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Reliability of Embedded Thin-Film NiP Resistors
The ultra-low profile copper is used with the very thin 8 and 12 micron thick dielectrics of the combined resistor-capacitor material.
By Bruce Mahler, Ohmega Technologies, Inc., Culver City, CA
The use of embedded resistors in multilayer printed circuit boards is increasing at the same time the industry shifts to dielectric materials for lead-free assembly. The compatibility and reliability of embedded resistors in laminates formulated for lead-free applications is therefore of major concern to designers, manufacturers and users of RoHS compliant circuit assemblies.
The reliability of nickel-phosphorous (NiP) thin-film resistive alloys used as embedded resistors has therefore become a major issue, and includes such concerns as bond strength and surface topography modifications needed for acceptable adhesion to dielectric materials for lead-free assembly. In addition, there are concerns about the stability of embedded resistors that are subjected to the higher temperatures needed for lead-free assembly.
The electro deposition of the NiP resistive alloy onto the matte surface of the copper foil is carefully controlled to allow for a uniform, sub-micron thick coating of the resistive material. The NiP thickness uniformity is key to controlling the sheet resistivity nominal ohmic value and percent tolerance, and ultimately, the tolerance of the finished embedded resistors. The surface topography of the copper foil is critical to this deposition process. The more uniform the copper matte surface, the more uniform the resistive coating and the less variation there is in sheet resistivity. Through collaboration with the copper foil manufacturer, three different copper foil types were developed for use in making the of the resistive material: ultra-low profile, low profile and a modified low profile copper).
The ultra-low profile copper is used with the very thin 8 and 12 micron thick dielectrics of the combined OhmegaPly
resistor-capacitor material. The low profile copper is used in microwave and other high frequency applications where the resistive material is designed primarily as power dividers on PTFE substrates. Electrical performance, especially insertion loss, is critical in these applications and the NiP resistive alloy exhibits excellent electrical performance over a wide range of frequencies.
Another application for the low profile copper is with the 16 and 24 micron thick dielectrics of the combined OhmegaPly/FaradFlex resistor-capacitor material. The modified low profile copper was especially developed to enhance the bond strength of the NiP resistive material to dielectrics for lead-free assembly, polyimide-glass substrates and ceramic filled resin systems.
The modified low profile copper increases bond strength by increasing the surface area of the copper matte surface while maintaining a low relative peak-to-valley difference in the copper tooth. Accordingly, both the low profile copper and the modified low profile copper have similar matte-side surface roughness (about 190 Rmax micro inches for half oz copper). The modified low profile copper provides excellent adhesion to dielectrics for lead-free assembly. The peel strength of the NiP/Copper foil is the same, if not greater, than standard copper foils laminated to lead-free dielectric materials. This is a critical feature, especially with the finer resistor line widths being used in high-density module applications.
To test the reliability of embedded NiP resistors undergoing lead-free assembly conditions, a 10 layer DRAM memory circuit was built using both a traditional FR4 laminate as a control as well as a phenolic-cured modified epoxy laminate especially formulated for lead-free assembly. One layer of NiP embedded material was used in the design for 22 ohm series termination resistors. The data demonstrated excellent resistor stability at lead-free assembly conditions through more than 25 cycles.
Standardized testing was per IPC-TM-650, using the standard methods for bond strength and thermal stress testing. Resistors were also electrically tested after thermal shock. Stability was defined in terms of the percent change in sheet resistivity after cooling with a maximum allowed change in resistivity of 0.5 percent to 1.0 percent.
The FR4 control circuit was tested at both conventional leaded assembly conditions (260°C for 20 seconds) and the lead-free assembly conditions (288°C for 10 seconds). The embedded resistors were stable through conventional assembly conditions but failed after only 2 cycles at the more stringent lead-free conditions. Of particular note is how the embedded thin film NiP resistor stability can be used to give some indication of circuit integrity prior to any circuit failure. In fact, the FR4 based DRAM circuit did not demonstrate any outward sign of delamination, blistering or open copper traces when the embedded resistors failed. The resistor failures are indicative of internal structural damage due to resin decomposition or micro-delamination. This circuit, if designed without embedded resistors, would likely pass standard electrical tests only to potentially become a field failure due to its weakened internal structure. In this case, the embedded resistor instability is indicative of internal damage that may not otherwise be apparent. It is clear that standard reliability testing using thermal shock cross-sectioning is insufficient for printed circuit boards with embedded resistors. The actual value of the resistors must be tested after thermal shock.
An OEM user of the NiP embedded resistive material conducted a HATS (Highly Accelerated Thermal Shock) test to evaluate the reliability of the NiP resistors using a specially formulated multifunctional epoxy resin for lead-free assembly. A multilayer board design with embedded NiP resistor networks was subjected to a thermal cycle from -40 to +145°C. After 1,000 cycles, there was essentially no change in resistance of the embedded resistors.
Although the subtractively processed thin-film NiP resistive alloy has been used in production applications for over 30 years with excellent field reliability, changes in electronic packaging and assembly have necessitated the development of alternative copper foils of different surface topographies for specialized applications. The NiP alloy on the modified low profile product is one of these and has demonstrated excellent reliability in lead-free assembly applications.
For more information, contact: Ohmega Technologies, Inc., 4031 Elenda St., Culver City, CA 90232-3799
310-559-4400 fax: 310-837-5268 E-mail: email@example.com Web:
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