(This column, which originally appeared in Global SMT & Packaging magazine 6.9 (October 2006), is also available as a free PDF.)

It appears that the threat posed by lead-free soldering to the reliability of the interconnect structures of printed circuit boards (PCBs) is perhaps more significant than the as yet not fully understood threat to the reliability of the solder joints themselves.

The threat to the reliability of printed circuit boards (PCBs) and their survival during the assembly process comes primarily from one source—the temperatures required during the soldering processes for the assembly of the components onto the PCB. These reliability threats were first discussed in this column in the October 2001 [GSMT&P 1.3] issue and more recently with emphasis on the high lead-free soldering process temperatures in the September 2005 [GSMT&P 5.8], August 2006 [GSMT&P 6.8] and September 2006 [GSMT&P 6.9] issues. As was shown in my columns of August 2001 and May 2003, the PCB reaches temperatures as high as 280°C when soldering with LF-solders.

In particular, the column published in September 2005 discussed the material changes required for PCBs to successfully survive lead-free soldering. Unless these changes are included in the design and specification of the PCBs1, they will not happen, because the PCB Fab houses will bid on the lowest costs and all these changes carry a cost increase.

The reliability of the plated-through hole (PTH) barrels can be analytically determined2,3; to determine the reliability of the inner-layer connections requires finite element analysis (FEA) or testing4.

In Table I, the resulting loss of life is given for a number of conditions. The basic input parameters for the reliability model in References 2 and 3 are:

PTH Barrel Copper:  

Modulus of Elasticity=83 GPa [12x106 psi]
Modulus of Plasticity=0.69 GPa [0.1x106 psi]
Tensile Strength=241 MPa [35,000 psi]
Yield Strength=172 MPa [25,000 psi]
Fatigue ductility=22%

PCB Resin: 

Modulus of Elasticity=3.5 GPa [0.5x106 psi]
CTE(z,<Tg)=65 ppm/°C
CTE(z,>Tg)=315 ppm/°C

PCB: 

PCB Thickness=3.7 mm [93 mils]
Drilled Hole Diameter=0.53 mm [13.5 mils]
PTH Copper Thickness=0.04 mm [1.0 mils]
Kb=1
Kc=95
KQ=7

Table I. Percent Loss of Life for Each Single Pass Reflow Process.

The data in Table I show that for SnPb soldering temperatures, five (5) reflow cycles would reduce the operational life of the PTH copper barrels by close to 15%; a rise to the soldering temperatures for the lead-free SAC solders increases the loss of life to a third.

An increase in the PCB resin glass transition temperature from 150°C to 170°C drops the loss of life to about 25%.

Figure 1—Weibull plots of failure data of plated-through hole barrels with and without pre-conditioning by SnPb-solder assembly simulation. These data show about a 30% loss of life resulting from the 5 simulated reflow cycles. Courtesy of Werner Engelmaier, Engelmaier Associates, L.C., USA.

Figure 1 shows the Weibull probability plots of failure data of plated-through hole barrels with and without pre-conditioning by SnPb-solder assembly simulation. These data show about a 30% loss of life resulting from the five simulated reflow cycles. These data show an even larger loss of life than is indicated by the results given in Table I.

In Figure 2 Weibull probability plots of failure data of plated-through hole barrels with and without pre-conditioning by Lead-Free solder assembly simulations are shown. These data show up to a 50% loss of life resulting from only three simulated reflow cycles to 230°C—for reflow soldering of SAC solders, the typical; PCB will reach temperatures of 260 to 280°C.

Figure 2—Weibull plots of failure data of plated-through hole barrels with and without pre-conditioning by Lead-Free solder assembly simulation. These data show up to a 50% loss of life resulting from only three simulated reflow cycles. Courtesy of Paul Reid, PWB Interconnect Solutions Inc., Canada.

Of course, one needs to put these losses of life during the soldering processes into perspective, however. The excursions to the soldering temperatures are far more damaging than product service operation cycles. Table II shows the mean cyclic life as well as the operation time to first failure for a number of different applications.

Table II. Estimated Mean Lives and Time to First PTH Failure2.

As shown in Table II even a loss of half of the life during soldering processes may not have significant reliability consequences except for the more severe applications such as automotive and military operations.

References:
[1] Engelmaier, W., “Recommendations for PCB FAB Notes and Specifications in Printed Circuit Board Drawings for SnPb and Lead-Free Soldering Assemblies, the Qualification of PCB Shops and Activities to Assure Continued Quality,” White Paper/Multi-Client Study, Engelmaier Associates, L.C., 2006.
[2] Engelmaier, W., “Interconnect Failures and Design for Reliability for Plated-Through Holes/Vias (PTHs/PTVs),” Workshop Notes, Engelmaier Associates, L.C., 2006.
[3] IPC-D-279 “Design Guidelines for Reliable Surface Mount Technology Printed Board Assemblies,” The Institute for Interconnecting and Packaging Electronic Circuits, Northbrook, IL, July 1996.
[4] IPC-TR-486 “Report on Round Robin Study to Correlate Interconnect Stress Test (IST) with Thermal Stress/Microsectioning Evaluations for Detecting the Presence of Inner-Layer Separations,” The Institute for Interconnecting and Packaging Electronic Circuits, Northbrook, IL, July 2001.

Werner Engelmaier has over 41 years experience in electronic packaging and interconnection technology. Known as ‘Mr. Reliability’ in the industry, he is the president of Engelmaier Associates, L.C., a firm providing consulting services on reliability, manufacturing and processing aspects of electronic packaging and interconnection technology. He is the chairman of the IPC Main Committee on Product Reliability. He was elected into the IPC Hall of Fame 2003, and was awarded the IPC President’s Award in 1996 and the IEPS Electronic Packaging Achievement Award in 1987. He also was named a Bell Telephone Laboratories Distinguished Member of Technical Staff in 1986 and an IMAPS Fellow in 1996. More information is available at www.engelmaier.com, and he can be reached at engelmaier@aol.com.

Keywords : Werner Engelmaier

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