Abstract:
Thin film, multi-layered components wherein the layers are hermetically sealed with a re-flowed conductive sealant (e.g. Pb/Sn solder). The sealant is applied to an endless ground conductor at the peripheral edge of at least one of each pair of opposed substrate layers prior to registering the conductors and re-flowing the sealant. The microstrip conductors comprise thin film adhesion and seed layers and a covering metalization. The signal and ground conductors are terminated with solder balls and the signal and ground conductors are connected with micro vias that extend through the substrates.

Description:
BACKGROUND OF INVENTION 
     The present invention relates to packaging assemblies for electrical circuits and, in particular, to multi-layered circuit assemblies that are hermetically sealed with a reflowed solder seal at the interface between each pair of overlapping layers. 
     A variety of methods exist for creating thin film circuits, such as transmission line type delay lines, oscillators, filters etc., using strip lines and micro-strip conductors. Many of these circuits are constructed as multi-layered structures from conductive materials that are deposited, patterned, etched and formed on suitable substrate materials, for example, ceramic or polymer sheets. Circuit terminations are provided that are compatible with the printed circuit mounting. 
     A primary necessity of the foregoing and many other types of multi-layer circuitry is that they must be mounted in environmentally insensitive packaging. Coatings of conventional encapsulants or outer covers are provided to protect the circuitry from the ambient environment. Most assemblies use polymer encapsulants that are conformally coated or molded to each layer and/or a cover assembly to protect the physical and electrical integrity of the circuitry. Covers may also be secured around all or a substantial portion of the assembled packages. 
     Problems can arise, however, if humidity is trapped inside or penetrates the packaging. For example, moisture can be trapped in the assembly. Moisture can also migrate into the assembly, if the encapsulant does not seal to the assembly or if pinholes are present in the encapsulant. Circuit reliability is thereby compromised due to possible open or short circuit conditions that can develop. 
     The encapsulant also adds undesired mass and bulk to the component packaging. Advantageous assemblies of reduced sizes can be constructed if the encapspulant can be avoided. Process complexity can then also be reduced with consequent savings in processing time and improved yields. 
     Depending upon the circuit assembly, package size is also affected by the necessary input and output circuit terminations that are distributed around the peripheral surfaces of the assembly. The number of terminations can add extra size, especially for products having J-shaped and gull wing-shaped terminations. Lead terminations also introduce possible impedance mismatch and unnecessary signal loss due to the extra length and complicated connections to the internal circuitry. 
     The present invention was developed to provide hermetically sealed circuit assemblies that don&#39;t require a conformal or molded encapsulant. Instead, hermetic seals are formed between the substrates of the laminated layers and at any cover pieces with a re-flowable material (e.g. metal alloy solders or other materials compatible with the substrate material and/or conductive paths). The internal layers and circuitry are connected with through vias that extend through the substrates. In one preferred surface mount package configuration, the conductors terminate at an array of spherical or ball shaped solder terminations known as a ball grid array (BGA) at the bottom surface of the assembly. Circuit integrity is thereby insured and package size is reduced with the present hermetically sealed, multi-layer assemblies. 
     SUMMARY OF INVENTION 
     It is a primary object of the invention to provide hermetically sealed multi layered circuit assemblies that do not require separate polymer encapsulation at the individual layers. 
     It is a further object of the invention to interconnect the layers and transmission line conductors with vias that extend through the substrates. 
     It is a further object of the invention to provide a hermetic seal using a re-flowable metal alloy solder (e.g. a Sn/Pb solder) that is thermally re-flowed and compressed under controlled conditions of time, temperature and pressure in the presence of suitable gas compositions. 
     It is a further object of the invention to seal the spaces between the layers of a multi-layer assembly with aligned endless paths of an appropriate material around the peripheries of two opposed layers (e.g. metal or metal alloy rings) and coating at least one of the endless paths with a sealant, such as a solder, and re-flowing the sealant to bond with the opposed path. 
     It is a further object of the invention that the seal be electrically conductive, for example, serving as a ground connector for the component. 
     The foregoing objects, advantages and distinctions of the invention are obtained in multi-layered thin film transmission line assemblies wherein at least one endless strip conductor is deposited around the periphery of one substrate at the interface between each pair of adjoining layers. A metal alloy solder compatible with the strip conductors and surface of the opposite substrate is introduced to flow between the adjoining layers to seal the interstitial space between the layers. The opposing layers can provide one or more endless conductive paths that overlap each other or an opposite ground plane or other endless, non-electrical structure compatible with the sealant. 
    
    
     DESCRIPTION OF DRAWINGS 
     FIG. 1 is an exploded assembly view of a typical  2 -layer, hermetically sealed circuit assembly of the invention, 
     FIG. 2 is an inverted exploded assembly view of FIG. 1 showing a BGA termination array at the bottom surface of the assembly and the backside of the top layer. 
     FIG. 3 is a circuit diagram of the signal conductor deposited on FIG.  1 . 
     FIG. 4 is a typical flow chart of the assembly fabrication and sealing process used to achieve the hermetic seal between the circuit layers. 
     FIG. 5 is a notated graph (i.e. time, temperature and chamber gas/pressure) of the reflow process used to achieve the hermetic seal between the circuit layers. 
     FIG. 6 is an exploded assembly view of a typical multi-layer, hermetically sealed circuit assembly of the invention shown without cover or bottom layers. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIGS. 1 and 2, an exemplary two layer, delay line circuit assembly  20  is shown. The corresponding circuit schematic and circuit terminations  1 - 14  are shown at FIG.  3 . The assembly  20  is constructed of two layers, a ground layer  22  and a signal layer  24 . Patterned conductive paths at the layers  22  and  24  are formed over dielectric substrates  26  and  28 . A ceramic material, such as an aluminum oxide, is used to form the substrates  26  and  28  for transmission line structures. A polymer resin board substrate material, among other conventional substrate materials, may also be used. A number of layers  22  and  24  are typically formed on larger size pieces of substrate material and from which the layers  22  and  24  are typically diced with a dicing wheel or scribed with a laser beam, prior to breaking the larger substrate into the individual layers  22  and  24 . 
     A ground plane  30  substantially covers the top surface of the ground top layer  22 . An endless strip conductor  32 , shown at FIG. 2, circumscribes the periphery of the bottom surface of the layer  22 . The strip conductor  32  can exhibit any desired shape. A second strip conductor  48  can also be formed at the layer  22  in adjacent registry (e.g. concentric) to the conductor  32 . 
     The transmission line delay line circuit  34  of FIG. 3 is formed on the upper surface of the layer  24  in the shape of a conductive serpentine pathway  36  and includes several parallel windings  38 . The conductive pattern can exhibit an infinite number of configurations. The width and thickness of the transmission line pathway  36  is tailored in relation to the substrate material and thickness of the substrate material to provide a  50  ohm distributed impedance. The length of the transmission line pathway  36  controls the delay time. A second endless strip conductor  40  circumscribes the periphery of the pathway  36 . A number of ground buss bars or terminations  42  extend from the strip conductor  40 . Although FIGS. 1-3 depict an example of a two-layer delay line, longer delay times can be achieved by adding additional layers. The circuit diagram of FIG. 3 does not change when more layers are added to increase the delay time. Each additional transmission line or signal layer  24  is then isolated with an intervening ground layer  22  and the layers are interconnected with vias that extend through the layers. FIG. 6 shows an exemplary multi-layer oscillator circuit that can be constructed in accordance with the invention. 
     The bottom surface of the layer  24  supports a number of solder balls  46  that form the circuit terminations  1 - 13 . The solder balls are arranged as a ball grid array (BGA) and define a relatively small footprint versus other leaded type terminations, such as “J” or gull wing shaped terminations. Some of the solder balls  46  are appropriately connected to the end terminations  3  and  10  of the transmission line pathway  36  with micro-vias or holes filled with conductive materials that extend through the ceramic substrates  26  and  28 . The remaining solder balls  46  are separately connected with micro vias to the strip conductors  32  and  40 , ground buss bars  42  and ground plane  30 . 
     The ground plane  30 , conductive pathway  36 , strip conductors  32  and  40  and ground buss bars  42  are defined by a suitable conductive material (e.g. copper) that is plated over one or more thin film seed layers and an adhesion layer, such as NiCr, in conventional fashion. A variety of other conventional conductive materials can be used to equal advantage. All conductive depositions are suitably patterned by selective photo etching, plating and sputtering techniques. 
     FIG. 4 depicts the general sequence of process steps used to construct the circuit assembly  20 . First, the ceramic substrates  26  and  28  are drilled to form the micro vias through the substrates. The vias are located at the strip conductors  32  and  40 , the connecting pads at the ends of the transmission line pathway  36  and ground buss bars  42 . The substrates  26  and  28  are next etched with scribe lines using a CO 2  laser. 
     Thin film adhesion and copper seed layers are sputtered over both sides of the ceramic substrates  22  and  24 . A dry film photo resist is laminated over both sides of the ceramic substrates  26  and  28 . Patterned circuitry masks appropriate for each layer  22  and  24  are overlaid over the photo resist covered substrates  26  and  28  and exposed with UV light. Unexposed portions of the resist are developed away and copper is plated over developed pattern areas to a depth of approximately 25-30 um. The residue photo resist is removed and the substrates  26  and  28  are etched to remove all undesired sputtered thin film to leave only the desired copper plated conductor on the substrates  26  and  28 . 
     The copper plated strip conductors  32  and  40  are next plated with a sealant material, such as solder. The sealant material can be any metal or metal alloy solder or other re-flowable material that can form a hermetic seal. A high temperature tin lead, Sn/Pb, solder sealant is used at the assembly  20  that has a high lead concentration and that is re-flowed under controlled time, pressure, temperature, and gas flow conditions in a graphite holder or boat and suitable oven, reference FIG.  5 . 
     One or both of the strip conductors  32  and  40  can be coated with a re-flowable sealant. When the registered solder coated conductive strips  32  and  40  are exposed under suitable conditions, the solder re-flows and a hermetic seal is formed at the opposed surfaces of each pair of layers. Although single bands of strip conductors  32  and  40  provide a sufficient seal, any additional conductive bands or rings  48  can augment each seal. The strip conductors  32 ,  40  and/or  48  may also directly contact a ground pane, if the conductor height is deposited to provide a sufficient separation from any intervening conductors. 
     The prepared substrates  26  and  28  are next broken into pieces at the scribe lines. The solder balls  46  are next deposited. 
     The substrate layers  22  and  24  of a number of circuit assemblies (i.e. 96 parts) are then collated and aligned in proper registry in a graphite support fixture or boat. A cover of the boat includes a number of portions that are spring biased to apply a 1-pound force on each layered stack. The boat is mounted in a suitable chamber and coupled to an electrical source. Current is passed through the boat that acts as a resistor and provides proper heating. The solder plating at the overlapping solder covered strip conductors  32  and  40  is re-flowed and fused. The particular time, temperature and gas compositions used in the re-flow step are shown at FIG. 5 
     With attention to the notated graph of FIG.  5  and after mounting the boat in the chamber, the chamber is evacuated and briefly purged with a suitable forming gas (e.g. 5% H 2  and 95% N 2 ). Current is applied to the boat at the start of a second purge sufficient to heat the boat at a rate on the order of 2.25-2.32° C./Sec. When the parts reach approximately 400° C. a continuous flow of forming gas is admitted to clean the exposed copper surfaces at the seal. Heating of the stacked layers is reinitiated at a rate of approximately 1.75° C./Sec to approximately 490° C. and the solder re-flows to the seal paths. After a suitable time, the cooling process is initiated. That is, current is disconnected from the boat and at approximately 10 minutes into the cycle, the exhaust and nitrogen gas ports are opened. Continuous streams of nitrogen and forming gas cool the parts over the next 5 minutes. 
     During the re-flow step of FIG. 5, the layers  22  and  24  at the layered circuit assemblies  20  are hermetically sealed together by re-flowing the solder sealant at the registered and overlapping strip conductors  32 ,  40  and/or  48 . The seal encloses the transmission line pathway  36  in the interstitial space between the layers  22  and  24 . The ground layer  30  and ground buss bars  42  are also connected to the strip conductors  32  and  40  at the micro-vias that extend through the ceramic substrates  26  and  28 . 
     Continuing with the flow chart of FIG. 4, the hermetic seal of each assembly  20  is next verified with MIL-Std based bubble test or dye penetration tests. Each assembly  20  is then tested for proper electric performance, inspected and final packaged such as on a reeled tape. A separate hard cover may also be mounted to each assembly  20 . The cover may be bonded to a suitable strip conductor  32  or  40 . 
     FIG. 6 shows another multi-layer circuit assembly  60  that is defined by several signal layers  62 ,  64  and  66  and intervening dielectric layers  68  and  70 . Solder plated, endless strip conductors  72  are formed on the layers  62 - 70  to appropriately register with each other and ground planes  74 ,  76  and  78 . Numerous micro vias  14  are provided at the layers to interconnect the patterned signal conductors  52 ,  54   a,b ,  56 ,  58   a,b  and  59  and ground layers  74 ,  76  and  78  with the sealed strip conductors  72 . During the solder reflow step, the strip conductors  72  bond to each other or the opposed ground plane  74 ,  76  or  78  to achieve the desired hermetic seal between each layer. 
     While the invention has been described with respect to presently preferred transmission line circuit assemblies, still other hermetic circuit assemblies can be constructed in accordance with the described invention. The following claims should therefore be construed to include all those equivalent embodiments within the scope thereof.