Standard test boards populated with common components including BGAs and QFPs were built with vapor phase and convection soldering technology and tested at Epic Technoligies, LLC facility. Reliability testing demonstrated that lead-free and tin-lead joints produced by vapor phase to be equally robust as those from convection reflow. The controllable, lower peak solder temperature makes vapor phase ideal for soldering complex assemblies having sensitive lead-free SMT components
Vapor Phase Soldered, Lead Free, ENIG Surface Finish (U26 partially removed in shear/tensile test)
Tests were conducted using the Vapor Phase Reflow Process described by Dan Coada at the August 2008 SMTA Orlando Florida presentation “Vapor Phase vs. in Lead Free Complaint Assembly” 1. EPIC Technologies LLC Test Vehicle boards were used with tin-lead HASL or lead free Immersion silver, Immersion tin or ENIG surface finishes, as appropriate. Boards were populated with tin-lead or lead-free components2, printed, assembled and soldered using standard reflow or vapor phase production equipment. The solder pastes selected for testing included tin-lead and lead-free no-clean and water soluble formulations. Assembled test boards were thermal shocked between–45oC and +125oC with 20 minutes duration at each limit for 500, 1000 and 2000 cycles in EPIC Technologies LLC Failure Analysis Laboratory. Other test boards were subjected to accelerated aging at 85oC and 85% relative humidity for 1000 hrs. The EPIC Test Vehicle is populated on only one side although it is equipped with plated through-holes (PTH) for mixed technology tests. Test boards were populated with dummy 402, 603, 805, 68 pin PLCC, TSOP32, SOIC TQFP QFP208 and daisy chained BGA169 and BGA352 components.
Convection Lead-Free Profile
Standard lead-free convection reflow profiles provided a peak temperature of 245oC and a time above liquidus (TAL) in the 60 – 90 second range recommended by paste suppliers. Vapor phase soldered boards were soldered in an IBL CM800 vapor phase soldering chambers using Galden LS/230 Perfluorinated heat transfer fluid. The vapor phase profiles developed provided a TAL of about 90 seconds and a maximum temperature of 230oC, a temperature that is governed by the vapor temperature. After a vapor phase profile is established, TAL can be modified to achieve any time required without exceeding the 230o maximum temperature. The vapor phase equipment first preheats the board using infrared. Next, the work is lowered into the vapors at a programmed rate to regulate ΔT and TAL. After the work reaches the maximum vapor temperature, the duration of its exposure is preprogrammed. Several soldering programs can be developed by the engineer and stored in memory to suit the needs of different lead-free or tin-lead board types. ΔT and TAL are controlled by the program developed by the engineer.
Vapor Phase Lead-Free Profile
Vapor Phase Tin-Lead Profile
Visual inspection for solder balls, tombstones, bridging, voids and dewetting indicated no apparent difference between the two methods of solder joint creation. No tombstones were experienced on the EPIC Test Vehicle boards in either case. Visual inspection indicates that while vapor phase created solder joint performance and microsection appearance on the board is very good, it might be a good idea to explore increasing the lead-free TAL above the 60 to 90 seconds recommended by solder paste manufacturers to accommodate thorough heat transfer to larger components, or clusters of large components. Larger thermal load components, especially in clusters tend to retard the complete melting of lead-free paste. It is more difficult to ensure good joints on components with high thermal mass in convection processing because while trying to achieve a sufficient TAL on larger components, smaller components in less populous areas may tend to overheat. There is no chance of overheating smaller or isolated components with vapor phase because vapor phase cannot heat a component higher than the vapor temperature. Much discussion in trade magazines and forums such as the IPC TechNet has focused on the question of soldering tin-copper and SAC alloy terminated BGAs and other components with standard tin-lead solders. Using a 230oC vapor phase system, even liquification of these terminations ceases to be a problem while posing little chance of overheating heat sensitive components. Similarly, risks associated with lower tg substrates and temperature sensitive components is reduced relative to lead-free convection processing. Since cleanliness had been studied using ion chromatography for a previously published report1, a cleanliness comparison was made for this report using ROSE techniques. An Omegameter operating above 100oF was employed. No differences were detected in ionic cleanliness between boards soldered using convection reflow and those soldered in vapor phase. Lead-free no-clean samples tended to have 50% higher contamination levels than standard tin-lead boards due to the type and level of flux used in lead-free pastes. All results were well below IPC limits. Resistance across soldered BGA daisy chain arrays of 40 and 80 joints were the same for convection and vapor phase reflowed test boards within the limits of experimental measurement. (Daisy chained dummy 169 and 352 termination BGAs containing 4 daisy chains each were used) Solder joint conductivity did not appear to deteriorate measurably after either 2000 thermal shock cycles or 1000 hours of accelerated aging at 85oC/85RH. During the 2000 thermal shock cycles and accelerated aging, the average absolute change in resistance on measured daisy chains is summarized in the following table. The difference between the performance of Vapor phase soldered and convection soldered test boards is insignificant considering the limited data set. The resistance change for each sample is reported as the average of absolute values of the changes in resistance for a set of samples. No special preparation or seasoning of samples was performed. Resistance values were recorded “blind”. A small amount of ohmmeter drift was experienced at the low resistances measured.
Average Percent Resistance Change (Absolute Value)
*Tin-Lead No-Clean resistance measurements tended to decrease slightly. Others increased or were mixed, hence, the use of absolute values to assess changes. The shear force required to cause SOIC joint failure was measured and found to be the same for convection and vapor phase reflowed test boards. Shear force was measured on an SOIC16 (U25 and/ U26) exerting a combination shear and tensile force which pushed the component parallel to the plane while lifting the component by means of a 30o wedge. These measurements did not deteriorate after shock and accelerated age. While thermal shock results were measured at 500 and 1000 shock cycles, only those from the 2000 cycle test are reported here. Results are summarized in the following table.
Shear /Tensile force required to remove SOIC16 (pounds of force)
Thermal profiles using vapor phase soldering equipment are controllable with the maximum temperature dictated by the specific thermal transfer fluid employed. The tin-lead and lead-free solder joints created using vapor phase technology have equivalent performance to those created using convection equipment, while offering a uniform fixed maximum temperature of controlled duration. Vapor phase solder joint creation offers a viable alternative to convection reflow. Convection reflow has less uniform maximum temperatures over complex circuit board surfaces. Restricted resources, rising energy cost, increased awareness on the environment, increased demand for quality at low operating cost, and the migration to PB-free components, urge a change towards vapor phase as the process of choice. Engineers are being challenged to establishes good processes up front, with minimal interference to operations. VP soldering process assists the Engineer to get it right the first time, minimizing production interruptions. VP reflow in inert gas atmosphere is not only a benchmark for other procedures but it defines an own unique standard. Vapor Phase reflow soldering is a technology from yesterday that will certainly see its comeback in the Lead Free, Lean manufacturing environment of today.
Source: EPIC TECHNOLOGIES