Deep Hole Drilling for Microelectronics Thermal Management: Vapour Chamber Wick Grooves, Heat Pipe Manufacturing, and Gun-Drilled Liquid Cold Plates for AI/GPU Cooling

A manufacturer of liquid cold plates for AI training servers (Al 6061-T6, 20 channels of 10 mm x 600 mm deep, cross-drilled serpentine circuit) used single-pass gun drilling (Vc = 250 m/min, f = 0.10 mm/rev, emulsified oil at 40 bar). Channel pitch accuracy was plus/minus 0.05 mm, Ra inside channels 0.4-0.6 microns, zero leakage at 10 bar. Compared to a vacuum-brazed microchannel plate of the same size, the gun-drilled plate had 0.045 C/W vs 0.025 C/W thermal resistance but at 40 percent of the cost. The gun-drilled plate was selected for 300 W per GPU cooling.

Gun-Drilled Liquid Cold Plates for AI and GPU Cooling ​

Liquid cold plates are the workhorse of high-power electronics cooling, and gun drilling is the most cost-effective method for producing them. In an AI data centre server rack, each GPU (300-700 W thermal design power for current-gen NVIDIA H100/B200 or AMD MI300X) is cooled by a liquid cold plate mounted directly on the chip package. The cold plate is a monolithic block of aluminium (6061-T6) or copper (C110), gun-drilled with a set of parallel cooling channels connected into a serpentine circuit by cross-drilling and plugging.

Serpentine Circuit Design The serpentine circuit is the standard configuration for gun-drilled cold plates. A set of parallel channels (typically 8-40 channels, 6-20 mm diameter, 300-2000 mm length) is gun-drilled through the plate. At each end of the plate, a cross-drilled hole connects adjacent channels — but only in a specific pattern that creates a single continuous flow path. The cross-drilled holes are then plugged (by threaded plugs or pressed-in interference-fit plugs) to seal the outer ends. The coolant enters at one corner of the plate, flows through the full serpentine path, and exits at the opposite corner, maximising the heat transfer contact length.

Thermal Performance Comparison The thermal resistance of a gun-drilled cold plate is determined by the channel diameter, the number of channels, the flow rate, and the wall thickness between channels. For a typical AI GPU cold plate (600 mm x 300 mm x 25 mm, 20 channels of 10 mm diameter), the thermal resistance at 10 L/min is approximately 0.045 C/W. This is adequate for cooling a 300-500 W GPU with a junction temperature below 85 °C. The key advantage of gun-drilled cold plates is the zero leak risk — the channels are machined from a solid block with no joints or seams inside the flow path, eliminating the primary failure mode of assembled cold plates.

Comparison Table: Cold Plate Manufacturing Technologies for Electronics Cooling ​

*At 10 L/min flow rate, 25 °C inlet temperature, measured with 50 x 50 mm heat source.

Heat Pipe Groove and Vapour Chamber Wick Manufacturing ​

Heat pipes and vapour chambers are passive two-phase heat transfer devices that use capillary action to circulate a working fluid between an evaporator and a condenser. The capillary wick structure — which provides the pumping force and distributes the working fluid — is produced by deep hole drilling or related machining processes.

Axial Groove Heat Pipes In grooved heat pipes, the wick consists of a set of axial grooves machined into the inner wall of a copper tube. The grooves are typically 0.05-0.15 mm wide, 0.1-0.3 mm deep, with 30-50 grooves per tube. For a standard 6 mm diameter heat pipe (200-400 mm length), the grooves are produced by a grooving tool that is pulled or pushed through the tube, displacing material to form the groove profile. This is mechanically similar to the button rifling process used in gun barrel manufacturing but at a much smaller scale. The grooving tool has a cutting edge with the inverse profile of the desired groove shape. As the tool passes through the tube, it simultaneously forms all grooves (30-50 grooves in a single pass). The tool is typically made from carbide or PCD, and the grooving speed is 1-5 m/min with a water-based lubricant.

Sintered Powder Wicks For higher capillary pressure, vapour chambers and some heat pipes use a sintered powder wick — a porous layer of copper powder (50-150 micron particle size) sintered onto the inner wall at 850-950 °C in a hydrogen/nitrogen reducing atmosphere. The sintered wick is not a drilling product, but the vapour chamber enclosure (the copper shell that contains the wick) requires deep hole drilling for the fill tube and mounting holes. The fill tube hole is typically 2-3 mm diameter drilled through the 1-2 mm thick copper sheet, and the mounting holes are gun-drilled through a solid copper or aluminium base plate.

Comparison Table: Heat Pipe and Vapour Chamber Wick Types ​

Cross-Drilling and Plugging for Serpentine Circuits ​

The serpentine circuit — the flow path that connects the parallel gun-drilled channels into a single continuous channel — is produced by cross-drilling and plugging. This is a two-operation process that is critical to the performance and reliability of the cold plate.

Cross-Drilling Operation After the parallel channels are gun-drilled, the cold plate is positioned on a CNC machining centre for cross-drilling. A carbide twist drill (diameter equal to the channel diameter, typically 6-20 mm) drills a perpendicular hole at each end of the plate, intersecting the adjacent parallel channels. The drill must pass through the centre of each parallel channel within +/- 0.2 mm to ensure the intersection is clean and not blocked by a wall. The cross-drilling depth is controlled by the CNC program, and the depth is verified by a touch probe after the first cross-hole. The cross-holes at the ends of the plate are spaced at the same pitch as the parallel channels, and alternate ends are connected to create the serpentine pattern: cross-hole 1 connects channels 1 and 2; cross-hole 2 connects channels 3 and 4; cross-hole 3 connects channels 5 and 6; and so on. At the opposite end, cross-holes connect channels 2 and 3, 4 and 5, 6 and 7, etc.

Plugging Methods The outer ends of the cross-drilled holes must be sealed to prevent coolant leakage. Three plugging methods are used. Threaded plugs (1/8-inch NPT) and thread-locking compound are the most common for aluminium cold plates and provide a reliable seal at up to 20 bar pressure. Press-fit plugs (interference fit of 0.02-0.05 mm) are used for copper cold plates and for applications where zero internal crevices are required. The press-fit plug is driven into the cross-hole by a hydraulic press, and the interference fit creates a permanent mechanical seal. Laser-welded plugs are used for high-reliability applications (aerospace, military) where the plug must be permanently sealed. The plug is fitted into the cross-hole and laser-welded around the circumference. The weld is then ground flush with the cold plate surface.

FAQ ​

How does thermal resistance of gun-drilled cold plates compare to other cooling technologies? ​

Gun-drilled cold plates occupy a specific position in the thermal performance spectrum — they are not the highest-performing option but offer the best balance of performance, cost, and reliability for moderate heat flux applications (below 100 W/cm2). A typical gun-drilled aluminium cold plate (20 channels of 10 mm, 600 mm length) achieves a thermal resistance of 0.040-0.060 C/W at 10 L/min flow rate. This is approximately 20-30% higher (worse) than a skived fin cold plate (0.035-0.055 C/W) and 50-70% higher than a vacuum-brazed microchannel cold plate (0.015-0.035 C/W). However, the gun-drilled cold plate costs 60-75% less than the microchannel plate and has zero leak risk (no joints inside the flow path). The skived fin cold plate has comparable cost to gun-drilled but is limited to lower aspect ratios (maximum 20:1 fin height-to-gap ratio) and cannot achieve the same channel length (skived fin plates are typically limited to 300-500 mm length due to tooling constraints). For AI GPU cooling at 300-700 W per GPU, the 0.045 C/W thermal resistance of a gun-drilled cold plate translates to a 13.5-31.5 °C temperature rise from the coolant to the cold plate surface. With a 25 °C coolant inlet temperature, the cold plate surface is at 38.5-56.5 °C — well within the 85 °C junction temperature limit of current GPUs. This explains why gun-drilled cold plates have become the standard solution for AI data centre cooling, even though they are not the thermal performance leader. The key decision factor is that the manufacturing cost of a gun-drilled cold plate ($80-150 for a typical AI GPU plate) is low enough to allow deployment at data centre scale (tens of thousands of plates per facility).

What are the limitations of gun-drilled cold plates for electronics cooling? ​

Gun-drilled cold plates have four primary limitations that designers must consider. First, channel geometry is restricted to straight, parallel holes. Gun drilling cannot produce curved, tapered, or branched channels — the drill follows a straight path. This limits the flow path design to a serpentine circuit, which creates a temperature gradient across the plate (the coolant heats up as it flows through the serpentine path, so the downstream section of the plate is warmer than the upstream section). For a 600 mm plate, the temperature gradient from inlet to outlet is typically 2-5 °C. Second, the heat flux capability is limited to approximately 100 W/cm2. At higher heat fluxes, the convective heat transfer coefficient inside the gun-drilled channels (typically 5,000-15,000 W/m2K for water at 1-3 m/s flow velocity) is insufficient to maintain the surface temperature below the component limit. Microchannel cold plates can achieve 50,000-100,000 W/m2K with the same flow rate by using much smaller channels (0.2-1.0 mm wide). Third, the wall thickness between adjacent channels is limited by the structural strength of the plate. For a 25 mm thick plate with 10 mm diameter channels at 15 mm pitch, the wall between channels is only 5 mm thick. If the coolant pressure is high (> 15 bar), the wall must be thickened, reducing the number of channels. Fourth, gun drilling is not practical for plates thicker than 100 mm or longer than 2500 mm, because the drill tube flexes and the hole deviation exceeds acceptable limits. For larger plates, a segmented design with multiple gun-drilled plates connected by external manifolds is required.

What is the difference between single-pass and two-pass BTA drilling for cold plates? ​

Gun-drilled cold plates typically use single-pass gun drilling, not BTA drilling, because the hole diameters are in the 6-20 mm range and the depth-to-diameter ratio is moderate (30-100:1). Gun drilling is faster (higher cutting speeds, 200-300 m/min for aluminium) and more economical for these hole sizes. In a single-pass gun drilling operation, the channel is drilled to its final diameter in one pass. The gun drill has a single cutting edge and two guide pads that burnish the bore wall. The surface finish after single-pass gun drilling in aluminium is typically Ra 0.4-0.8 microns, which is acceptable for liquid cooling without any finishing operation. BTA drilling — which uses a multi-edged cutting head with internal chip evacuation — is used for larger diameters (above 20 mm) or when the hole must be produced in a single pass at a faster penetration rate. For cold plate applications, BTA drilling is occasionally used for very large channels (20-40 mm diameter) where the material removal rate of gun drilling would be too slow. Two-pass BTA drilling (rough BTA followed by finish BTA reaming) is used only for the highest surface finish requirements (Ra < 0.2 microns) or for materials that are difficult to gun drill in a single pass (such as copper alloys with high oxygen content). Two-pass drilling is also used when the channel must be produced in a hardened material (such as 7075-T6 aluminium or C18200 copper), where the rough BTA pass removes the bulk of the material and the finish BTA ream achieves the final tolerance.

How are the cross-drilled serpentine holes sealed against leaks? ​

The cross-drilled holes at the ends of the serpentine circuit are sealed using one of three methods, depending on the application pressure, material, and cleanliness requirements. Threaded plugs (typically 1/8-inch NPT) with anaerobic thread-locking compound are the most common and economical method for aluminium cold plates. The seal integrity depends on the thread engagement (minimum 4 full threads) and the thread-locking compound curing time (typically 6-24 hours at room temperature). The maximum working pressure is 15-20 bar. Press-fit plugs (interference fit of 0.02-0.05 mm) are used for copper cold plates and for applications where zero internal crevices are required (such as medical or semiconductor cooling). The press-fit plug is a smooth cylinder that is hydraulically pressed into the cross-drilled hole. The interference fit creates a permanent metal-to-metal seal that can withstand pressures up to 30 bar. The press-fit method requires a smooth, precisely sized cross-hole (tolerance +/- 0.005 mm) and a clean, dry surface. Laser-welded plugs are the most reliable sealing method, used for aerospace, military, and high-reliability applications. A tapered or stepped plug is inserted into the cross-hole, and a pulsed laser (typically Nd:YAG or fibre laser) welds the plug circumference to the cold plate body. The weld is then ground flush and the area is inspected by dye penetrant testing or X-ray. The laser welding process does not create any internal crevices or gaps, and the weld strength exceeds the burst pressure of the cold plate (typically 40-50 bar). The choice of sealing method has a significant impact on cost: threaded plugs add $3-5 per cold plate, press-fit plugs add $8-15, and laser-welded plugs add $25-50.

How does gun-drilled cold plate design differ for single-phase vs two-phase cooling? ​

Gun-drilled cold plates can be designed for either single-phase cooling (liquid remains liquid throughout the circuit) or two-phase cooling (liquid boils inside the channels, absorbing heat through latent heat of vaporisation). The design parameters differ significantly between the two modes. For single-phase cooling, the channel diameter is selected to achieve turbulent flow (Reynolds number > 4000) at the available flow rate. For a typical AI GPU cold plate at 10 L/min, a 10 mm diameter channel achieves Re = 21,000, which is fully turbulent and provides good heat transfer. The channel length to diameter ratio (L/D) is typically 50-80 for single-phase plates. The pressure drop is calculated by the Darcy-Weisbach equation and is typically 0.5-1.5 bar for a serpentine circuit. For two-phase cooling, the channel design must account for the two-phase flow regime (slug flow, annular flow, or mist flow) that develops inside the channel as the liquid boils. Two-phase cold plates typically use smaller diameter channels (3-6 mm) to promote annular flow, which provides the highest heat transfer coefficients. The channel length is shorter (150-300 mm) to limit the vapour quality at the outlet (typically < 0.8, meaning less than 80% of the liquid is vaporised). Two-phase cold plates can achieve heat transfer coefficients of 20,000-50,000 W/m2K (2-5x higher than single-phase) but require a more complex system (condenser, pump, reservoir, and precise flow control). Two-phase gun-drilled cold plates are used in high-performance computing and military electronics where the heat flux exceeds 100 W/cm2 and the system volume and weight are constrained. The gun drilling process for two-phase cold plates uses the same single-pass method but with tighter diameter tolerance (+/- 0.02 mm vs +/- 0.05 mm for single-phase) because the two-phase heat transfer coefficient is sensitive to the channel diameter.

The information provided in this article is for general informational purposes only. Data and recommendations are based on published research and industry experience as of 2026.