| Posted by Werner Engelmaier on 29 August 2006 at 14:08
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(This column, which originally appeared in Global SMT & Packaging magazine 6.6 (Jun/Jul 2006), is also available as a free PDF.)
“The common assumption that removal of non-functional lands has no reliability impact needs to be rethought in light of the higher lead-free soldering temperatures." - Werner Engelmaier
The threat to the reliability of printed circuit boards (PCBs) 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 issue.
In neither column was the issue of non-functional lands (NFLs) addressed. The reason for this came from the assumption that it was essentially a non-issue. This common assumption that removal of non-functional lands has no reliability impact needs to be rethought in light of the higher lead-free soldering temperatures. As was shown in my column of May 2003 (GSMT&P 3.4), the PCB reaches temperatures of 265 to 280°C when soldering with LF-solders.
Early work showed that there was no significant impact on barrel stresses when NFLs were removed, as can be seen in Figure 1.
Figure 1 - Graph showing the z-direction tension stresses on the copper barrel of a plated-through hole[1].
Experience has shown that the reliability issues primarily related to plated-through hole (PTH)/via copper barrel fractures for small-diameter holes or inner-layer separation (ILS) for larger-diameter holes[2-5]. ‘Resin recession,’ as illustrated in Figure 2, did not have any real reliability consequences.
Figure 2 - Micro-photograph of plated-through hole copper barrel crack and some ‘resin recession’[2].
Further, studies and testing concentrated on the long-term threat to reliability from the cyclic operating conditions[1,3,5], and assembly conditions, to the extent considered at all, where simply pre-conditioning to the main effort.
However, recent experience shows things to be nowhere near as benign for assemblies undergoing soldering processes utilizing the lead-free solders as had been tacitly assumed. Figure 3 shows some recent industry experience with PTHs in PCBs that reached nowhere the ‘Red Flag’-level of an aspect ratio of 4-to-1.
Figure 3 - Micro-photograph of plated-through hole without any inner-layer connections/lands and resulting full hole-wall pull-away after lead-free wave soldering process[2].
In Figure 4, the same PCB is shown with a full complement of non-functional lands (NFLs) and showing no wall separation or ‘resin recession.’
Figure 4 - Micro-photograph of plated-through hole with full complement of inner-layer connections/lands[2].
The physical processes that cause what is seen in Figure 3, and to a lesser degree in Figure 2, need to be understood. For an understanding of what is going on, it is perhaps best to visualize the PTH as a submerged submarine. The surrounding resin is the equivalent of the ocean water surrounding the sub: as the temperature is increased to soldering temperatures, the ‘hydro’-static pressure on the copper barrel ‘submarine’ increases.
In Physics 101 we learned that if one drills a hole in a plate of steel and then heats the plate, the size of the hole increases with the thermally expanding steel. Unfortunately, the isotropic steel plate is not the equivalent of the PCB, with its glass-fiber sheets constraining the resin in the x/y-directions. This makes the PCB highly anisotropic, and as the resin thermally expands almost exclusively in the z-direction, it tries to expand into the space of the PTHs as well.
Coming back to our Gedankenbild of the submarine, clearly a ‘sub’ with a smaller diameter is stronger than one with a larger diameter, a ‘sub’ with closely spaced reinforcement ribs is stronger than one with wider spaced ones or none at all. For a PTH, the functional and non-functional lands serve as ‘reinforcement ribs;’ thus, removing them clearly weakens the PTH barrel against collapse—not against barrel cracks, however—as can be seen in Figure 3. The higher temperatures required for lead-free soldering clearly exceeded the maximum ‘dive depth’ of this PTH.
What is actually happening is that on heating the PTH copper barrel wall bulges inward and plastically deforms more or less depending on the inner-layer land separation and the copper plating thickness for a given maximum temperature and PCB resin content.
When one looks at micro-photographs of PTHs with barrel cracks and/or resin recession, one notices that the vast majority of these occurrences are at prepreg layers and not laminate layers within the PCB (see Figure 2, also Ref. 5).
On cooling, when the resin thermally contracts to its original volume, the plastically deformed barrel walls cannot return to their former geometry, particularly if the inner-layer ‘anchor points’ are missing—the result: ‘resin recession’ or even full hole-wall separation, as illustrated in Figure 3.
Thus, the conclusion is clear: in order for PCBs to be compatible with the higher soldering temperatures required for lead-free assemblies, non-functional land should not be removed.
I have just finished a multi-client study/white paper: "WHITE PAPER REPORT: 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." As indicated by the title, in this report recommendations are made regarding appropriate specifications and ‘FAB Notes’ on drawings for printed circuit boards (PCBs), general procedures to qualify PCB shops and to assure they would be producing PCBs of good quality, and testing procedures to verify quality and reliability. It contains examples—one for SnPb solder assemblies and one for RoHS-compliant Pb-free solder assemblies—of ‘FAB Notes’ serving as general specifications on PCB drawings, a basic questionnaire for new PCB shops to be qualified meant as a supplement to IPC-1710, as well as recommendations for ongoing activities to assure that qualified PCB shops maintain the quality of the PCBs produced by them. All the recommendations are fully researched and referenced.
References:
[1] Bhandarkar, S. M., A. Dasgupta, D. Barker, M. Pecht, and W. Engelmaier, “Thermo-Mechanical Stress Analysis of Plated Through Hole Structures,” ASME J. Electronic Packaging, Vol. 114, No. 1, March 1992, pp. 8-13.
[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.
[5] Birch, B. “Discussion on Non Functional Pad Removal/Backdrilling and PCB Reliability,” Monograph, PWB Interconnect Solutions Inc., www.pwbcorp.com, 2005.
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.
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