(This column, which originally appeared in Global SMT & Packaging magazine 6.8 (September 2006), is also available as a free PDF.)

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] and August 2006 [GSMT&P 6.8] issues.

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

To assure the survival of PCBs during the high soldering temperatures, and their subsequent reliability afterward, improvements in PCB resin properties are necessary. The integrity of the PCB interconnect structures—PTV copper barrels and barrel/inner-layer interconnects—requires improved Tg and CTE, and improvements in Tg and Td are necessary for the thermal stability of the PCB resin.

The PCB base and prepreg materials are frequently specified according to industry document IPC-4101[1,2]. It should be noted that specifying using IPC-4101 slash sheets for the various material choices is not sufficient to specify specific properties. The ranges of properties contained within these slash sheets are too broad for this purpose; therefore it is recommended that specific materials and their equivalents be specified for critical applications.

Whether to use Dicy-cured (Dicyandiamide) FR-4 materials is being debated in the industry. Some are adamant about not using Dicy-cured materials in high-density high-reliability applications, while others have good experience and like the economy achievable.

The glass transition temperature should be determined using the Thermo-Mechanical Analysis (TMA) method as per IPC-TM-650, 2.4.24C[3]. The TMA method is preferred over the other two methods sometimes used to determine the glass transition temperature, DSC[4] and DMA[5], because the thermal expansion of the PCB is a critical parameter, which is given by TMA as a function of temperature.

For PCBs subject to soldering processes using the more elevated soldering temperatures required for lead-free solders, specifying a glass transition temperature, Tg, of 140°C will not be adequate for PCBs thicker than about 50 mils.

Furthermore, for thicker PCBs, it is recommended that a minimum decomposition temperature, Td, determined as per IPC-TM-650, 2.4.24.6[6] as well as a maximum thermal expansion coefficient in the PCB thickness direction, CTE(z), determined as per IPC-TM-650, 2.4.41[7] be specified. CTE(z) values should be given separately for temperatures below Tg and above Tg; however, frequently the thermal expansion, TE in %, is lumped together from 50 to 260°C or even 50 to 288°C. Typically, the decomposition temperature is given as Td(5%) to a 5% weight loss; the decomposition temperature, Td(2%), to a 2% weight loss, has been found a very good indicator, but is not as yet widely available.

Frequently, the time to delamination, either T-288 or T–260[8], are specified either in addition to Td or instead of it. The T-288 delamination time provides a more appropriate level of performance given the process temperature required for LF-soldering. The delamination time is sometimes combined with the requirement that that temperature needs to be survived for 4 to 5 excursions.

Unfortunately, the data sheets from the various laminate/prepreg suppliers are neither consistent not complete. What is worse, on the occasions where property measurements were performed by commercial laboratories, because the values given in the data sheets did not appear to be credible, values of the CTE(x) and CTE(y) were found to be nearly double those given in the data sheets.

The easiest way to specify the three properties critical for the survival of the PCB and the PTH/via interconnect structure—Tg, Td, thermal expansion (TE) —is by specifying a minimum Soldering Temperature Impact Index, STII, which is defined as
STII = Tg/2 + Td/2 — (TE%(50 to 260°C) x 10).

For PCBs with thicknesses of 0.06 inches (1.5 mm) or more, an STII-value of 215 or larger is recommended. However, the STII-concept is not widely used as yet.

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." In this report, detailed 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. IPC-4101 “Laminate/Prepreg Materials Standard for Printed Boards,” The Institute for Interconnecting and Packaging Electronic Circuits, Northbrook, IL, December 1997.
2. IPC-4101A “Specifications for Base Materials for Rigid and Multilayer Printed Boards,” The Institute for Interconnecting and Packaging Electronic Circuits, Northbrook, IL, June 2002.
3. IPC-Test Methods Manual, IPC-TM-650, 2.4.24C, “Glass Transition Temperature and Cure Factor by DSC,” The Institute for Interconnecting and Packaging Electronic Circuits, Northbrook, IL, December 1994.
4. IPC-Test Methods Manual, IPC-TM-650, 2.4.25C, “Glass Transition Temperature and Z-Axis Thermal Expansion by TMA,” The Institute for Interconnecting and Packaging Electronic Circuits, Northbrook, IL, December 1994.
5. IPC-Test Methods Manual, IPC-TM-650, 2.4.24.4, “Glass Transition Temperature and Thermal Expansion of Materials Used In High Density Interconnection (HDI) and Microvias-DMA Method,” The Institute for Interconnecting and Packaging Electronic Circuits, Northbrook, IL, November 1998.
6. IPC-Test Methods Manual, IPC-TM-650, 2.4.24.6, “Decomposition Temperature Td) of Laminate Material Using TGA,” The Institute for Interconnecting and Packaging Electronic Circuits, Northbrook, IL, April 2006.
7. IPC-Test Methods Manual, IPC-TM-650, 2.4.41, “Coefficient of Linear Thermal Expansion of Electrical Insulating Boards,” The Institute for Interconnecting and Packaging Electronic Circuits, Northbrook, IL, March 1986.
8. IPC-Test Methods Manual, IPC-TM-650, 2.4.24.1, “Time to Delamination (TMA Method),” The Institute for Interconnecting and Packaging Electronic Circuits, Northbrook, IL, December 1994.


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 : reliability, PCB reliability, lead-free reliability

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