July 13, 2011
John Coonrod is a Market Development Engineer for Rogers Corporation, Advanced Circuit Materials Division. John has 23 years of experience in the Printed Circuit Board industry. About half of this time was spent in the Flexible Printed Circuit Board industry doing circuit design, applications, processing and materials engineering. The past ten years have been spent supporting circuit fabrication, providing application support and conducting electrical characterization studies of High Frequency Rigid Printed Circuit Board materials made by Rogers. John has a Bachelor of Science, Electrical Engineering degree from Arizona State University. www.rogerscorp.com/acm. This blog is part of Microwave Journal's guest blog series.
The goals of the Restriction of Hazardous Substances (RoHS) directive by the European Union (EU) for lead-free printed-circuit-board (PCB) processing are worthwhile, but have impacted how electronic circuits are designed and manufactured. For one thing, lead-based solders had lower melting points than lead-free solders. The peak solder temperature for lead-free solders is typically +260°C. As a result, PCB materials must handle higher temperatures during lead-free-solder processing and rework. Understanding what happens to a PCB at those elevated temperatures can help guide the task of selecting PCB materials for lead-free-solder processing.
A main concern for circuit laminates at high temperatures is maintaining a strong copper/dielectric bond. That bond can weaken when exposed to elevated temperatures or when subjected to the thermal gradients that occur during processing. Strong copper/dielectric bonds are essential for PCB reliability. Another concern, particularly in multilayer-board (MLB) assemblies, is the electrical and mechanical stability of the PCB materials with exposure to elevated and changing temperatures.
Often, mixed dielectric materials may be used to form a MLB assembly. In a high-temperature lead-free-solder process, RO4000® PCB materials from Rogers Corporation, for example, are popular laminates when high performance is required, and are often combined with high-temperature FR-4-type materials for handling less critical signal chores in the MLB. The RO4000 series of materials includes RO4003C™ and RO4350B™ laminates as well as companion bondply materials.
What are the dangers when mixing and matching different materials such as RO4000 and FR-4 laminates using a lead-free-solder process? The answers can be found by evaluating how temperature affects different PCB materials.
Suppliers of high-frequency PCB materials provide numerous data-sheet parameters to describe the electrical and mechanical characteristics of their materials. Some of these parameters can also be used as “predictors” to determine the suitability of a circuit laminate for lead-free-solder processes. Some of these lead-free predictors include the glass transition temperature (Tg), the coefficient of thermal expansion (CTE) in the z-axis of the material, the decomposition temperature (Td), and the time for the material to delaminate at a specific temperature. What must be known about a PCB material is how many reflow cycles it can withstand at higher temperatures and whether multiple temperature cycles have additive detrimental effects on the material, for example, by reducing plated-through-hole (PTH) reliability.
The Tg of a material is the temperature at which the material undergoes a transition from a rigid state to a soft state. While it is a useful parameter for evaluating the thermal robustness of a material, it is not by itself a good indicator of a material’s suitability for lead-free-solder processing. Typically, circuit-board materials for lead-free-solder processing have a Tg of greater than +175°C, although Tg values for FR-4 like materials can range from +125°C to +220°C. In contrast, both RO4003C and RO4350B laminates have Tg values in excess of +280°C, as determined by differential scanning calorimetric (DSC) analysis.
The decomposition temperature, Td, of a material is defined as that temperature during a steady-state ramp at which the material’s mass has been reduced by 5%. This temperature marks the permanent thermal degradation of the material, and it may be accompanied by other effects to the material, including blistering, measling, delamination, and even loss of the copper/dielectric bond. For high-Tg FR-4-like materials intended for lead-free-solder processing, the Td is typically in the range of +300°C to +350°C. In contrast, for RO4000 materials, the Td ranges from +390°C to +425°C.
The CTE in the z-axis of the material is yet another temperature-dependent parameter which can shed light on the expected behavior of a material in a high-temperature, lead-free-solder process. The CTE in the z-axis refers to the rate at which the thickness of the material changes in parts per million (ppm) with changes in temperature, over a specified temperature range. A material’s z-axis CTE directly correlates to the PTH reliability of the material. Lower CTE values translate to increased PTH reliability. The RO4000 materials, for example, are characterized for z-axis CTE of 46 ppm/°C or less from -55°C to +288°C. In a lead-free-solder process, the major concern for CTE is at temperatures approaching or exceeding a material’s Tg value, where the CTE value is considerably higher than for its specified temperature range.
Not only are high temperatures a concern for a PCB material to be used in a lead-free-solder process, but also the manner in which the temperatures change or cycle. High temperatures and temperature cycling both contribute to weakening of a laminate’s copper/dielectric bond, often resulting in delamination. PCB materials are characterized in terms of their time to delaminate at different temperatures, such as T-245 for the time at +245°C, T-260 for the time at +260°C, and T-288 for the time at +288°C. The onset of delamination is usually evidenced by blistering, measling, and voiding in a material. For materials to be used in a lead-free-solder process, typical goals for T-260 might be 30 minutes and for T-288 about 10 minutes.
To evaluate their suitability for high-temperature lead-free-solder processes, 0.125-in.-thick RO4000 materials were subjected to T-288 tests. Tests included 90 minutes at +288°C, with the sample then removed from the furnace for evaluation. The samples were cross-sectioned, and no defects were found. They were then returned to the furnace for another 90 minutes at +288°C, removed, and again cut into cross-sectioned slices for examination. Again, no defects were found. For comparison, high (+175°C) Tg FR-4-like materials were placed in the furnace at +288°C to determine their T-288 values. Although these are nominally high-temperature materials, with Td of +300°C to +350°C, they showed clear signs of delamination in less than 10 minutes.
The high-Tg FR-4-like materials did not fare well in various studies of temperature cycling as might be found in PCB rework processes. For example, the material’s copper peel strength dropped dramatically at temperatures above +150°C. This is a fairly standard test for evaluating the reworkability of a material, even though lead-free-solder rework may be performed at temperatures exceeding +370°C.
In contrast, evaluation of RO4000 series materials for reworkability was conducted through simulated rework with typical circuit elements, such as capacitors and resistors, on circuit boards populated with Sn/Ag/Cu solder paste with a peak reflow temperature of +365°C. The boards were reworked through three cycles, using a variety of extraction techniques, including solder tips, solder tip fixtures, hot air, infrared (IR) heating, and a convection oven. Reattachment of circuit components was performed at +371°C using a solder tip. In short, the RO4000 materials survived through three cycles. When high-Tg FR-4-like materials were subjected to the same sequence of events, some failed during the first cycle and many during the second cycle.
Finally, the RO4000 circuit materials were subjected to different series of thermal shock tests, with pressure to simulate the fabrication of MLBs. Various temperature gradients and dwell times between temperatures were applied. In all cases, MLBs fabricated with these materials produced PTHs that passed with flying colors mechanically and electrically even after as many as 1000 temperature shock cycles from -55°C to +125°C, regardless of the core thickness, prepreg, copper type, MLB thickness, and hole diameter.