Circuit Board Laminates Research Articles

Control of ignition delay in microfabricated hot-wire igniters: Simulation, fabrication, and experimental results

Abstract The presented work studied thermal interactions between laminated, metal-foil, hot-wire igniters and exothermic solid-state, composite chemical systems in order to demonstrate precise timing control of a thermal ignition process. The study includes FEA modeling, device fabrication, and characterization to demonstrate control microthruster ignition delays to within 2 ms. The modeling included studies of total ignition delay as well as the ignition delay variation versus process variations. Microthrusters were then fabricated via printed circuit board lamination, and the ignition performance was characterized. The characterization showed agreement with modeling to within 2-sigma for most cases. And the characterization demonstrated that the ignition delay could be controlled to within 0.36 and 0.84 ms for the best case. Furthermore, this performance was demonstrated with a small battery supply (200–600 mAh) and minimal electronics in the ignition system. This work extends the use of current microthrusters to short-lifetime applications that need high forces delivered in millisecond time intervals.

Comparison of Printed Circuit Board Property Variations in Response to Simulated Lead-Free Soldering

Use of lead-free solders such as Sn/Ag/Cu results in exposure of printed circuit boards to higher temperatures during assembly compared with eutectic tin-lead solder. If the thermo-mechanical and electrical properties of the laminate materials get affected by exposure to this higher temperature, that may impact the performance and reliability of the circuit board. Variations, if any, in laminate material properties before and after board assembly should be considered in the selection of appropriate laminates. The board and system designers need to be cognizant of such variations and account for them during laminate selection for an application. This paper presents guidelines for laminate selection along with the process used to derive the guidelines. The process includes measurement of key material properties (glass transition temperature, coefficient of thermal expansion, decomposition temperature, time-to-delamination, water absorption, flammability, dielectric constant, and dissipation factor), and their responses to lead-free soldering assembly conditions. A range of commercially available FR-4 printed circuit board laminate materials, classified on the basis of glass transition temperature (high, medium, and low), curing agents (dicyandiamide and phenolic), flame retardants (halogenated and halogen-free), and the presence of fillers, are included in the measurements. The measurements are conducted in accordance with IPC-TM-650 test methods before and after exposure to multiple lead-free soldering profiles. The extent of variations in the properties due to lead-free soldering exposures are reported and analyzed as a function of classification parameters. The causes behind the variations in material properties are investigated by Fourier transform infrared spectroscopy analysis and a conjunctional property analysis. This study also suggests that the preconditioning steps specified in the IPC test methods should address the initial moisture content of the laminate test samples in material property measurement tests, otherwise significant errors can be introduced.

Dynamic Mechanical Analysis of Printed Circuit Board Laminates

Printed circuit boards must meet stringent requirements imposed by elevated temperature processes required for mixed-solder and/or Pb-free assembly. To meet these requirements, laminate manufacturers offer a variety of resin formulations, reactive additives, and glass styles designed to impart specific properties. Both the coefficient of thermal expansion (CTE) and the glass transition temperature (Tg) have received considerable attention with respect to design of high-temperature laminates. CTE mismatch between the copper and the laminate within a PCB results in stress upon the copper that may manifest itself as opens within vias, at the interfaces between internal lands and plated-through hole barrels, as well as open traces. Since the CTE of resin materials below the Tg is typically on the order of 5X lower than the CTE above Tg, a typical laminate design strategy is to produce a resin that exhibits a high Tg without adversely impacting other properties. Numerous factors affect the ultimate Tg of the resin, including the functionality of the monomer(s), crosslink density, the cure profile, and absorbed moisture. Within the electronics industry, Tg is determined via differential scanning calorimetry (DSC) as per IPC-TM-650. However, due to the multilayer construction of current circuit boards coupled with sample size limitations, DSC has been shown to be an inadequate technique for measurement of the glass transition temperature. The endotherm in the DSC is often ill defined, of marginal quality, and may be convoluted with stress relaxation and/or volatile outgassing at elevated temperature. Dynamic mechanical analysis (DMA) has been demonstrated to provide far greater information relative to not only the Tg, but also physical property depression due to moisture plasticization and incomplete resin conversion in various high-Tg laminate systems. Several case studies regarding phenolic-cured epoxy resins, cyanate ester/epoxy blends, and/or polyphenylene oxide/triallylisocyanurate blends will be discussed.

Effect of Lead-Free Soldering on Key Material Properties of FR-4 Printed Circuit Board Laminates

The transition to lead-free soldering of printed circuit boards (PCBs) using solder alloys such as SnAgCu has resulted in higher temperature exposures during assembly compared with eutectic SnPb solders. The knowledge of PCB laminate material properties and their dependence on the material constituents, combined with their possible variations due to lead-free soldering temperature exposures, is an essential input in the laminate selection process. This paper provides laminate selection guidelines that were arrived at by assessing key material properties (glass transition temperature, coefficient of thermal expansion, decomposition temperature, and water absorption), and their responses to lead-free soldering assembly conditions. A range of commercially available FR-4 PCB laminate materials, classified on the basis of glass transition temperature (high, mid, and low), curing agents (dicyandiamide and phenolic), flame retardants (halogenated and halogen-free), and fillers (presence or absence) were studied. The laminate material properties under investigation were measured as per the IPC-TM-650 test methods before and after exposure to multiple lead-free soldering cycles. Combinatorial property analysis was conducted to investigate the causes behind variations in material properties.

Modeling of Resin Flow in Reinforced Dielectric Prepregs

Manufacturing of printed circuit boards or chip-packaging substrates involves the use of resin-filled reinforcement materials, known as prepregs, to bond together laminates with patterned copper layers and serve as dielectric material. In the circuit board or substrate lamination process, the prepreg sheet is placed on top of the conductive copper patterns and pressure is applied to squeeze the resin out of the prepreg to flow and fill the gaps between the baseboard and the copper as well as drilled holes and vias. The primary processing requirement is for resin to fill all the gaps within reasonable time and pressure limits, before the resin cures to a hardened thermoset material. As the resin flow path may be nontrivial, it is desirable to model resin flow and filling of the gap as a function of applied pressure and lamination press closing rate so that one can successfully manufacture a variety of circuit board designs with different material systems. In this work, we model the flow during the filling of the gaps and justify noteworthy simplifications to provide a solution in closed form. This allows us to relate the material and process parameters such as prepreg thickness and applied pressure to the circuit board design. It also permits prediction of the transient development of gap filling. We illustrate the factors that influence the flow and fill process and discuss their importance. Finally, we analyze the process with typical material and processing parameters and compare it with laboratory scale and industrial experiments.

An introduction to high performance laminates and the importance of using optimised chemical processes in PCB fabrication

With the ever increasing demands for high performance electronic devices there is a need for circuit board laminates that have enhanced properties when compared to conventional materials such as the widely used epoxide‐based FR4 laminates. Equipment manufacturers require boards with better mechanical stability and improved electrical characteristics. At the same time, new environmental legislation is set to drive electronics assembly temperatures much higher as manufacturers start to use lead‐free soldering processes. The legislation is also raising questions about the long‐term viability of brominated resins as the basis for imparting flame retardancy to laminates. Fortunately, laminate manufacturers have responded to these challenges by developing and introducing a wide range of new laminates that address these issues. This paper describes some of these challenges and gives an introduction to the new high performance laminates that are finding increasing use. It also highlights the need for chemical processes used in the manufacture of interconnects with laminates to be specifically optimised for the chosen substrate material.

Fine Pitch Surface Mount Technology Assembly with Lead‐free, Low Residue Solder Paste

Fine pitch (0.4 mm) surface mount assembly studies were conducted with several lead‐free solder pastes formulated with both traditional RMA (∼6% residue level) and low residue (1%) flux vehicles. The lead‐free solder alloys evaluated included the two baseline eutectic binary alloys, Sn‐Bi and Sn‐Ag, and three new lead‐free solder compositions: (1)91.8Sn–4.8Bi–3.4Ag (wt%) developed at Sandia Laboratories, (2) 77.2Sn–20ln–2.8Ag (Indalloy 227) developed at Indium Corporation of America and (3) 96.2Sn–2.5Ag–0.8Cu–0.5Sb (Castin) provided by AIM, Inc. The basic physical properties pertinent to assembly performance (melting temperature and wetting behaviour) were determined for each of the new alloys. Assembly performance was assessed as a function of circuit board surface finishes, thermal reflow profiles and solder paste flux composition. The feasibility of 0.4 mm pitch assembly was established with each of the lead‐free solder alloys investigated. No issues particular to the combined use of low residue flux vehicles and lead‐free solder powders were identified. The circuit board laminates did not suffer any thermal degradation effects (reflow was performed in an inert atmosphere). All lead‐free solders, compared with the Sn‐Pb eutectic solder, exhibited reduced spreading on the circuit board lands after reflow. It was concluded that the performance of the new solder formulations is adequate for surface mount applications. Further differentiation among these solders will have to be based on their long‐term reliability performance. These studies are currently under way.

Method of making multi-layer metal core circuit board laminate with a controlled thermal coefficient of expansion: Hanson, J.R, Hauser, J.L., Kilfeather, J.F. and Hendriks, H.B. (General Electric Company, Schenectady, NY, USA) US Pat 4 522 667 (11 June 1985)

Method of making multi-layer metal core circuit board laminate with a controlled thermal coefficient of expansion: Hanson, J.R, Hauser, J.L., Kilfeather, J.F. and Hendriks, H.B. (General Electric Company, Schenectady, NY, USA) US Pat 4 522 667 (11 June 1985)