Military Grade PCB

Project Detail:

Material：FR4+Rogers 4350B

10 Layer 1.6mm PCB

Line width/space:4mil/mil

Min hole:0.2mm

Military is one sectors that is highly demanding when it comes to their applications. The systems or devices that are used in these sectors have to be of superior quality. The printed circuit boards developed at RayPCB fulfill such quality requirements and thus they are chosen for various applications pertaining to the military sector.

Military Grade PCB require high reliability with long service times under extreme conditions. We produce military PCBs for aviation, ground activities, defense, naval applications, and space weapons. RayMing provides printed circuit boards that offer a wide range of materials, composites, and construction, that are very effective in military applications.

High temperature laminates, copper and aluminum substrates can perform very well in these challenging environments. Anodized aluminum can be utilized to eliminate the effects heat induced oxidation. Aluminum substrates combined with thermally conductive pre-pregs gives the military designer performance with weight advantages.

The following represents a few military applications:
Over horizon enforcement networks (Cyber counterintelligence systems)
Cross targeting systems (Cryptanalysis systems)
Automation networks (Command and control systems)
Air cartographic photography (Automatic search jamming systems)
Radio communication systems (AMRAAM – Advanced Medium Range Air to Air Missiles)
Radar installations (APU – Auxiliary Power Units)
Power converters (ASRAAM – Advanced Short Range Air to Air Missiles)
Power supplies (AWACS – Airborne Warning and Control System)
LED lighting systems (AEW&C – Airborne Early Warning & Control)
Instrumentation (CIWS – Close In Weapons Systems)
Control tower systems (ECCM – Electronic Counter-Countermeasures)
Temperature sensors (FLIR – Forward Looking Infrared)
Audio interface applications (IADS – Integrated Air Defense System)

All about the Design Rules and Specifications for Military PCB Manufacturing

Printed circuit boards have diverse applications owing to their significance in electronics. Key among the application areas include the military sector. However, you may know very little about military-grade PCB due to the nature and sensitivity of this unique sector. So what does it all entail? This article aims at answering all the potential areas of significance you may want to understand about military manufacturing and the military PCB. This is the only article about military-grade capacitors which provides an overview of the PCB industry for military use. It reveals how military prototypes and PCBs have been designed and manufactured with the special requirements that this industry demands. This is a necessary read for anyone interested in designing, manufacturing, or servicing PCBs manufactured for defense applications.

Military-grade components require a very high degree of quality control and rigorous tolerances, which may not be possible to achieve using conventional manufacturing methods. The military PCB industry has evolved to produce components and boards for demanding applications. This section describes state of the art in military PCB manufacturing. In addition to discussing the basics of PCB manufacture, we will also discuss factory layout and design considerations.

The Military PCB

Military applications need printed circuit boards. However, the harsh environment that characterizes the military requires a highly reliable PCB. The typically printed circuit board is unheard of to work in an environment with constant contact and interaction with contaminants, extreme temperatures, and chemicals without problems. It is where the military printed circuit board comes to play.

The military PCB has to meet or exceed the requirements and specifications set by the respective military. Because of this, such printed circuit boards are of diverse types based on the requirements and the specific application area specified by the military. Consequently, the type of military PCB will determine the board specifications. Additionally, you have to consider specific design features from conceptualization all through the manufacturing process.

Additionally, getting the right military PCB manufacturer becomes important. The company will source the materials and components desired for the military PCB design and fabricate and mass-produce the PCB according to your needs. Remember, military-grade PCBs require a high level of quality and reliability, and picking a top manufacturer starts the process.   

What better entity to cater to your PCB military manufacturing needs than us at RayMing PCB and Assembly? The company guarantees top-quality military printed circuit boards, including aviation, defense, space, ground activities, and naval military equipment. What’s more, the company offers a range of composites, materials, and construction, which are effective in almost every military application.  

Designing a Military Printed Circuit Board

The conceptualization and design of the PCB often represent the first step in the entire production process of circuit boards, and as such, not any different from a military PCB. It includes diverse aspects that you must consider, including the type of military PCB. So what are these considerations?

• Board materials. It is an instrumental part of a military PCB design as you need to ascertain that every material is of a military-grade standard and specification.  Materials used for developing military PCBs need to prove heat resistant because of the application of the PCB in harsh weather conditions. Additionally, the deployment of thermal compounds becomes crucial in aiding heat dissipation.

Remember, a printed circuit board needs non-conductive and conductive materials to work. As much as the material considerations need adhering, it should happen in line with base material needs (ceramic or FR-4, comprising glass-reinforced epoxy) and the conductive copper layer. Since most military PCB are high-performance circuits, the base materials will comprise mainly ceramic or PTFE bases.

• PCB stack-up. In this case, the printed circuit board stack-up infers to the layer number that a military PCB will need. It is crucial to determine this in the design phase to help you understand the necessary impedance for each layer, besides the correct fabrication process to pursue. Such determinations help figure out the anticipated yield, minimize the cost of production, and ensure reliability.
• Copper traces. A printed circuit board circuit’s copper traces are important in transmitting electrical and other types of signals in the PCB. It is vital to comprehend the attributes of the copper traces besides its limitation during the design phase.
• Via types

The type of via becomes instrumental in connecting the diverse military components in the military PCB. It includes through-hole, blind, micro, and buried vias. However, all top-quality PCBs have a reduced number of vias. Additionally, the current carrying capacity of the via becomes a crucial factor to consider when designing the PCB.

• Picking components. A military PCB contains not only one military component but a myriad of them. Consequently, you will have to factor this in your design requires diverse types of military component types. It becomes crucial to pick the correct components for your military PCB to reduce errors and ensure high standards of PCB quality.
• Design rules. It becomes essential to consider the design rules post the considerations already elucidated. Here, aspects such as component placement, DFM, layer allocation cross-talk budgets, and flight time should get considered. Other components that will work in close correlation with the PCB, such as a military grade capacitors, need to be factored in the military PCB design.
• Power and signal integrity

A quality military printed circuit board possesses sound power and signal integrity. For this aspect, key considerations should include the signal rise and signal fall times, characteristic impedance, rack lengths, and drive strength. However, the signal integrity simulation has to get underway before and after the layout for a quality performance guarantee. Additionally, the power distribution’s DC and AC performance needs to get considered as well.         

Prototyping a Military PCB

Military printed circuit boards, like other PCBs, need a prototyping phase. It should include the following process steps.

• Design. It represents an important step, and as discussed above, sets the tone for the prototyping and production process of the military PCB. Here, it becomes pivotal to deploy a relevant printed circuit board design software to make your prototype. Additionally, it is crucial to indicate the design software used when sending the military design file to your prototype contract manufacturer.  
• Schematic design. The stage encompasses describing useful information that the engineers will use to prototype your military printed circuit board. It is crucial to detail the materials, military components (PCB), component placement, and the circuit board’s function. Additionally, the panel and grid size needs to get picked at this stage.

While acting as a segment of the initial design phase, this stage also encompasses checks through simulations to ensure the accuracy and functionality of the military PCB design.  

• Bill of materials. It is crucial to develop a listing of all the components and materials required in the prototyping process. It is especially pivotal when trying to self-sourced for the military PCB materials. The bill of materials also becomes crucial for the contract military PCB prototype manufacturer, especially in sourcing your printed circuit board materials. It enhances your military PCB’s accuracy per the schematic. A BOM encompasses details like:
• Value. It specifies the specifications in units of the component. It includes units like ohms, etc.
• Quantity. Encompasses the military PCB components needed
• Reference designators. It covers codes deployed in the identification of certain parts
• Footprint. It is a precise location of particular components on the printed circuit board prototype.
• Manufacturer parts number. The number indicates the part’s number according to the component manufacturer.

Upon finalizing the design schematic and BOM, a check to ascertain that you have all the applicable parts becomes necessary. To enlist the assistance of an expert like a component engineer in ascertaining the parts selection vis-à-vis their suitability to the budget, military PCB prototype design, and size requirements.

• Routing design. It is the next step and entails the design of the military PCB prototype routing. It gets accomplished by deploying traces to link or connect diverse PCB components. Additionally, it becomes useful to remember the diverse factors that impact the routing approach. It entails signal noise generation, noise sensitivity, and power levels.

However, you have to note that most PCB designs use the netlist information to plan the military printed circuit board routing. Using a software program, it becomes simple to automatically compute the optimal routes of the printed circuit board using aspects like layer number, etc. However, it can take a long time, especially when the military PCB proves larger than the standard ones besides having a lot of components.

• Checks. The fabrication phase can always feature plenty of functionality issues. However, checking the PCB prototype design constantly while undergoing the prototyping phase reduces these issues. Some of the standard printed circuit board problems you must factor in include thermal issues such as heat spots. It is also crucial to maintain the military printed circuit board at consistent temperatures. Therefore, the need to address all arising issues from the design attributes such as thermal paths, copper thickness, the large printed circuit board size, layer number, and inconsistent temperatures.  
• DRC (Design Rule Check). Besides the DRC, you should also do the LVS (layout-versus-schematic), an antenna check, and ERC (electrical rule check). Additionally, other manufacturers conduct other QAA (quality assurance assessments). Upon the conclusion of this step, you can submit the military printed circuit board prototype design for fabrication.

While this process step ends the design and testing phase, the next phase entails the fabrication stage.

• Photo film development.  Here, RayMing PCB company experts like our engineers will deploy your military prototype design in developing a photo film consistent with your design needs using a printer or plotter. The printer ensures the printing of the military PCB layers and solder mask. Remember, the film often refers to printed photo negative plastic sheet of the board. It marks the conductive copper sections and the non-conductive sections of the military PCB prototype.  
• Printing the inner layers

It features a stage where a copper layer gets applied to the military grade PCB substrate material. The process commences with the pre-bonding process of the copper on the substrate before applying the photoresist. The photoresist often comes as a photosensitive film, which hardens when exposed to UV light. Additionally, blocked sections (often with black ink) stay soft. The last sub-step entails the removal of the soft photoresist while leaving the hardened sections to shield the copper. It is then followed closely with removing the hardened part or UV-exposed photoresist to expose the copper layers in the designated spots of your military PCB design.

• Aligning the military PCB prototype’s layers

Most military PCBs have multiple layers, and this can exceed ten layers to twenty-eight layers. In such instances, it is prudent to align these layers accurately before proceeding with the precise punching of registration holes. It is significant to ensure accuracy in the alignment process to prevent revision of the process, especially in the inner layers, which proves a tough ask post the fusion of the layers.

• Fusing the Military PCB layers

It encompasses the fusion of Prepreg and the original substrate, often clad with copper foil traces. The process step entails two steps. Firstly, the layering up phase, before the bonding phase ensues.

Fusion of the layers begins by putting the Prepreg layer atop the alignment basin. The substrate layers then get stacked before incorporating the copper sheet and another Prepreg layer. Atop the Prepreg sheet, the incorporation of another aluminum layer gets underway before topping it all off with the copper press plate. After the incorporation of the layers, everything gets fitted into the pin-attached steel table.

After this, a computer-controlled bonding press ensues. It heats the PCB stack, applies pressure before cooling it down afresh. After that, the pins get removed to uncouple the pressure plate before unpacking and obtaining your military PCB prototype.

• Drilling holes. It is prudent to note that the process continues, and at this point, you have to drill holes into the stack. It is instrumental whenever you begin adding the components. However, precision becomes key, and computer-controlled equipment eliminates the hustle you would otherwise have to go through.   
• Copper plating. It encompasses the incorporation of a copper layer on the military PCB surface through a chemical bath phase. The copper acts to shield the entire panel, including the hole’s interiors. It is also controlled by programmed machines to increase and ascertain precision.  
• Outer layer imaging. It follows the copper plating process closely. It entails applying another layer of photoresist on the printed circuit board panel, which allows for the outer layer imaging of the military PCB prototype. Remember, the process proves similar to the already discussed process of creating the inner layer’s inversion.  
• Tin and copper plating

It follows the outer layer imaging process step and entails the addition of another copper layer. However, unlike in earlier instances, the photoresist will ensure the copper deposition exclusively on the anticipated PCB board sections. After that, the military PCB board receives a round of tin plating to protect the copper layer in subsequent steps.

• Final etching. It encompasses the removal of any excess copper by utilizing a chemical process reaction. However, the tin plating protects the fundamental copper traces within the conductive sections. Hooray! You will have successfully established your PCB’s conductive connections.
• Applying the solder mask

The process entails the panel or board cleaning before subjecting it to the solder mask application. It then passes through a hardening phase through exposure to UV radiation to toughen up the solder’s photo film. Elimination of undesired soft parts gets carried out at this point.  

• Surface finish application

It encompasses the deposition of extra gold or silver plating. What’s more, the process also involves the hot-air leveling of the pads to ensure uniformity.

• Applying silkscreen

The military PCB surface gets a silkscreen through ink-jet writing, ensuring the proper conveyance of vital information concerning the printed circuit board.  

• Cutting. PCBs often get extracted from a larger panel at this point. To ensure this, it has to get cut from this panel using a router or v-groove cut. However, before you extract the PCB from the larger panel, please ensure that it passes the electrical test, and its functionality is as stipulated in the design. After this, pop the segment cut to obtain your PCB board.  
• Sourcing. Preparing for the assembly process requires sourcing crucial military PCB components, something you can achieve either by relying on the PCB prototype solutions company or by yourself. The bill of materials becomes crucial as it guides the solutions provider with the sourcing of the military PCB components. What’s more? The BOM will ensure that the components sourced are of satisfactory quality according to your military PCB design.   
• Assembly. It closely follows the component sourcing stage immediately after it finalizes. It encompasses attaching the necessary components to your military circuit board.  
• Stenciling of the solder paste. The process step entails the solder paste application onto the circuit board. It combines the solder with the flux and proves helpful in melting the solder and bonding it with the circuit board. In this case of stainless steel, the stencil gets placed on the PCB to permit the solder paste application, especially in precise areas where components will get stationed. The stencil ensures the even spread of the solder paste, after which it gets removed to expose the even and correctly applied solder paste.  
• Pick and place. It ensures the perfect placing components on the prototype board’s surface through a mechanism of picking and placing, often aided by a computerized machine. It also ensures the placement of the non-connector parts on top of the solder paste in the designated spots.
• Reflow soldering. It proves pivotal in attaching the surface-mount components to the military PCB. The process gets carried by placing the circuit board on a conveyor belt before passing it through an oven (reflow). Reflow ovens have heaters that steadily and slowly heat the circuit board to about 480 degrees Fahrenheit. It is essential as the heat ends up melting the solder before cooling and solidifying the solder, which ensures the permanent placement of the SMDs to the military PCB.  It is vital to use a stencil before carrying out the reflow process independently for double-sided printed circuit boards.
• Quality control and inspection

Accidental movements at times happen during the process of oven reflow, which has the untidy outcome of poor connectivity, electrical shorts, or a lack of connection. Therefore, it is prudent to ascertain that the circuit board possesses no errors after completing the reflow phase. It gets carried out through manual checks, automatic optical inspection, and x-ray inspection. Additionally, checking by testing the functionality and quality of the PCB is crucial.

• Slotting in through-hole components

Some military prototype boards require more components aside from the SMDs. It entails the plated through-hole parts, which get plated all over to improve transmission and receptivity of electrical signals, either side of the board. However, please note that solder paste will not work on PTH parts as it runs through the PTH without adhering. It, therefore, requires manual soldering or wave soldering, where the circuit board atop a conveyor passes through the oven to apply molten solder on the board’s base. The process ensures the simultaneous soldering of the PCB pins. However, it lacks effectiveness when it comes to double-sided boards.  

• Functionality tests

The last stage of the prototyping process entails a myriad of tests to ascertain the functionality of the military PCB. Here, the board gets tested under the respective simulated setting mimicking the anticipated operating environment of the military PCB, once mass-produced and deployed for use.

Military PCB Manufacturer

Mass manufacturing and assembly of military PCBs come after the prototyping stage. However, it becomes essential to pick an ideal manufacturer for your military PCB needs. Some of the standard considerations you should take into account before picking a military PCB manufacturer include the following.

• The military PCB manufacturer needs to have a solid industry reputation. It includes having a demonstrated history and experience of sourcing for military-grade components and materials and producing quality military PCBs. Additionally, the manufacturer must demonstrate a history and reputation of delivering on time and honoring its contractual obligation.
• The military PCB manufacturer also needs to have a simple and easily accessible system for making military PCB orders and submitting Gerber files.
• The manufacturer will also have to possess top-of-the-range manufacturing equipment and the latest technologies besides skilled personnel to handle your military PCB needs.

What are the Applications of Military Printed Circuit Boards in the Present-Day World?    

The military deploys the military printed circuit boards in a myriad of ways. Some of the vital applications of the military PCB include the following.

• Unmanned vehicles
• Defense navigation systems
• Robotic systems
• Field computers
• Artillery and motors
• Communication systems
• Security equipment
• Satellite subsystems

Final Thoughts

Military PCB proves crucial in the endeavors of the military to safeguard the national security interests of any country. Because of this, anyone who needs a military-grade PCB has to understand what it all implies, the design, prototyping, and production process, besides the ideal manufacturer for these important electronic pieces.  We hope that the article has addressed some of the pertinent areas you had plenty of problems with regarding military PCBs.  

Understanding Hybrid Circuit Boards

Definition and Basic Concept

A hybrid printed circuit board (PCB) multilayer is a specialized PCB that incorporates dissimilar materials to optimize electrical performance and enhance system reliability, particularly for high-frequency RF applications. The primary challenge in manufacturing such PCBs lies in managing the differing coefficients of thermal expansion (CTE) among the materials used, both during the fabrication process and component assembly.

These designs often combine FR-4 material with PTFE laminates, enabling designers to integrate RF functionality and digital circuitry on a single PCB. This integration not only reduces the device’s footprint but also lowers overall costs. Leading laminate manufacturers, such as Isola, Rogers, Arlon, and Taconic, have extensively documented the properties and performance of their materials, providing valuable technical insights for designers.

In the sections that follow, we will highlight key capabilities we offer and outline potential pitfalls to avoid when designing RF hybrid PCBs. Leveraging our extensive experience and investment in advanced production technologies, we are equipped to deliver the most comprehensive circuit board manufacturing services for these complex products.

Key Characteristics

The primary characteristics that set hybrid circuit boards apart include:

1. Combination of technologies: Hybrid circuits integrate different types of electronic components and manufacturing techniques within a single package.
2. Customization: They offer greater design flexibility, allowing for tailored solutions to specific application requirements.
3. Miniaturization: Hybrid circuits often achieve higher component density than standard PCBs.
4. Performance: In certain applications, hybrid circuits can offer improved electrical performance due to their compact design and specialized components.

Components of Hybrid Circuit Boards

Substrate

The substrate serves as the foundation for the hybrid circuit board. Common materials used for substrates include:

• Ceramics (e.g., alumina)
• Glass
• Metal-core PCBs

The choice of substrate material depends on factors such as thermal management requirements, electrical properties, and cost considerations.

Conductors

Conductors in hybrid circuits are typically made of thick-film or thin-film metal layers that form the circuit paths. Common materials include:

• Gold
• Silver
• Copper
• Platinum-gold alloys

The selection of conductor material impacts the circuit’s electrical performance and reliability.

Passive Components

Passive components in hybrid circuits can be either printed directly onto the substrate or attached as discrete components. These include:

• Resistors
• Capacitors
• Inductors

Printed passive components offer space savings and improved reliability, while discrete components may be used for higher precision or specific performance requirements.

Active Components

Active components in hybrid circuits are usually semiconductor devices such as:

• Transistors
• Diodes
• Integrated circuits

These components are typically attached to the substrate using wire bonding or flip-chip techniques.

Interconnects

Interconnects form the connections between various components and layers of the hybrid circuit. They may include:

• Wire bonds
• Conductive adhesives
• Solder joints

The design of interconnects is crucial for the overall performance and reliability of the hybrid circuit board.

Protective Enclosure

To shield the circuit from environmental factors, hybrid circuit boards often include a protective package or coating. This may be:

• A hermetic seal
• A plastic or ceramic encapsulation
• A conformal coating

Manufacturing Processes for Hybrid Circuit Boards

The production of hybrid circuit boards involves several specialized processes:

1. Substrate Preparation

The base material is cleaned and prepared for circuit deposition. This may involve:

• Surface polishing
• Chemical cleaning
• Application of adhesion promoters

2. Thick-film Printing

Conductive, resistive, or dielectric pastes are screen-printed onto the substrate to form circuit patterns and passive components. This process involves:

• Preparing the screen or stencil
• Applying the paste
• Drying the printed layer

3. Firing

The printed layers are fired in a high-temperature oven to cure and bond the materials to the substrate. This process typically involves:

• Controlled temperature ramping
• Peak temperature hold
• Controlled cooling

4. Thin-film Deposition

In some cases, thin-film layers may be added using techniques such as:

• Sputtering
• Evaporation
• Chemical vapor deposition

These methods allow for higher precision components and finer circuit features.

5. Component Attachment

Discrete components and ICs are attached to the substrate using various methods:

• Soldering
• Wire bonding
• Flip-chip bonding

The choice of attachment method depends on the component type and performance requirements.

6. Trimming and Tuning

Laser trimming may be used to adjust the values of printed resistors or capacitors for precise performance. This process involves:

• Measuring component values
• Selectively removing material to adjust resistance or capacitance
• Re-testing to confirm desired values

7. Testing

The completed hybrid circuit undergoes electrical testing to ensure proper functionality. This may include:

• Continuity testing
• Functional testing
• Environmental stress testing

8. Packaging

The circuit is encased in a protective package or coating to shield it from environmental factors. This final step may involve:

• Hermetic sealing
• Plastic or ceramic encapsulation
• Application of conformal coatings

Applications of Hybrid Circuit Boards

Hybrid circuit boards find use in a wide range of applications where high performance, reliability, and miniaturization are crucial:

Aerospace and Defense

• Avionics systems
• Radar equipment
• Satellite communications

Medical Devices

• Implantable medical devices
• Hearing aids
• Diagnostic equipment

Telecommunications

• High-frequency communication systems
• Microwave circuits
• Optical network components

Automotive Electronics

• Engine control units
• Sensor interfaces
• Safety systems

Industrial Controls

• Process control systems
• Robotics
• Instrumentation

Consumer Electronics

• High-end audio equipment
• Specialized consumer devices

Advantages of Hybrid Circuit Boards

Hybrid circuit boards offer several advantages over traditional PCBs and monolithic ICs:

1. Size reduction
2. Improved performance
3. Customization capabilities
4. Enhanced reliability
5. High-frequency operation
6. Mixed-technology integration
7. Improved thermal management

Challenges and Limitations

Despite their advantages, hybrid circuit boards also face some challenges:

1. Higher production costs
2. Design complexity
3. Limited repairability
4. Scalability issues for high-volume production

Future Trends in Hybrid Circuit Board Technology

As electronics continue to evolve, hybrid circuit boards are likely to see advancements in several areas:

Advanced Materials

New substrate materials and conductive pastes may offer improved performance, reliability, and cost-effectiveness. Researchers are exploring:

• Novel ceramic composites
• Polymer-based substrates
• Nanomaterial-enhanced conductors

3D Printing Technologies

Additive manufacturing techniques could revolutionize hybrid circuit production, allowing for:

• Greater design flexibility
• Faster prototyping
• Reduced material waste

Integration with Flexible Electronics

Combining hybrid circuit technology with flexible substrates could lead to new applications in:

• Wearable devices
• Conformable electronics
• Stretchable circuits

Enhanced Simulation and Design Tools

Improved software for designing and simulating hybrid circuits will:

• Streamline the development process
• Optimize performance
• Reduce time-to-market

Nanotechnology Integration

Incorporating nanomaterials and nanostructures into hybrid circuits may lead to:

• Further miniaturization
• Enhanced electrical properties
• Novel functionalities

Increased Automation

Advancements in robotic assembly and artificial intelligence could:

• Reduce manufacturing costs
• Improve consistency in production
• Enable more complex designs

Conclusion

Hybrid circuit boards represent a powerful and versatile approach to electronic circuit design and manufacturing. By combining the best aspects of printed circuit boards and integrated circuits, hybrids offer unique advantages in terms of size, performance, and customization. While they may not be suitable for every application, hybrid circuits continue to play a crucial role in industries where high reliability, compact size, and specialized functionality are paramount.

As technology advances, we can expect to see further innovations in hybrid circuit board design and manufacturing. These developments will likely expand the range of applications for hybrid circuits and potentially reduce their cost, making them more accessible for a wider variety of electronic products.

For engineers and designers working on cutting-edge electronic systems, understanding the capabilities and limitations of hybrid circuit boards is essential. As the boundaries between different types of electronic assemblies continue to blur, hybrid circuits stand as a testament to the ingenuity and adaptability of the electronics industry. By leveraging the strengths of multiple technologies, hybrid circuits pave the way for new possibilities in electronic design, pushing the boundaries of what’s possible in miniaturization, performance, and functionality.