (room temperature in air or post reflow operation) for an extended period of time prior to wave soldering. An oxi-dized metal surface inhibits solderability. Once the oxides are removed, the solder flows over to the metallization and forms a metallurgical solder joint. A clean non-oxidized surface has a higher surface energy than an oxidized one and hence a liquid will more readily wet to a clean sur-face.
Forming gas (3% hydrogen, balance nitrogen) fluxless soldering is a process in which liquid flux is replaced by a fluxless process in which hydrogen is added to an inert carrier gas (nitrogen). By definition, the forming gas is a flux or reactive species in that its function is to reduce sur-face metal oxides in order to enhance solderability. The reactive nature of hydrogen is well documented in the lit-erature. Reduction of solder oxides by hydrogen in form-ing gas has been observed in other microelectronics appli-cations.[1, 2] Experimental results indicate that an initiation temperature exists for hydrogen to reduce solder oxides, below which the reduction is insignificant and above which the reduction process is accelerated. This initiation tem-perature is found to be dependent on the area of solder to be reduced or the volume of oxide present. The initiation temperature is about 319°C for Pb oxide and 400°C for Sn oxide. The isothermal reduction rate is found to be a func-tion of forming gas concentration. The reduction rate is directly proportional to the hydrogen concentration in the forming gas, consistent with a theoretical model.[3, 4, 5] This model predicts that the average reduction rate at 370°C in pure hydrogen is about 3.3 nm per minute, in terms of de-crease in oxide thickness. Another plus in using forming gas as a fluxing agent is it leaves no residue therefore no post solder cleaning is required.
The experimental techniques and solderability results dis-cussed here show that forming gas soldering as a replace-ment for liquid flux has promise as a production process.
Experimental Procedure The fluxless soldering process trials used the following standard production equipment and modifications:
An Electrovert wave soldering machine, model
Econopack I SMT was used to evaluate the forming gas soldering process. The wave soldering machine is equipped with a long inerting tunnel, two bottom IR preheaters and an in house fabrication of a contour with a wave type "A" (see Figure 1 and 2).
To perform forming gas fluxless soldering, three require-ments must be met:
1. The forming gas must reduce the critical surface oxides in a vaporized form at soldering tem-perature (260 degree Celsius).
2. The forming gas must react quickly, on the or-der of a few seconds to provide sufficient manufacturing throughput.
3. The forming gas must be reactive enough to re-duce metal surface oxides but not cause a detri-mental effect on the soldered assembly.
The premixture (high pressure cylinders) of hydrogen and nitrogen is gaseous at room temperature and can be eas-ily introduced into the wave soldering equipment for the solderability tests.
The concentration of the hydrogen in the inert gas carrier (nitrogen) is on the order of 3 percent. As mentioned above the reduction rate of oxides is more efficient in the presence of pure hydrogen. Due to safety concerns and since the wave solder system was not equipped to proc-ess 100% hydrogen, the mixture of 3% hydrogen was chosen for this test program since this concentration is below the flammability limits of hydrogen in air.
Solderability tests with 63SN - 37Pb solder were per-formed with the wave soldering equipment to explore the effect of wettability and solder joint formation for a variety of forming gas process variations. The solder pot tem-perature was at 260 degrees Celsius. The metal intercon-nects tested were copper and tin/lead. The test parame-ters and results are shown in Table 1.
Results At a soldering temperature of 260 degree Celsius and the forming gas injected through the contour diffusers, the forming gas reacts with the metal oxides but not enough to form an acceptable solder fillet on the top side of the board. Research has shown that SnO2 is reduced to SnO in an hydrogen atmosphere at temperatures starting at 200°C.[3, 4] SnO has greater wettability characteristics than SnO2. The forming gas injected in the second pre-heater at higher temperature (537 degree Celsius), reacts rapidly with the metal oxides and, consequently, results in suitable soldering quality (top and bottom solder fillets). The temperature in the preheat zone is above the activa-tion temperature for hydrogen to reduce the metal oxides. The temperature of the top of the board was about 115 degree Celsius before contact with the solder wave. The following is a detailed discussion of the test results.
To determine baseline characteristics of the printed circuit boards with the Value solder mask, the testing of Group A was performed using forming gas introduced into the
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hood enclosing the solder pot (pre and post wave) and injecting nitrogen in the contour shroud (Figures 1 and 2). An acceptable bottom side fillet was observed with minimal second side (top) wetting (Figure 3). Forming gas was in-troduced into Preheat Zone No. 1 via a 0.250" OD stainless steel tube (Group B). The temperature of Pre-heat Zone No. 1 was set at 1,050°F (565°C). There was no wetting to the second side (Figure 4). For Group C the nitrogen was shut off to the contour shroud. The sol-dering results were similar to the baseline run with im-proved wetting to the second side (Figure 5). The wetting observed was probably due to the temperature of the preheat atmosphere which was above the initiation point to reduce SnO2 to Sn.
A second family of boards with the CIBA 52 solder mask were processed. These boards were processed in nitro-gen only to determine baseline characteristics and demon-strated fair bottom side wetting but no second side wet-ting. A series of baseline runs were completed that had forming gas introduced into the hood area as in Group C with no nitrogen flow to the contour shroud. Wetting was acceptable on the bottom side; however, no second side wetting was observed. Injection of forming gas only into Preheat Zone No. 1 with nitrogen flowing to the other ar-eas gave reduced wetting to the bottom side with no sec-ond side wetting. To promote wetting on the second side of the board forming gas was introduced in the diffuser system to the contour. This change in the injection pattern of the forming gas gave a minimal improvement to the bot-tom side wetting but did not promote second side wetting. The improved wetting on the bottom side may be attribut-able to the possible reduction of SnO2 to SnO. The tem-perature of the atmosphere with the solder pot area is greater than 200°C and with forming gas diffusing in this area, some reduction probably occurred.
In an effort to maximize reduction of SnO2, the injection points for the forming gas were changed from the above trials. For Group D the forming gas was introduced into Preheat Zone No. 2 and the diffuser system of the contour shroud. The printed circuit board demonstrated good bot-tom side fillet with 100% side hole wetting and good sec-ond side wetting (Figure 6). With the introduction of form-ing gas into the Preheat Zone No. 2 at a temperature of 563°C the reactivity time was increased due to the ex-tended exposure in an hydrogen atmosphere. The forming gas in the wave area could have possibly contributed in two ways. Initially the forming gas diffused into the Pre-heat Zone No. 1 to maintain the reduced state of the tin and secondarily in the wave area to reduce SnO2 to SnO or to maintain the oxide as SnO.
One final experiment was run that involved bare copper boards with organic solder preservative (OSP) coating. Forming gas was introduced in the hood area (pre and post wave) and through the contour shroud diffusers. Ni-trogen was injected into Preheat Zone 1. Mixed results were observed. Group E demonstrated good bottom fillets with 100% side hole wetting and fair second side wetting to the pads (Figure 7).
The forming gas was not properly diffused in the preheater zones and variations were observed from board to board
with the same set-up. The forming gas can be a suitable option if it is properly diffused in the preheater zones and in the contour zone for fluxless soldering in wave soldering equipment. Modifications of the diffuser locations within the preheater area and possible additional diffuser assem-blies will allow for a more accurate introduction of the forming gas into the equipment. A separate flow panel can be used to allow individual control of the forming gas flow rates in the different areas and for ease in switch-over be-tween reactive gas (forming gas) and nitrogen inerting blanket for the wave area. The forming gas also acts as a blanket around the wave area that displaces oxygen and prevents oxidation during soldering.
Conclusion Forming gas can be used in a fluxless soldering process to reduce metal oxides of copper and tin/lead and to facili-tate the formation of high quality solder joints if it is prop-erly diffused in the preheater zones and the contour zone of a wave soldering machine. The use of elevated prehea-ter temperatures (535 degree Celsius) results in rapid re-duction of the metal oxides.
The forming gas leaves no residue after soldering. Equip-ment modifications, including additional diffusers to permit a more consistent flow of forming gas into the preheat zones and contour zone and separate flow controls for the forming gas, are planned and develop an optimized at-mosphere system for production use. Although initial trials showed variation and non-optimized results, the positive results in oxide reduction capability demand further test-ing. Additional trials will be run to expand on the initial re-sults obtained in this study.
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Acknowledgments The authors gratefully acknowledge Bruce Adams, Ralph Richardson and Christine C. Dong of Air Products and Chemicals, Inc. for their support and technical discus-sions, and Jacques Bechard and Mark Legros of Air Prod-ucts, Canada for providing technical assistance during the experimental runs.
References 1. R.D. Deshmukh, M.F. Brady, R.A. Roll, and L.A.
King, "Active Atmosphere Solder Self-Alignment and Bonding of Optical Components," The International Journal of Microcircuits and Electronic Packaging, 16[2], pp. 97-107 (1993).
2. Rao Bonda and Kenneth Kaskoun, "Flip Chip As-sembly of 34K Thunderbolt Die on Glass Substrate for the THUNDER Build," Proceedings of the 1995 Summer Motorola AMT Symposium, pp. 269-276, July 1995.
3. W.A. Oates and D.D. Todd, "Kinetics of the Reduc-tion of Oxides," The Journal of Australian Institute of Metals, 7[2], pp. 109-114 (1962).
4. G.B. Hoflund, "Characterization Study of Oxidized Polycrystalline Tin Oxide Surfaces before and after Reduction in H2," Chemical Material, 6[4], pp. 562-568 (1994).
5. D. Morgan, D.P. Anderson and P. Kim, Rockwell International Science Center, "Solderability As-sessment via Sequential Electrochemical Reduc-tion Analysis," accepted for publication in Journal of Applied Electrochemistry.
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Figure 1. Contour Control Panel
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Figure 2. Contour With "A" Wave
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Figure 3. Test Group A
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Figure 4. Test Group B
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Figure 5. Test Group C
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Figure 6. Test Group D
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Figure 7. Test Group E
©Air Products and Chemicals, Inc., 2010 (32578)
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