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The Global Assembly Journal for SMT and   Volume 9 Number 4 April 2009
Advanced Packaging Profession...
Vapor phase vs. convection reflow in RoHS-compliant assembly




Vapor phase vs. convection
reflow in RoHS-compliant
assem...
Vapor phase vs. convection reflow in RoHS-compliant assembly




                                                         ...
Vapor phase vs. convection reflow in RoHS-compliant assembly


                                                           ...
Conquering SMT stencil printing challenges with today’s miniature components


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Vapor Phase Versus Convection Reflow

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Vapor Phase Versus Convection Reflow

  1. 1. www.globalsmt.net The Global Assembly Journal for SMT and Volume 9 Number 4 April 2009 Advanced Packaging Professionals ISSN 1474 - 0893 Krassy Petkov Interview Inside Hot air solder leveling in tHe lead-free era NEW PRODUCTS vapor pHase vs. convection reflow in roHs-compliant assembly INDUSTRY NEWS conquering smt stencil printing INTERNATIONAL DIARY cHallenges witH today’s miniature components
  2. 2. Vapor phase vs. convection reflow in RoHS-compliant assembly Vapor phase vs. convection reflow in RoHS-compliant assembly by Dan Coada, EPIC Technologies, Norwalk, OH USA introduction to consider in volume manufacturing. The contract manufacturing in- Vapor phase (VP) reflow technology has Today’s VP reflow process makes use of dustry is changing rapidly from been in existence since the early 1970s as a the heat produced by a boiling fluorinated lead-based soldering to lead-free reflow process for surface mount technol- polymer or fluid. This boiling fluid pro- soldering. There is no stopping ogy (SMT) assemblies. While used for some duces a uniform temperature zone (vapor defense applications and in smaller volume blanket) in which the PCBA is exposed for the transition or the reality that production settings, the disadvantages lead-free components are going solder purposes. Heat is transferred to the associated with the initial processing tech- PCBA as it is immersed into the vapor area to be introduced in lead-based nology limited its widespread acceptance. until the PCBA reaches temperature equi- processes. This challenge to These disadvantages included environmen- librium with the boiling point of the fluid. engineering and quality is a tal concerns about the fluids being used, The primary soldering benefits of VP in huge concern and one that needs throughput limitations, applicability only comparison to infrared (IR) or convection scrutiny and a watchful eye. to single-sided printed circuit board as- include an oxygen free (inert) environment EMS providers rely on compo- semblies (PCBAs) and an inherent problem without the need for nitrogen, fixed upper nent suppliers to ensure that the with tombstoning. temperature exposure and superior heat lead-free transition on the com- Advances in VP technology have transfer on thermally challenged PCBAs. ponent terminations is seamless addressed many of these shortcomings VP also offers distinct advantages through continued development of im- in the realm of lead-free soldering. Key to their soldering processes, but proved machines, chemical selections and that rarely happens. Termina- benefits include a lower peak reflow process controls. Doublesided PCBAs are temperature, an inert environment without tion changes require additional easily processed in current equipment. As a nitrogen, improved solder wetting and flow modification to solder profiles result, VP is becoming a viable alternative and a reduction in profiling time and flux chemistries in order to ensure proper wetting of the sol- der to the lead-free termination. The need for nitrogen to be used in convection reflow is becoming a requirement more than an op- tion, and nitrogen is costly. This paper will look at the advantages and disadvantages of vapor phase (VP) and convec- tion reflow in RoHS-compliant processing and discuss associ- ated design for manufacturing (DFM) issues. Keywords: Vapor Phase, Convection Reflow, IR Reflow, Lead-Free This paper was originally published in the proceedings of the SMTA International Figure 1. Vapor phase soldered, lead free, ENIG surface fnish (U26 partially removed in shear/tensile test). Conference, Orlando, Florida, August 2008. 20 – Global SMT & Packaging – March 2009 www.globalsmt.net
  3. 3. Vapor phase vs. convection reflow in RoHS-compliant assembly Figure 3. Vapor phase lead-free profile. Figure 4. Vapor phase tin-lead profile. and complexity to get the profile close so that process development time is kept to a minimum. Conversely, VP reflow profiling can be classified by process type to the point Figure 2. Convection lead-free profile. where there are fewer profiles to develop. For instance, standard multi-layer 0.062” thick PCBAs can follow the same lead- lower peak reflow processing improved solder wetting and flow based VP profile regardless of component VP reflow requires a lower processing Visual inspection of micro sections complexity. With advances in VP process temperature. In convection or IR process, indicates that VP creates good solder joint systems, monitoring of heat load during temperatures can reach 245˚C to 265˚C performance. When larger thermal load the soldering process allows the systems to at the component level. VP temperatures components or clusters of components are profile almost automatically. Ramp rates stay at the boiling point of the fluid, present, time above liquidus (TAL) should and soak times at peak temperature can be typically established between 230˚C and be increased beyond the 60-90 seconds rec- defined by the engineer and controlled by 240˚C. The lower temperatures make it ommended by solder paste manufacturers the systems regardless of the product mix possible to reduce the cost of the PCB by to accommodate thorough heat transfer. during the process. In a true one-piece flow using lower Tg/Td material selections for Comparatively, in convection processing it on a prototype, it is much easier to get it SMT assemblies. Savings of 10-15% and can be more difficult to ensure good joints right the first time using VP processing. even more could be seen on laminate costs on components with high thermal mass be- The days of inadequate reflow tempera- alone. VP also offers processing advantages cause achieving TAL in larger components ture or over temperature on the first piece with large mass components, such as con- may result in smaller components overheat- are virtually eliminated by use of the VP nectors, because the thermal equilibrium ing. There is no chance of overheating systems. is better. In convection reflow, particularly smaller or isolated components with VP in higher temperature lead-free processes, because VP cannot heat a component comparative data on lead-free and correctly soldering large mass connectors higher than vapor temperature. tin-lead solder joint creation may overheat the rest of the PCBA. Standard test boards (Figure 1) populated reduction in profiling time with common components, including inert environment Another advantage of VP’s heat transfer BGAs and QFPs, were built with vapor The inert environment and consistency of characteristics and the uniformity at which phase and convection soldering technology heat transfer allow VP to be more forgiving it accomplishes heat transfer is that it and tested at an EPIC facility. Reliability with lead-free component terminations. makes it easier to understand the profiling testing demonstrated that lead-free and Less active no clean flux chemistries have relationship between PCBAs. In traditional tin-lead joints produced by vapor phase to proven to be adequate in soldering lead- profile development for a new PCBA, be equally robust as those from convection free terminations that demand high activ- sample assemblies with thermocouples are reflow. The controllable, lower peak solder ity fluxes in the convection reflow process. run through the reflow process numerous temperature makes vapor phase ideal for There is also cost savings due to lower times in order to get the right profile, with soldering complex assemblies having sensi- energy consumption. In addition to the manual inspection of the solder joints tive lead-free SMT components. elimination of nitrogen, electricity usage and flux residue used to determine if the with VP is much lower. target profile is correct. Engineers develop matrix charts on board size, layer count www.globalsmt.net Global SMT & Packaging – March 2009 – 21
  4. 4. Vapor phase vs. convection reflow in RoHS-compliant assembly 2000 1000 hrs. Visual inspection for solder balls, Cycles Accel. Age tombstones, bridging, voids and dewetting SnPb No Clean Convection indicated no apparent difference between 19.5 * 9.34 * the two methods of solder joint creation. Vapor Phase 6.67 16.8 No tombstones were experienced on the Lead-free No Clean Convection JOCY test vehicle boards in either case. 2.11 9.16 Visual inspection indicates that while Vapor Phase 2.59 5.5 vapor phase created solder joint perfor- SnPb Water Soluble Convection 14.5 No data mance and micro-section appearance on the board is very good, it might be a good Vapor Phase 10.2 7.96 idea to explore increasing the lead-free Lead-free Water Soluble Convection 1.31 23.7 TAL above the 60 to 90 seconds recom- Vapor Phase mended by solder paste manufacturers 4.16 5.16 to accommodate thorough heat transfer to larger components or clusters of large * Tin-Lead No-Clean resistance measurements tended to decrease slightly. Others in- creased or were mixed hence, absolute values to were used to assess changes. components. Larger thermal load compo- nents, especially in clusters, tend to retard Table 1. Average percent resistance change (absolute value). the complete melting of lead-free paste. It is more difficult to ensure good joints on components with high thermal mass Initial 2000 Cycles 1000 hrs. in convection processing because while Thermal Shock Accel. Age trying to achieve a sufficient TAL on larger SnPb No Clean Convection 19.1 23.1 23.6 components, smaller components in less populous areas may tend to overheat. Vapor Phase 22.8 21.6 19.0 Much discussion in trade magazines Lead-free No Clean Convection 27.6 25.6 26.6 and forums such as the IPC TechNet has focused on the question of soldering tin- Vapor Phase 27.3 27.6 26.2 copper and SAC-alloy-terminated BGAs SnPb Water Soluble Convection 23.7 20.3 and other components with standard tin- Vapor Phase lead solders. Using a 230˚C vapor phase 23.5 20.5 system, even liquification of these termina- Lead-free Water Soluble Convection 26.3 27.6 25.0 tions ceases to be a problem while posing Vapor Phase 26.9 28.0 25.7 little chance of overheating heat sensitive components. Similarly, risks associated Table 2. Shear/tensile force required to remove SOIC16 (pounds of force). with lower Tg substrates and temperature sensitive components is reduced relative to lead-free convection processing. Test Description profiles (Figure 2) provided a peak tem- Since cleanliness had been studied Tests were conducted using a VP reflow perature of 245˚C and a TAL in the 60-90 using ion chromatography for a previously process. Vehicle boards were used with tin- second range recommended by paste sup- published report2, a cleanliness compari- lead HASL or lead free immersion silver, pliers. Vapor phase soldered boards were son was made for this report using ROSE immersion tin or ENIG surface finishes, as soldered in an EPM-IBL SLC500 vapor techniques. An Omegameter operating appropriate. Boards were populated with phase soldering chamber using Galden above 100˚F was employed. No differences tin-lead or lead-free components, printed, LS/230 Perfluorinated heat transfer were detected in ionic cleanliness between assembled and soldered using standard fluid. The vapor phase profiles developed boards soldered using convection reflow reflow or VP production equipment. The provided a TAL of about 90 seconds and and those soldered in vapor phase. Lead- solder pastes selected for testing included a maximum temperature of 230˚C, a free no-clean samples tended to have 50% tin-lead and lead-free no-clean and water temperature that is governed by the vapor higher contamination levels than standard soluble formulations. Assembled test temperature. After a vapor phase profile tin-lead boards due to the type and level boards were thermal shocked between is established, TAL can be modified to of flux used in lead-free pastes. All results -45˚C and +125˚C with 20 minutes dura- achieve any time required without exceed- were well below IPC limits. tion at each limit for 500, 1000 and 2000 ing the 230˚C maximum temperature. Resistance across soldered BGA daisy cycles in EPIC’s Failure Analysis Labora- The vapor phase equipment first chain arrays of 40 and 80 joints were the tory. Other test boards were subjected to preheats the board using infrared. Next, same for convection and vapor phase accelerated aging at 85˚C and 85% relative the work is lowered into the vapors at a reflowed test boards within the limits of humidity for 1,000 hrs. The JOCY test programmed rate to regulate ΔT and TAL. experimental measurement. (Daisy chained vehicle is populated on only one side, al- After the work reaches the maximum va- dummy 169 and 352 termination BGAs though it is equipped with plated through- por temperature, the duration of its expo- containing four daisy chains each were holes (PTH) for mixed technology tests. sure is preprogrammed. Several soldering used.) Solder joint conductivity did not ap- Test boards were populated with dum- programs can be developed by the engineer pear to deteriorate measurably after either my 402, 603, 805, 68 pin PLCC, TSOP32, and stored in memory to suit the needs of 2000 thermal shock cycles or 1000 hours SOIC TQFP QFP208 and daisy-chained different lead-free or tin-lead board types. of accelerated aging at 85˚C/85RH. Dur- BGA169 and BGA352 components. ΔT and TAL are controlled by the program ing the 2000 thermal shock cycles and Standard lead-free convection reflow developed by the engineer. Continued on page 34 22 – Global SMT & Packaging – March 2009 www.globalsmt.net
  5. 5. Conquering SMT stencil printing challenges with today’s miniature components Minimum Aperature Vapor phase vs. convection reflow in surface registration RoHS-compliant assembly, continued Technology area ratio Cost Material accuracy from page 22 accelerated aging, the average absolute Chemical etch 0.66 Low SS, Alloy Moderate change in resistance on measured daisy Traditional laser-cut 0.66 Low SS, Alloy Very High chains is summarized in Table 1. The differ- Traditional laser-cut 0.55 Low Slic™ Very High ence between the performance of VP sol- dered and convection soldered test boards Electroformed 0.5 High Electroformed High is insignificant considering the limited data Nickel set. The resistance change for each sample Advanced laser-cut 0.45 Medium Fine Grain Very High is reported as the average of absolute values of the changes in resistance for a set of sam- Table 1: Stencil technology summary. ples. No special preparation or seasoning of samples was performed. Resistance values were recorded ‘blind.’ A small amount of mounted into a stencil frame, tension The answer to these important ques- ohmmeter drift was experienced at the low is applied to electroformed foil by the sten- tions is in our view an unequivocal ‘yes.’ resistances measured. cil frame’s polyester mesh. This tension Stencil laser and material technologies The shear force required to cause SOIC pulls on the foil causing slight shifts in the have advanced to the point where laser-cut joint failure was measured and found to locations of the stencil apertures. In most stencil performance is beyond that of be the same for convection and vapor cases, the electroformed stencil aperture current electroformed technology. Using phase reflowed test boards. Shear force was locations will be long, or further away the new LPKF high-power short-pulse fiber measured on an SOIC16 (U25 and U26) from their expected locations. If the PCB laser technology and the new Fine Grain exerting a combination shear and tensile has SMT pad locations that are short of material, stencil performance is significant- force that pushed the component parallel expected locations and the electroformed ly improved over electroformed, especially to the plane while lifting the component stencil has aperture locations further away when printing miniature components. by means of a 30˚ wedge. These measure- than expected, there can be a significant Improvements in stencil laser and mate- ments did not deteriorate after shock and shift, or misalignment, between the stencil rial technologies have lead to significant accelerated age. While thermal shock re- apertures and PCB pads. improvements in solder paste release down sults were measured at 500 and 1000 shock A shift between the stencil aperture to a surface area ratio of 0.45 as well as cycles, only those from the 2000 cycle test are reported here. Results are summarized and PCB pad reduces the amount of solder improved aperture registration accuracy. in Table 2. paste in contact with the surface of the These improvements are critical to meet- PCB pad. This lowers the adhesive force ing future requirements when printing conclusion between the solder paste and PCB pad, miniature components like 01005s. The VP key benefits include: effectively reducing the ability of the board technology summary is as follows: • Lower peak reflow temperature to pull the paste from the stencil. Minia- At a cost savings of 30-50 percent • Inert environment without ture components already have very low compared to electroformed, the ability to nitrogen surface area ratios. The lower the surface produce multi-thickness (step) stencils, • Improved solder wetting and flow area ratio, the more critical the alignment and the option of same day turn times, • Reduction in profiling time. between the stencil aperture and PCB Fine Grain stencils, cut with the new pad. The Fine Grain stencil in this DOE fiber lasers, are a marked improvement As the data above indicates, thermal was cut in the frame on the new LPKF compared to the high-performance stencil profiles using vapor phase soldering equip- high-power short-pulse fiber laser. The solutions available today. OEMs and CMs ment are controllable with the maximum intent was to minimize stencil aperture can get the performance they need while temperature dictated by the specific ther- registration errors, thereby increasing the reducing costs and meeting critical delivery mal transfer fluid employed. The tin-lead alignment accuracy between the stencil and schedules. The new stencil laser and mate- and lead-free solder joints created using PCB. The results (27 position errors for rial technologies available today give stencil vapor phase technology have equivalent the Fine Grain stencil and 2,307 position manufacturers the tools and materials performance to those created using convec- errors for the electroformed stencil) below needed to supply an ever-changing industry tion equipment, while offering a uniform show a marked improvement in aperture for many years to come. fixed maximum temperature of controlled registration when compared to an electro- duration. Vapor phase solder joint creation Acknowledgement offers a viable alternative to convection formed stencil. reflow. Convection reflow has less uniform The authors would like to thank Stephan maximum temperatures over complex conclusion Schmidt and Sebastian Gerberding of circuit board surfaces. As advancements continue in component LPKF Laser Electronics (www.lpkfusa.com) and PCB technologies, will the stencil tech- for their contribution to this article. references nology of today provide current and future 1. Munroe, C., “Beating the RoHS Heat,” solutions to the challenging assembly issues Circuits Assembly Magazine,” March faced by OEMs and CMs? Is electroformed Robert F. Dervaes is V.P. technology and engi- 2005, pp. 38-47. technology the right solution or have new neering for Fine Line Stencil, Inc. Jeff Poulos, 2. Fraser, S. and Munroe, C, “Lead-Free developments in stencil laser and material is V.P. of manufacturing and sales, Alternative Using Vapor Phase Reflow in Lead- technologies caught up with and surpassed Solutions, Inc. Scott Williams is product/ac- free Processing,” SMT Magazine, April the electroformed technology of today? count manager with Ed Fagan, Inc. 2005, pp. 48-49. 36 – Global SMT & Packaging – March 2009 www.globalsmt.net

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