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Time to Delamination : 4 min . 300 280 260 240 220 200 180 160 140 120 100 80 60 40 20 0
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0.16 0.14 0.12 Displacement (mm) 0.10 0.08 0.06 0.04 0.02 0.00 5 10 15 20 Time (min) 25 30 35 T288 by TMA 33.1 min. 28.8 min .
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Temperature ( C)
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TMA plot: time to delamination at 288 C.
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recorded (Td 2 percent), and the temperature to a 5 percent weight decrease is subsequently recorded (Td 5 percent). IPC-4101 RoHS-compliant laminates pass Td 5 percent at 340 C minimum. Delta Td (Td 5 percent Td 2 percent) is considered by some as an indicator for the onset and rapidity of decomposition. For laminates with identical Td 5 percent values but differing Td 2 percent values, the lower the delta Td the more thermally resistant the material. Note that Tg and Td do not correlate. A low Tg material can have a high Td and vice versa. Materials with identical Tg can have different Td. See Fig. 27.2 for an example of a TGA plot. Coefficient Of Thermal Expansion (CTE). The CTE of a laminate is comprised of values for x-y axis, or in-plane expansion, and z-axis, or vertical expansion (which is further comprised of the percent expansion below and above the glass transition temperature). The data are reported in ppm/ C; a typical plot is shown in Fig. 27.3. The x-y CTE for standard FR-4 at 14 to 20 ppm/ C is higher than for ceramic or silicon. The resulting thermal expansion mismatch between an ML-PWB and the attached devices can lead to solder joint fatigue failures when the system undergoes multiple heat cycles during power-up and power-down. Packages with compliant leads can accommodate the CTE mismatch and so can be used with standard ML-PWB material systems. The z-axis CTE for standard 130 to 135 C Tg FR-4 is typically about 40 ppm/ C Tg and 250 ppm/ C > Tg. At LFA temperatures of 245 to 260 C, the material expansion is so dramatic that the traditional materials experience delamination and PTH ruptures. The new generation of Lead Free Assembly Capable( LFAC) FR-4 laminates generally utilizes one or both of the following approaches to minimize the CTE above Tg: a change to non-dicy curing agents, typically phenolic, and/or the dispersion in the resin of some amount of inorganic fillers to restrain the z-axis expansion.
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100 95 90 Weight (%) 85 80 75 70 65 60
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50.5 C
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WL Loss: 2.072% 317.5 C WL Loss: 5.060% 319.5 C
Decomposition Temperature by TGA
Temperature ( C)
FIGURE 27.2 TGA plot: decomposition temperature.
0.11 0.10 0.09 0.08 Displacement (mm) 0.07 0.06 0.05 0.04 0.03 0.02 0.01 0.00 0.01 20
Tg by TMA
240.57 C CTE: 289.55 u m/m C 201.06 C
Tg = 161.90 C 105.91 C CTE: 46.37 u m/m C 80 100 120 140 160 Temperature ( C) 180 200 220 240
38.38 C 40 60
TMA plot: Tg and CTE.
PRINTED CIRCUITS HANDBOOK Moisture Absorption. Moisture is the enemy of an ML-PWB. Absorbed water raises Dk somewhat and can raise tan d significantly, hampering the functioning of the circuit at high frequencies. For applications in humid environments, a low moisture-absorbing material should be selected. Additionally, moisture absorption increases leakage current, which in turn degrades the ability of the material to resist CAF formation under bias. Trapped moisture expands with temperature, so the board sees more thermal expansion (additive to the effects of the laminate CTE) at assembly, which causes thermal defects such as delamination, blistering, or barrel cracking. The severity of these problems depends on the storage environment prior to soldering and the peak temperature of the reflow. If storage times are short, humidity is low, and reflow temperature is moderate, moisture will not be a serious issue. However, if the ML-PWB material has high moisture absorption, or is subjected to high humidity for an extended period of time, or if the assembly temperature is especially high, special care must be taken. This can include storing with a desiccant, baking prior to assembly, or both.
Properties of the Resin Systems Materials with highly cross-linked epoxy resin reinforced with woven fiber glass are the most common in use. Bromine is reacted with the epoxy matrix and is used to provide fire retardancy. Most epoxy-based materials satisfy the Underwriters Laboratories (UL) classification of V-0 for fire retardancy. The generic term for this family of epoxy resin materials is FR-4, with FR standing for flame retardant and 4 an assigned number indicating epoxy. Epoxyfiberglass materials are sold by many suppliers and have become a commodity material. Epoxies. A variety of epoxies are available for use, including halogen-free epoxies and materials with enhanced thermal, electrical, and mechanical properties. Standard Epoxies. Two resin systems are used to make GF laminate: difunctional and tetrafunctional. These systems are distinguished by the nature of the epoxy crosslinking. In a difunctional system, the epoxide molecule has two cross-linking sites, and the cured epoxy contains long linear molecular chains. Pure difunctional laminates have excellent physical properties, and for many years were the mainstay of the industry. They have a Tg of 120 C, which is adequate for most use environments, but is low for some applications and too low for LFA. The epoxide molecule of a tetrafunctional epoxy has more than two cross-linking sites. This allows a high cross-link density and a high Tg. A pure tetrafunctional system is expensive and difficult to work with. To meet the need for a Tg, above 120 C, laminators blend difunctional and tetrafunctional resins, producing a mixture referred to as multifunctional. Around 1985, some laminators began to sell a multifunctional epoxy blend with a Tg between 130 and 145 C. This blend was called tetrafunctional, even though it actually contained both difunctional and tetrafunctional epoxies. This blend is available at little or no price premium over a difunctional laminate. Laminates with a higher fraction of tetrafunctional resin are available. These systems, which are called multifunctional, have Tg values in excess of 170 C and are sold at a price premium of approximately 10 percent over difunctional systems. Multifunctional blends often have lower moisture absorption and higher thermal degradation temperature than difunctional systems. However, in some multifunctional systems, these properties are not improved. Care must be taken in selecting a multifunctional system to ensure that all properties of importance are enhanced. Modern ML-PWBs often operate at elevated temperatures due to the heat output from semiconductor devices. As boards become thicker and holes smaller, these thermal cycles result in an increasing threat to the reliability of plated holes. For example, plated holes have been shown to fail when subjected to multiple thermal cycles up to temperatures near the Tg of the base material. These cycles can easily occur when a high-power device is turned on and off. One solution to this problem is to use materials with a higher Tg.
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