Custom Heat Pipe Heat Sinks: Design, CNC Manufacturing, and Supplier Guide

High-power electronic components often generate concentrated heat within a small contact area. A conventional solid aluminum heat sink may provide adequate cooling when the heat source is directly below a large fin array, but its performance can decline when heat must travel across a long base or reach a remote cooling area. Custom heat pipe heat sinks address this challenge by transferring heat through evaporation, vapor movement, condensation, and capillary return. However, the heat pipe alone does not determine the final result. Base flatness, machined grooves, fin contact, mounting pressure, pipe orientation, airflow, and assembly quality all affect thermal resistance. This guide explains how heat pipe cooling systems work, how they are manufactured, and what engineers should specify when developing a custom thermal management solution.

What Is a Heat Pipe Heat Sink?

A heat pipe heat sink combines one or more sealed heat pipes with a base, heat spreader, fin structure, or remote cooling assembly. The heat pipe transports thermal energy away from a concentrated heat source, while the fins release that energy into the surrounding air. This division of functions allows engineers to position the cooling surface where more space or airflow is available.

Unlike a simple metal bar, a heat pipe uses a phase-change cycle inside a sealed container. This allows it to move relatively large amounts of heat with a small temperature difference between the hot and cold sections. It is especially useful when the heat source and fin stack cannot be positioned directly above one another.

How Does a Heat Pipe Transfer Heat?

The thermal cycle begins at the evaporator section, which is positioned near the heat source. The working fluid inside the pipe absorbs heat and changes from liquid to vapor. Because vapor pressure is higher in the hot region, the vapor moves through the internal passage toward the cooler condenser section.

At the condenser, the vapor releases latent heat to the pipe wall and changes back into liquid. The condenser wall then transfers the heat into a fin stack, heat sink base, chassis, cold plate, or another cooling structure. A wick structure lining the internal wall uses capillary force to return the condensed liquid to the evaporator.

This cycle continues as long as there is a temperature difference between the evaporator and condenser. The heat pipe therefore acts mainly as a heat transport device. The fins, airflow path, and surrounding cooling structure remain responsible for rejecting the heat into the environment.

Why Is It More Effective Than a Solid Metal Conductor?

A solid copper or aluminum conductor transfers heat through ordinary conduction. As the transport distance increases, a larger temperature difference is generally required to move the same heat load. Increasing the cross-sectional area can reduce this resistance, but it also increases weight, material use, and packaging volume.

A heat pipe transports much of its energy through liquid-to-vapor and vapor-to-liquid phase changes. This makes it possible to connect a small heat source to a larger or more conveniently located cooling area without requiring a massive metal block. The advantage is especially important in compact electronic equipment, where the heat-generating component may be separated from the available fan, ventilation opening, or fin stack.

What Components Determine Heat Pipe Performance?

A heat pipe may appear to be a simple metal tube, but its performance depends on several internal and external elements. Container material, working fluid, wick design, pipe diameter, length, orientation, bending, flattening, and condenser contact all influence how much heat the device can transport.

Metal Container

The outer container maintains the sealed internal environment and provides a path between the working fluid and the external heat source. Copper is commonly used because it offers good thermal conductivity, practical forming characteristics, and compatibility with working fluids used in many electronic cooling applications. Aluminum and stainless steel may also be used when weight, corrosion resistance, temperature, or fluid compatibility requires a different material.

The container must remain leak-tight throughout forming, installation, thermal cycling, and service. It must also withstand internal pressure changes without excessive deformation. A small leak can introduce air or release working fluid, reducing the effectiveness of evaporation and condensation.

Working Fluid

The working fluid determines the temperature range in which the heat pipe can operate effectively. Water is widely used in many electronics applications, while fluids such as methanol or ammonia may be selected for other operating conditions. The correct choice depends on the expected evaporator temperature, ambient conditions, startup temperature, storage requirements, material compatibility, and service life.

The working fluid must be compatible with the container and wick. An unsuitable combination can encourage corrosion, contamination, or the formation of non-condensable gases. These gases occupy internal volume and obstruct vapor flow, causing the thermal resistance of the heat pipe to increase over time.

Wick Structure

The wick structure returns liquid from the condenser to the evaporator. Its capillary pressure must overcome pressure losses in both the liquid and vapor paths. Common wick structures include sintered powder, internal grooves, and wire mesh.

• Sintered powder wicks can provide strong capillary pumping and are often suitable for products that operate in different orientations. Their internal resistance and manufacturing complexity must still be considered.
• Grooved wicks can provide lower flow resistance and a relatively simple structure, but their performance may depend more strongly on orientation.
• Wire mesh wicks offer another balance between capillary force, permeability, cost, and manufacturing flexibility.

No wick type is best for every product. The selection must match heat load, transport length, pipe orientation, diameter, bending requirements, and the expected operating environment.

Evaporator, Adiabatic, and Condenser Sections

The evaporator receives heat, the condenser rejects heat, and the adiabatic section transports vapor between them. Although the term “adiabatic” suggests no heat exchange, some heat loss or gain may still occur in an actual assembly.

Mechanical processing can change the performance of these sections. Aggressive flattening may reduce the vapor passage. Tight bends may distort the wick or restrict liquid return. Excessive clamping force may deform the pipe wall. For this reason, bending and flattening should be planned according to the pipe construction rather than treated as ordinary tube-forming operations.

What Operating Limits Can Cause Heat Pipe Failure?

A heat pipe does not have unlimited transport capacity. Its maximum useful performance depends on several physical limits. When a design approaches one of these limits, the evaporator temperature can rise rapidly even if the condenser and fan appear adequate.

Capillary Limit

The capillary limit occurs when the wick cannot return enough liquid to the evaporator. Pressure losses caused by long transport distances, high heat loads, small liquid passages, unfavorable orientation, or complex bends can exceed the pumping force of the wick.

When liquid return becomes insufficient, part of the evaporator may dry out. The dry area loses effective phase-change contact, causing local temperature to increase. This is one reason engineers must evaluate orientation and transport distance instead of selecting heat pipes only by diameter or total wattage.

Boiling Limit

The boiling limit is associated with excessive heat flux in the evaporator. If vapor forms within the wick faster than it can escape, vapor pockets may obstruct liquid flow and reduce contact between the liquid and heated wall.

This issue is influenced by the size of the heat source, not only by total power. A small semiconductor device can create a more severe evaporator condition than a larger component producing the same total heat because the heat is concentrated within a smaller area.

Sonic and Viscous Limits

The sonic limit occurs when vapor flow reaches a speed that restricts additional mass transport. It can become important during startup or under conditions involving high vapor velocity.

The viscous limit is more relevant when vapor pressure is low and viscous forces dominate the internal flow. Low-temperature operation can therefore reduce heat transport even when the same pipe performs well at a higher temperature.

Entrainment Limit

At high vapor velocity, the vapor stream can disturb the returning liquid and carry droplets toward the condenser. This process interferes with liquid return and can eventually contribute to evaporator dry-out.

These limits demonstrate why a reliable heat pipe cooling system must be selected using heat flux, operating temperature, length, diameter, wick type, orientation, bends, and condenser conditions. A single nominal heat-transfer value is not enough to describe performance in every installation.

Heat Pipe vs Vapor Chamber: Which One Should You Use?

Heat pipes and vapor chambers rely on similar phase-change principles, but their geometries support different thermal management goals. A heat pipe is generally optimized for moving heat along a defined path, while a vapor chamber is designed to spread heat across a larger planar area.

When Is a Cylindrical Heat Pipe Better?

A cylindrical heat pipe is often preferable when heat must be transported around other components or transferred to a fin stack that is separated from the heat source. It can be bent into a three-dimensional route, provided that bend radius and forming limits are respected.

This makes heat pipes useful in tower heat sinks, industrial computers, power electronics, lighting systems, communication equipment, and compact enclosures where the available airflow is not directly above the hot component.

When Is a Vapor Chamber Better?

A vapor chamber may be more suitable when a small, high-flux heat source must be spread across a wider heat sink base. It can help reduce local hot spots and produce a more uniform base temperature before heat enters the fin structure.

However, a vapor chamber is not automatically better than a heat pipe. It may require more planar space, different joining methods, additional structural support, and careful control of external pressure. The correct choice depends on whether the main problem is long-distance heat transport or local two-dimensional heat spreading.

Which Materials Are Used in Custom Heat Pipe Heat Sinks?

Material selection affects thermal conductivity, weight, corrosion behavior, manufacturability, joining, cost, and service reliability. Many custom heat pipe heat sinks use a hybrid structure rather than a single material throughout the assembly.

Copper Heat Pipes

Copper is commonly used for heat pipe containers because it combines good thermal conductivity with practical bending and flattening characteristics. It also supports several joining methods used to connect pipes with bases and fin stacks.

The main disadvantages are weight and cost. A full copper heat sink can become unnecessarily heavy for products that only require copper near the heat source or inside the heat transport path. Engineers often reserve copper for the pipe and critical thermal contact areas while using aluminum for larger structural sections.

Aluminum Heat Sink Bases and Fins

Aluminum alloys are widely used for heat sink bases, housings, brackets, and fin structures. They provide a useful balance of thermal conductivity, low density, CNC machinability, corrosion resistance, and cost.

Aluminum bases can be machined with heat pipe grooves, threaded holes, mounting pockets, alignment features, sensor locations, and airflow-related geometry. Extruded, bonded, folded, or machined fins may then be incorporated according to the required volume and thermal performance.

Copper and Aluminum Hybrid Structures

A common configuration combines copper heat pipes with an aluminum heat sink base or fin stack. This arrangement uses copper where rapid heat transport is important and aluminum where a larger, lightweight cooling area is required.

The interface between the two materials must be designed carefully. Gaps, thick adhesive layers, oxide contamination, poor mechanical contact, or incomplete joining can add thermal resistance. Galvanic corrosion may also require consideration when dissimilar metals are exposed to moisture or conductive contaminants.

Material and Working Fluid Compatibility

The internal material-fluid combination must remain stable over the intended operating life. Corrosion products or non-condensable gases can reduce the effective condenser area and obstruct vapor movement.

Compatibility must therefore be evaluated using the actual operating temperature, fluid purity, container material, wick material, sealing process, and expected lifetime. General material guidelines are useful during early design, but final combinations should be validated for the specific heat pipe construction.

How Are Heat Pipes Integrated into Heat Sinks?

The interface between the heat pipe and heat sink determines how efficiently heat enters and leaves the pipe. Even a correctly selected heat pipe can perform poorly if the contact surfaces, groove geometry, joining layer, or fin connection introduce excessive thermal resistance.

Direct Touch Heat Pipe

In a direct touch design, several heat pipes are partially flattened and positioned so that their surfaces contact the heat source or thermal interface material. This creates a relatively short heat path and can reduce the number of separate components.

Direct touch structures can also reduce cost because they may not require a thick copper spreader. However, maintaining a flat, continuous contact surface across multiple pipes is difficult. Differences in pipe height, gaps between adjacent pipes, tool marks, and deformation during assembly can cause uneven pressure.

Excessive flattening may also compress the wick or reduce the internal vapor passage. The required final profile should therefore be coordinated with the heat pipe supplier before machining or forming begins.

Embedded Heat Pipe Base

An embedded heat pipe heat sink uses precision grooves machined into an aluminum or copper base. The pipes are seated inside the grooves and fixed using an appropriate mechanical, adhesive, soldering, brazing, or bonding process.

The groove must support adequate pipe contact without forcing the pipe into damaging deformation. Groove width, depth, bottom radius, position, and spacing affect both assembly and thermal performance. A groove that is too wide may leave gaps and require a thick joining layer. A groove that is too narrow may prevent full seating or create excessive stress.

After assembly, the base may require controlled finishing to achieve the specified contact surface. This operation must not remove too much material above the pipe or expose the sealed container.

Tower and Remote Heat Sink Assemblies

In a tower heat sink, the evaporator ends of the pipes are connected to a base, while the condenser sections pass through or attach to a remote fin stack. This arrangement increases fin area without placing the entire heat sink directly over the component.

The pipe-to-fin interface must provide repeatable contact. Oversized holes, poor expansion, incomplete soldering, or inconsistent fin spacing can reduce heat transfer from the pipe into the air-cooled structure. The fan and fin stack must also be matched so that airflow reaches the active condenser area.

Why Is CNC Machining Important for Heat Pipe Heat Sinks?

CNC machining is important because the external thermal interfaces must be geometrically controlled even though the heat pipes themselves contain delicate sealed structures. Precision machining helps position the heat pipes, maintain base contact, align mounting features, and support repeatable assembly.

Machining Heat Pipe Grooves

Machined grooves determine where each pipe is positioned inside the base. CNC milling can control groove width, depth, spacing, alignment, length, and end geometry. These features affect pipe seating and the thickness of the joining layer.

A groove that is too shallow may leave the pipe above the surrounding base surface. A groove that is too deep may reduce contact pressure or leave unnecessary material between the heat source and pipe. Incorrect spacing can interfere with mounting holes or create uneven heat spreading.

The groove bottom should also match the intended pipe geometry. A round pipe, flattened pipe, or formed pipe may require a different seat profile. Designers should avoid specifying an idealized groove without considering the pipe’s actual post-forming dimensions.

Controlling Base Flatness

The base must make consistent contact with the heat source or thermal interface material. Warping, local high points, deep cutter marks, assembly distortion, and uneven joint thickness can create air gaps or require a thicker thermal interface layer.

Flatness requirements should be based on the contact area, component package, mounting method, and compressibility of the interface material. Extremely strict values can increase machining and inspection cost, while loose requirements may prevent uniform contact. The drawing should define the functional surface and the datum system used to inspect it.

Machining Mounting Features

Heat sink bases may include threaded holes, through-holes, counterbores, countersinks, locating pins, alignment bores, shoulders, cable clearances, sensor pockets, or connector mounting features. Their positions affect how load is distributed across the heat source.

If the mounting pattern is misaligned, screws may pull the base to one side or introduce bending. Holes positioned too close to heat pipe grooves can weaken the surrounding wall or interfere with pipe installation. These relationships should be reviewed before production rather than corrected during final assembly.

Protecting Heat Pipes During Final Machining

A sealed heat pipe must not be treated like a solid rod. Cutting through the wall, using excessive clamping pressure, or removing too much material can damage the seal, wick, or vapor passage.

Fixtures should support the assembly without crushing the pipes. Machining allowances must be established before joining. Tool paths should avoid uncertain pipe locations, and the drawing should clearly identify areas where post-assembly material removal is permitted.

How Should You Select a Heat Pipe for Your Application?

Heat pipe selection should begin with the complete thermal system rather than the pipe alone. Engineers need to understand how heat enters the evaporator, how far it must travel, how it leaves the condenser, and how the surrounding airflow removes it.

Define the Heat Load

Specify normal operating power, short-duration peak power, duty cycle, and heat source dimensions. The source area is important because it determines heat flux at the evaporator. A small component can create a demanding local condition even when total power appears moderate.

Determine the Temperature Range

Define the desired component temperature, maximum ambient temperature, minimum startup temperature, storage range, and short-term peak conditions. The working fluid must function within the real operating range rather than only at room temperature.

The condenser temperature also matters. A pipe cannot continue transferring heat effectively if the fin stack remains nearly as hot as the evaporator.

Measure the Transport Distance

Longer pipes create additional vapor and liquid pressure losses. Transport distance may also require more bends or longer unsupported sections. The distance should be measured along the planned pipe path, not only as a straight line between the heat source and fin stack.

Evaluate Orientation

A gravity-assisted arrangement can help liquid return when the condenser is above the evaporator. Horizontal installation removes much of this assistance, while an evaporator located above the condenser may force the wick to pump liquid against gravity.

Products that can be installed in several directions require a heat pipe and wick structure capable of supporting the least favorable orientation.

Check Bending and Flattening Requirements

Define bend radius, bend angle, number of bends, distance between bends, final pipe thickness, and three-dimensional route. Tight bends or severe flattening can reduce internal flow area and disturb the wick.

Designers should not apply one universal bend rule to every pipe. Allowable forming depends on diameter, wall thickness, wick construction, internal support, working fluid, and forming method.

Confirm Available Airflow

The heat pipe transports heat but does not eliminate it. Airflow, fin area, fin spacing, pressure drop, fan curve, duct geometry, and exhaust path determine how effectively the condenser rejects heat.

A larger fin stack may provide little benefit if air bypasses it. Likewise, closely spaced fins may increase surface area but restrict airflow when the fan cannot overcome the pressure drop.

What Design Trade-Offs Affect Heat Pipe Heat Sinks?

Custom heat sink design involves balancing thermal performance against manufacturing complexity, packaging constraints, weight, cost, and long-term reliability. Improving one characteristic can create disadvantages elsewhere in the system.

Thermal Performance vs Cost

A full copper structure may improve heat spreading but increases material cost and weight. A hybrid copper-and-aluminum structure often provides a more practical balance, but it introduces additional interfaces and joining requirements.

Custom pipe shapes, complex embedded routes, and tightly controlled assemblies can improve packaging efficiency while increasing tooling, forming, machining, and inspection cost. Standard pipe diameters and simpler base geometry may reduce cost when the available space allows them.

Compact Size vs Cooling Capacity

Reducing overall size limits fin area, fan diameter, pipe quantity, airflow channel size, and condenser length. A compact structure may therefore require higher fan speed or a larger allowable temperature rise.

Thermal targets should be reviewed together with the installation envelope. A heat sink cannot be optimized effectively when the mechanical design fixes an unrealistic volume before the heat load is understood.

Low Weight vs Mechanical Strength

Thin bases and fins reduce mass, but they can become more vulnerable to warping, vibration, handling damage, and mounting deformation. Heat pipes can also be damaged if the assembly lacks adequate structural support.

Weight reduction should focus on nonfunctional material while maintaining sufficient thickness around mounting points, pipe grooves, contact surfaces, and unsupported spans.

Manufacturing Simplicity vs Thermal Resistance

Reducing the number of interfaces can lower thermal resistance, but it may require more difficult machining or forming. A direct touch heat pipe design removes a separate base layer, yet creates challenges in surface flatness and pipe consistency.

An embedded base is easier to mount and may provide better structural support, but the base wall and joining layer add resistance between the heat source and pipe. The preferred design depends on heat flux, production volume, acceptable cost, and assembly capability.

How Does Altitude Affect Forced-Air Heat Pipe Heat Sinks?

Heat pipe operation is mainly governed by internal temperature and pressure conditions, but the air-cooled condenser can be affected by altitude. As altitude increases, air density decreases. A fan delivering a similar volumetric flow rate may therefore move less air mass through the fin stack.

Lower air mass flow can reduce convective heat transfer and increase the temperature difference required to reject the same heat load. Fan performance may also change because system resistance and operating conditions are different from those at sea level.

Possible compensation methods include increasing fan speed, selecting a larger fan, increasing fin area, adjusting fin spacing, reducing airflow bypass, improving duct geometry, lowering heat load, or allowing a higher operating temperature. The appropriate adjustment should be based on thermal simulation or testing under representative pressure, temperature, and airflow conditions rather than a universal correction factor.

What Information Should You Send to a Heat Sink Manufacturer?

A complete request for quotation allows the heat sink manufacturer and CNC supplier to evaluate thermal, mechanical, and production requirements together. Incomplete data often leads to repeated revisions because heat pipe quantity, base dimensions, mounting features, or fan selection cannot be confirmed.

• Heat source dimensions and location
• Normal and peak heat load
• Maximum allowable component temperature
• Ambient and startup temperature ranges
• Available installation envelope
• Airflow direction and fan specifications
• Required heat pipe orientation
• Transport distance between evaporator and condenser
• Maximum assembly weight
• Preferred base and fin materials
• Mounting hole layout and allowable mounting pressure
• Base flatness and surface finish requirements
• Thermal interface material
• Surface treatment requirements
• Prototype and production quantities
• 2D engineering drawings and 3D CAD files

It is also helpful to identify restricted areas where pipes, fins, screws, or tools cannot be placed. If the design will operate in multiple orientations or at high altitude, these conditions should be stated at the beginning of the project.

How Does Tuofa CNC Germany Support Custom Heat Pipe Heat Sink Projects?

Custom heat pipe assemblies frequently require precision-machined metal components even when the sealed heat pipes are sourced separately. Tuofa CNC Germany supports projects involving aluminum and copper heat sink bases, mounting structures, heat pipe grooves, contact surfaces, and related metal parts.

CNC-Machined Heat Sink Bases

Heat sink bases can be machined with pipe seats, mounting holes, threaded features, alignment bores, pockets, cable clearances, and controlled contact surfaces. Machining plans should account for material removal, fixture pressure, wall thickness, groove position, and the risk of distortion.

For hybrid assemblies, the base design can also be reviewed to determine whether pipes can be installed without interfering with fasteners, sensors, brackets, or surrounding product components.

Prototype and Low-Volume Production

Prototype and low-volume machining allows engineers to evaluate pipe layout, mounting geometry, base thickness, contact area, assembly clearance, fan location, and enclosure compatibility before committing to higher-volume tooling or production.

Physical prototypes are particularly useful when thermal and mechanical requirements interact. A pipe route that appears suitable in a CAD model may become difficult to bend, install, clamp, or inspect after the other parts are assembled.

DFM Review

A design for manufacturability review can identify grooves that are inaccessible to standard tools, walls that are too thin for reliable machining, threaded holes positioned too close to pipe channels, and surfaces that may distort during clamping.

The review can also help distinguish functional tolerances from noncritical dimensions. This allows tighter control to be concentrated on contact surfaces, locating features, pipe positions, and mounting geometry rather than applied unnecessarily to the entire component.

Precision Components for Heat Pipe Assemblies

In addition to bases, projects may require machined brackets, fin supports, fan mounts, covers, frames, housings, clamps, and interface plates. These components must align with the heat pipe route and protect the assembly from excessive mechanical loading.

For project evaluation, customers can provide 2D drawings, 3D models, material requirements, estimated quantities, heat load information, and assembly details. This information helps determine whether the machined components are suitable for the intended heat pipe configuration.

Domande frequenti

The following questions address several common design and sourcing concerns related to custom heat pipe heat sinks.

Are heat pipe heat sinks better than solid aluminum heat sinks?

Heat pipe heat sinks are not always necessary. A solid aluminum heat sink may be sufficient when the heat source is distributed across a large area, the transport distance is short, and the fin stack can be positioned directly above the component. Heat pipes become more useful when heat is concentrated, must travel across a long base, or needs to reach a remote fin stack. The final result still depends on airflow, fin area, mounting contact, and condenser design. A heat pipe cannot compensate for an undersized fan or blocked airflow path.

Can heat pipes be bent into custom shapes?

Heat pipes can be formed into custom routes, but the allowable geometry depends on pipe diameter, wall thickness, wick structure, bend radius, bend position, and flattening requirements. Tight bends can restrict vapor flow or damage the wick. Multiple nearby bends may also make installation difficult. The route should be reviewed before finalizing the surrounding enclosure. Designers should use the forming limits provided for the specific heat pipe rather than relying on a general tube-bending rule.

Can CNC machining damage a heat pipe?

Yes. A heat pipe contains a thin sealed wall, working fluid, internal vapor space, and wick structure. Cutting too deeply, clamping too tightly, or flattening the pipe beyond its approved limit can reduce performance or cause leakage. Post-assembly machining should only be performed in defined areas with known material allowances. Fixtures must support the base without crushing the pipe, and tool paths should not pass through uncertain pipe locations.

What materials are commonly used for heat pipe heat sinks?

Copper is widely used for heat pipe containers, while aluminum alloys are commonly used for heat sink bases, fins, brackets, and housings. Some designs use copper bases or spreaders for high local heat flux, but this increases weight and cost. Hybrid copper-and-aluminum structures are common because they combine efficient heat transport with lightweight cooling surfaces. Material selection should also account for joining, corrosion exposure, surface treatment, structural strength, and compatibility with the internal working fluid.

What files are needed for a custom heat pipe heat sink quotation?

A quotation should include 2D engineering drawings, 3D CAD files, heat source dimensions, normal and peak power, temperature limits, installation orientation, airflow information, material requirements, surface treatment, mounting details, and expected quantity. The drawing should identify critical contact surfaces, datums, flatness requirements, thread specifications, and restricted areas. Providing the enclosure model or surrounding component layout can also help identify pipe-routing and assembly conflicts before machining begins.

Conclusione

Custom heat pipe heat sinks move concentrated heat from an evaporator to a larger or remote cooling area through a sealed phase-change cycle. Their performance depends on working fluid, wick structure, orientation, transport distance, pipe geometry, condenser design, and airflow. CNC machining supports reliable assembly by controlling heat pipe grooves, mounting features, contact surfaces, and base flatness. Engineers should evaluate heat load, heat flux, temperature, available space, airflow, weight, manufacturing complexity, and cost as one connected system. Tuofa CNC Germany can support custom projects with CNC-machined aluminum or copper bases, precision grooves, mounting structures, prototypes, and related metal components based on customer drawings and assembly requirements.