Executive Summary

Standard test boards populated with common components including BGAs and QFPs

were built with vapor phase and convection soldering technology and tested at Epic

Technologies, 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)

Test Description

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 liquefaction 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)

Conclusion

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 establish 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 LLC