LCP PCB-based Packaging
10/01/2006
For High-performance Protection
BY LINAS JAUNISKIS, BRIAN FARRELL, ANDREW HARVEY, Foster Miller, Inc. AND SCOTT KENNEDY, Rogers Corp.
avorable electrical properties have made liquid crystal polymer (LCP) an optimal electronic substrate and packaging material for high-frequency circuit boards. Injection molding and associated techniques, such as co-molding and over-molding, are relatively mature technologies as well.
 Figure 1. LCP PCB packaging platform hypothetical structure. |
Using a LCP PCB platform approach, fiber-optic transceiver, micro-fluidic chemical sensor, and GaAs, MMIC components have been prototyped to demonstrate the value proposition of this LCP material system and PCB-based architecture. Multiple functions such as optical I/O, micro-fluidics, high-frequency electronics, digital-control electronics, MEMS sensors, and thermal management are integrated with injection-molded lid assemblies (Figure 1).
While this approach provides a route to highly integrated, high-performance components and modules, a common concern with organic packaging is reliability, particularly with respect to moisture ingress, as compared to metal and ceramic packaging.
The LCP PCB-based platform has demonstrated functionally hermetic performance with respect to moisture ingress. Theoretical and empirical analysis shows LCP PCB-based test vehicles maintain internal moisture content well below the MIL-STD-883-1018-allowed 5000-ppm level after exposure to 1000 hours of 85oC/85% relative humidity (RH).
Much of the technology maturation involved with deploying LCP PCB-based technology are issues with process and materials compatibility in complex assemblies. Therefore, various lid sealing techniques were refined, together with leak testing protocols. Plastic welding processes are used to join plastic lids to LCP PCB substrates. Solder-sealing of cavity packages is accomplished straightforwardly with metal lids, while injection-molded lids require appropriate metallization and soldering processes. Chip I/O interconnection processes, such as wire bonding and flip chip, need to be adapted to the LCP material system through proper materials selection and process development.
In working these three prototype development efforts, a number of unique practical design and execution advantages of this LCP PCB packaging and interconnection approach have been defined. First of all, the LCP material system is lighter than ceramic and metal packaging simply by virtue of material density, with LCP at 1.4 to 1.6 g/cm3 versus alumina at 3.97 g/cm3 and Kovar at 8.36 g/cm3. LCP PCB dielectrics come in thicknesses down to 1 mil; compared to a minimum thickness of 4 mils in low-temperature co-fired ceramic (LTCC), the LCP PCB dielectrics facilitate thinner multi-layer circuit boards.
In production, injection molding can produce complex, tight-tolerance parts for lid assemblies at lower cost than machined-metal counterparts. The design of system-in-package (SiP) modules with this LCP PCB-based approach is less inhibited by physical and cost constraints as compared to traditional metal and ceramic approaches. Lastly, a multi-layer PCB design cycle is simpler, with shorter lead time and lower cost than a comparable iteration in LTCC. While injection molding has a cost/lead time barrier to entry associated with the tooling, this can be mitigated with the use of moldflow analysis and rapid prototyping to facilitate efficient part design. These advantages indicate that LCP PCB is suited for SiP applications where high performance, small form factor, and cost are concurrently critical factors.
Material Permeability and Hermeticity
Based on its permeability properties, LCP has grown as an alternative to traditional metal and ceramic packaging materials for cavity packages. LCP low moisture and oxygen permeability data, as compared to other organics, justifies LCP use as a “near-hermetic” or “quasi-hermetic” packaging material.
Figure 2 shows the calculated time to reach 5000 ppm moisture through a 0.04" thick barrier with 85°C/85% RH exposure based on steady-state permeability of various film materials. This shows that in normal film geometry, LCP alone cannot attain 1,000-hour resistance, while composite structures of LCP with copper foil can. In a laminated PCB format, LCP films can be arranged together with such copper-foil ground planes so as to allow only lateral ingress through the laminate, not normal to the film. By applying proper design approaches, LCP PCB-based cavity packages can provide sufficient resistance to moisture ingress to pass 1,000-hour 85°C/85% RH exposure tests.
Model Calculation
Most data for moisture permeability of materials is presented in a test arrangement where a relatively large area (about 3" diameter) of relatively thin film (a few mils) presents a barrier between wet and dry sides. The transfer rates are allowed to equilibrate, and steady-state permeability is measured. This procedure ignores the transition period before equilibrium, which is short for relatively thin, large-area films.
 Figure 3. Normal film and lateral laminate geometries for moisture ingress. |
Cavity packages were designed using the LCP PCB-based packaging approach where moisture ingress occurs only laterally through the dielectric. The lids used for this experiment were solid copper so that moisture resistance performance of the LCP PCB could be evaluated. Figure 3 illustrates the normal film and lateral laminated moisture ingress geometries. The test vehicle for this design approach consists of 4-mil LCP with half-ounce copper cladding. The back side is a continuous copper ground plane. The top side is patterned with a 0.25" wide, 1.5" diameter annular solder ring. A 1.5" diameter and 0.25" deep copper lid is soldered to the solder ring with SnAg solder.
In this arrangement, multiple factors work to prevent moisture ingress. First, the permeability of a barrier is proportional to the thickness and inversely proportional to the area of the barrier in the direction of gas migration. This simple change in geometry has a significant effect. Assuming steady-state film material permeability, this barrier would allow the inside of the cavity to reach approximately 500 ppm after 1,000 hours of 85°C/100% RH exposure. This is already well below the 5,000 ppm benchmark.
The second factor is that the transit time - the time required for the first water molecule to traverse the barrier from the wet side to the dry side - is no longer relatively negligible as it would be for normal film permeability geometry. The transport of moisture through barriers can be calculated using Fick’s law. Exact solutions to differential equations are cast in graphical form from which transit time can be extracted. To extract estimates for transit time, diffusivity of the barrier material is required, which is calculated from permeability and solubility. Using this approach for our test vehicle, we calculate 1,700 hours for the transit period with exposure to 85°C/85% RH .
This calculation implies that the cavity will not begin to see an influx of moisture until 1,700 hours elapse in 85°C/85% RH. There will then be an equilibrating period as permeability increases as moderated by the balance between the fixed wet side moisture density and increasing dry side moisture density. Making a conservative assumption that permeability is taken as the steady state value, an additional 1,000 hours at 85°C/85% RH would only increase internal moisture density to 500 ppm.
Experiment
The test vehicles were vacuum baked, then sealed in a nitrogen atmosphere of <50 ppm moisture. Devices were helium bombed, then fine- and gross-leak tested. Together with the LCP test devices, metal cavity controls were fabricated. These did not have a patterned top layer, but a continuous copper foil layer with a soldered copper lid, resulting in an all-metal cavity with no moisture ingress path. Devices that passed leak tests were divided into various groups. Zero-hour controls (test vehicles and metal control devices) were sent to an outside testing service for internal vapor analysis (IVA) to measure moisture content in the cavity to confirm fidelity of the sealing process. Remaining test devices were put in an environmental chamber at 85°C/85% RH. At intervals of 172 hours and 1,000 hours, LCP test and metal cavity controls were pulled and sent for IVA. Table 1 shows the IVA results. Most of the data points were reported by the test service as <100 ppm, below the measurement sensitivity of the test, with the balance primarily nitrogen and trace amounts of other gasses.
 Table 1. Test group results for exposure to 85°C/85% RH. |
Helium fine-leak results are plotted in Figure 4 for 0-, 1-, and 6-hour ambient dwell times after helium bomb. The plot also includes optical leak test data taken effectively with no ambient dwell time. It is presumed that these organic materials absorb helium and register a high leak rate initially, which, over time, outgasses rather than absorbing helium or transmitting it through the material. The helium fine-leak test data reflects this behavior, being initially at high levels and over a number of hours falling to lower levels. MIL Standard 883 Test Method 1014 allows a maximum time between release from helium bomb and fine-leak test of 1 hour, hence this behavior complicates helium-based leak testing. But optical leak testing confirms that the cavities are below the 5×1-8 level independent of an outgas period. Therefore, the LCP cavities pass the 5×1-8 leak criteria, surface adsorption of helium on the LCP confounds the mass spectrometer helium-based fine-leak testing, and optical leak testing avoids this problem.
 Figure 4. Helium fine-leak test compared to optical leak test results. |
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Conclusion
The calculations of moisture ingress laterally through a laminated LCP structure and empirical data of such test vehicles corroborate that LCP PCB-based packaging can functionally meet hermeticity standards for moisture ingress. The Fick’s law calculation estimates that even the transient time for moisture to traverse the laminated barrier is in excess of 1,000 hours at 85°C/85% RH. Our 1,000-hour 85°C/85% RH test vehicles measured below the detection limit of 100 ppm, supporting the calculations. Helium fine-leak test data, with sufficient ambient dwell to allow surface-adsorbed helium to outgas, also meets the MIL Standard rates of <5×10-8 atm cc/sec He. Optical leak testing shows 5×10-8 performance without an outgas period.
LCP PCB-based packaging provides functionally hermetic performance with respect to moisture ingress as well as helium leak testing for cavity packaging applications. Additionally, LCP’s unique combination of electrical properties, the ability to be precision injection molded and metallized by various techniques and various part joining techniques, makes it a valuable packaging platform for cavity package applications, particularly where a complex package envelope is required for high-performance and cost-sensitive applications.
ACKNOWLEDGEMENTS
This work sponsored in part by the United States Army Aviation and Missile Research Development and Engineering Center, Contract W31P4Q-06-C-0084 and US Naval Air Systems Command through Penn State EOC, Contract N00421-03-D-0044 Delivery Order 01.
REFERENCES
Contact the authors for a complete list of references.
LINAS JAUNISKIS, technology manager, BRIAN FARREL, assistant group director, and ANDREW HARVEY, senior staff engineer, may be contacted at Foster Miller Inc., 350 Second Ave., Waltham, MA 02451; 781/684-4139; [email protected]. SCOTT KENNEDY, senior engineer, may be contacted at Rogers Corp. Lurie R&D Center, One Technology Drive, Rogers, CT 06263; 860/779-4769; [email protected].