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Novel Material for High-Layer Count & High-Reliability PCBs
November 6, 2012 |Estimated reading time: 4 minutes
Editor’s Note: This paper was first published in the IPC APEX EXPO 2012 technical conference proceedings and in the October 2012 issue of The PCB Magazine.
Abstract
To meet the demand of lead-free soldering processes, a thermoplastic resin-modified multifunctional epoxy system material has been developed. Silica fillers have been carefully selected to ensure a lower CTE. The material shows an outstanding heat resistance, reliability and toughness performance with complex BGA design (pitch 0.8 mm, through-hole diameter 0.25 mm) on thicker PWB boards (4 mm thick PTH boards and 3.2 mm thick HDI boards) compared to currently available lead-free compatible materials. The dielectric properties and other properties of this material are presented here.
Introduction
With the advent of lead-free PCB assembly soldering, the laminate integrity of the lead-free materials after lead-free assembly was significantly better than two years ago, showing that the materials industry is learning and maturing relative to materials’ ability to survive multiple cycles through lead-free assembly. Many of the materials have been found to perform satisfactorily for consumer products where reflow temperatures peak around the 245°C level, withstanding 90 seconds over 217°C.
However, the high-end PWB designs for the telecommunications and IT equipments typically tend to be more complex, thicker, and therefore have a much higher thermal mass than consumer products. To apply sufficient heat to enable satisfactory solder reflow of surface mounted devices, the temperature of the printed wiring board can peak at up to 260°C. Add to that the complexity of the board (typically double-side assembled) and the requirement to be able to repair boards, then it is highly possible that boards throughout their manufacturing cycle could see up to five thermal excursions up to this 260°C level withstanding 20~30 seconds over 255°C, and withstanding 120~150 seconds over 217°C. The ability of laminate systems to withstand the combined thermal exposure without degradation or delamination and at the same time exhibiting consistent electrical properties is essential.
Heat Resistance Test Board
The PTH and HDI boards with three different thickness printed circuit board design used for this heat resistance study are shown in Table 1. The BGA coupons feature 0.35 mm (0.014 inch) and 0.30 mm (0.012 inch) diameter drilled via holes. Three coupons have 1 mm (0.040 inch) and 0.8 mm (0.032 inch) via-to-via spacing and one coupon has a 0.65 mm (0.026 inch) via-to-via spacing.
The thermal via coupons feature 0.35 mm (0.014 inch) and 0.30 mm (0.012 inch) diameter drilled via holes. Three coupons have 1 mm (0.040 inch) and 0.8 mm (0.032 inch) via-to-via spacing and one coupon has a 0.65 mm (0.026 inch) via-to-via spacing.
There are 20-hole X 20-hole array FBGAs and 10-hole X 10-hole array for thermal vias.
Table 1: Test coupon design.
Material Stack-ups
For the 24-layer PTH test board constructions, the standard resin content constructions are used, measuring 4 mm (0.158 inches) in overall thickness (Figure 1).
Figure 1: 24-layers PTH test board construction.
For the 24-layer HDI test board constructions, the high-resin content constructions are used, measuring 3.20 mm (0.126 inches) in overall thickness (Figure 2).
Figure 2: 24-layer HDI test board construction.
Table 2: Material properties.
Board Pre-conditioning and Simulated Lead-free Assembly
Pre-conditioning profile used the parameters as follows:
1. Time above 217°C liquidus: Target 120 to 150 seconds.2. Target peak temperature: 260°C minimum +5°C/-0°C.3. Time within 5°C of max peak temp: 20-30 seconds.
Boards were not baked prior to reflow. After each cycle through the reflow oven, each panel was visually inspected to determine if surface material delamination was present. Table 3 provides an overview of the results of traditional cross-sections completed after both 3X reflow at 260°C and 5X reflow at 260°C.
Table 3: Cross-section check.
The novel material has passed the severe lead-free reflow 5 without any heat resistance defect indicated.
Cross-section of the Material
Figure 3: 24 layers, 0.8 mm BGA, reflow five times (no delamination).
Figure 4: 24 layers, 1.0 mm BGA, thermal shock 288°C, three times (no delamination).
The novel material could pass the severe lead-free reflow five times without any typical defects, which happens on high-Tg, FR-4 as delamination inside core or between pre-pregs, cigar voids, hole-wall separation, eyebrowing crack. Cross-sections of the material are shown in Figure 3 and Figure 4.
The horizontal cross-section of the novel material (Figure 5 and Figure 7) and high-Tg, FR-4 D (Figure 6 and Figure 8) have shown us the apple-to-apple comparison of the heat resistance and toughness performance.
Figure 5: 24 layers, 1.0 mm BGA, thermal shock 288°C, three times (no delamination).Figure 6: 24 layers, 1.0 mm BGA, thermal shock 288°C, three times (resin cracks).
Figure 7: 24 layers, 1.0 mm BGA, thermal shock 288°C, three times (no delamination).
Figure 8: 24 layers, 1.0 mm BGA, thermal shock 288°C, three times (wicking).
Table 4: Dielectric properties.
The test results above show that the novel material is a standard loss material.
Summary
This article illustrates how a novel thermoplastic resin-modified multifunctional epoxy system material shows an outstanding heat resistance, reliability and toughness performance with complex BGA design (pitch 0.8 mm, through-hole diameter 0.25 mm) on thicker (4 mm PTH board and 3.2 mm HDI board) PWBs, compared with currently available lead-free compatible materials. The dielectric properties of this material would be able to get Dk of 4.2 and Df of 0.015 under 1GHz based on SPDR method.
Reference:
1. Junqi Tang, Jie Wan, Xianping Zeng, “A High Heat Resistance and High Reliability Material for High Layer Count Printed Circuit Boards,” CCLA, Zhuhai China, 2011.