数据中心液冷纯铜冷板表面处理技术与验证方法

英文正文

 

English Version

Surface Treatment Technology and Verification Methods for Pure Copper Cold Plates in Data Center Liquid Cooling

R&D Team: Hefei Huaqing Gaoke Surface Technology R&D Center, Copper Foil Surface Treatment Technology Project Group

Host Institution: Hefei Huaqing Gaoke Surface Technology Co., Ltd., Hefei 230000, Anhui, China

Contact: Tel: 15956911638; Email: 664520171@qq.com

Abstract

1. Technical Application Background

Liquid cooling removes chip heat directly through the coolant, substantially improving heat dissipation while reducing rack-level power consumption.

The pure copper cold plate carries the primary heat-exchange duty in this architecture: copper has a far higher thermal conductivity than aluminum, allowing thinner walls and smaller hydraulic diameters when microchannels are machined, while offering excellent brazing and solderability. It is therefore widely adopted in scenarios above 40 kW per rack.

Yet the corrosion resistance of copper alone cannot support long-term service. Its standard electrode potential of +0.34 V means that in ethylene glycol aqueous solution, in the presence of long-chain carboxylic acid inhibitors and isothiazolinone-class biocides, it continuously dissolves. The speciation of copper ions in the coolant determines the orientation of the entire surface treatment strategy.

1.1 The Copper-Soap Adhesive Bed: The Real Cause of Microchannel Blockage

In alkaline coolant at pH 8.5, long-chain carboxylic acid inhibitors exist as carboxylate anions and react with released Cu²⁺ to form insoluble copper soaps, waxy and tacky copper carboxylate salts. These deposit as a continuous adhesive bed on the copper surface, firmly capturing passing micron particles, metal debris and hydroxide precipitates, which subsequently bridge and accumulate within the microchannels.

The true causal chain is: copper dissolves to form copper soap, the soap forms an adhesive bed, particles are captured, particles bridge into a blockage, and continued copper dissolution itself accounts for roughly thirty percent of the deposit in the form of corrosion products.

1.2 Amplification in Mixed-Metal Loops

Data center liquid cooling systems are rarely single-metal. Copper cold plates, stainless piping, aluminum quick-disconnect housings and copper-aluminum transition fittings coexist. The potential difference between copper and aluminum is significant: industry data show that aluminum cold plates directly coupled to copper fittings corrode 8 to 12 times faster than single-metal aluminum systems.

With a copper cold plate acting as cathode and an aluminum quick-disconnect housing as anode, area ratios reach 1:15 to 1:30, and aluminum housings develop localized pitting and perforation within 12 to 24 months. This unfavorable large-cathode-to-small-anode ratio is the classic condition for accelerated galvanic corrosion.

This yields a frequently overlooked implication: the anti-fouling coating on the copper cold plate has a second, independent function, cutting off continuous dissolution of copper as cathode and thereby reducing accelerated corrosion of the aluminum anode.

2. Types and Limitations of Mainstream Pure Copper Cold Plate Surface Treatment Technologies

Five categories of surface treatment are currently used industrially for liquid cooling pure copper cold plates: electrophoretic organic thick films, chromate passivation, ultra-thin ceramic and diamond-like carbon films, nickel-phosphorus composite coatings, and pure sol-gel coatings. The characteristics and limitations of each are shown in Table 1.

Table 1 Comparison of Mainstream Surface Treatment Technologies for Pure Copper Cold Plates

Type	Main Components	Advantages	Limitations
Electrophoretic	Cathodic epoxy electrophoretic paint, 15 to 50 μm	Established process, strong sealing, Grade 0 adhesion	Thermal resistance reaches 26.7% of the liquid-side convective resistance, exceeding the fouling it avoids; nucleophilic ring-opening with isothiazolinone
Chromate	Hexavalent or trivalent chromate bath	Best corrosion resistance, self-healing	Cr⁶⁺ fully restricted under RoHS and REACH; trivalent chromium dissolves in alkali and its electrochemical polarity becomes a corrosion site
Ultra-thin ceramic and DLC	Al₂O₃ or TiO₂ atomic layer deposition 50 to 100 nm; amorphous carbon film 2 μm	Negligible thermal resistance, excellent inertness, high hardness	Line-of-sight and chamber-limited geometry cannot reach enclosed microchannels; alumina dissolves at roughly 100 nm/h in 1 mol/L NaOH
Ni-P composite coating	Low-phosphorus electroless nickel-phosphorus 30 to 35 μm plus a PTFE co-deposited layer 8 to 10 μm	Single-bath continuation, shortest route, dense and pore-free, contact angle 110 to 120 degrees	The co-deposited layer is a porous dispersed structure relying entirely on the base layer for protection; relatively low contact angle
Pure sol-gel	Co-condensed perfluoropolyether silane and tetraethyl orthosilicate network 1 to 3 μm	Contact angle up to 140 degrees, dense continuous film, environmentally benign	Si-O-Si bonds hydrolyze rapidly in alkaline pH 8.5 medium and cannot alone perform the barrier function

Note: Table 1 is compiled from public literature and industry practice. Composition and performance ranges are typical values, and process windows vary between suppliers; claims that involve regulation or measured data are sourced separately in the body text.

Electrophoretic coating is the most frequently underestimated option. Cathodic epoxy electrodeposition is an established process in the microchannel heat exchanger industry, and industry literature commonly reports heat-transfer degradation below 3%.

That figure, however, derives from air-cooled condenser duty, where air-side thermal resistance greatly exceeds liquid-side resistance, and it cannot be extrapolated to liquid cooling internal passages.

Under liquid cooling conditions, the thermal resistance of a 20 μm epoxy film reaches 26.7% of the liquid-side convective resistance, already exceeding the 20.8% of the 25 μm copper-soap fouling layer it is meant to avoid. The coating itself harms heat transfer more than the fouling does.

Moreover, vendor selection manuals explicitly state that electrophoretically coated coils are unsuitable for liquid immersion, and the epoxy-amine curing system undergoes nucleophilic ring-opening with the isothiazolinone biocide in the coolant, deactivating the biocide while eroding the coating matrix in reverse.

The limitations of the remaining three routes are equally clear. Chromate passivation offers the best corrosion resistance of the five and is self-healing, but hexavalent chromium is fully restricted under RoHS and REACH, while trivalent chromium dissolves in alkaline media and its electrochemical polarity makes it a corrosion site in itself, so both the regulatory path and the alkaline stability fail.

Nickel-phosphorus composite coatings are the only dense, pore-free route that can continue an existing process within a single bath, with contact angles of 110 to 120 degrees, yet their anti-fouling function depends entirely on the base layer and protection is lost the moment that base layer is damaged, while the contact angle stays low relative to a purely anti-fouling coating.

Pure sol-gel coatings reach contact angles of 140 degrees with dense, continuous and environmentally benign films, but siloxane bonds hydrolyze rapidly in alkaline coolant at pH 8.5 and cannot independently perform the barrier function.

Ultra-thin ceramic and diamond-like carbon films are in fact excellent in material performance. Atomic layer deposited alumina can theoretically produce pinhole-free barrier films with conformal coverage of high aspect-ratio structures, and patent literature explicitly positions it as a passivation scheme for copper microchannel coolers.

Diamond-like carbon reaches 2,500 to 4,500 HV hardness with excellent inertness and negligible thermal resistance. The problem is not the material but the geometry: physical vapor deposition is a line-of-sight process, plasma-enhanced chemical vapor deposition remains chamber-limited, and cold plate microchannels are enclosed blind cavities that neither can effectively reach.

Alumina is also amphoteric and leaches as hydroxide in alkaline media, with measured dissolution around 100 nm/h in 1 mol/L NaOH and a reported etch rate of roughly 50 nm/min in pH 9 to 12 alkaline solution at 60 °C, guaranteeing complete thinning over a ten-year continuous service. Alumina and titania are also hydrophilic surfaces, so even if dissolution were solved, particle adhesion would remain unaddressed.

3. Mechanism of Insufficient Anti-Fouling and Corrosion Resistance

This chapter follows two threads. The first traces the dissolution of copper, the formation of copper soap and the maturation of the adhesive bed, and therefore answers where the deposit comes from. The second traces the failure paths of existing protective layers in liquid cooling coolant, and therefore answers why barriers are breached.

Both threads converge on a single conclusion: anti-fouling and corrosion resistance are not two attributes of one layer, but two duties that must be carried separately by different layers.

3.1 Copper Dissolution and Copper-Soap Formation

Corrosion of copper in coolant is not simple uniform dissolution but a three-stage serial mechanism. The first stage is dissolution of the cuprous oxide film: the Cu₂O protective film on copper dissolves continuously in coolant containing chloride ions and organic acids, releasing Cu²⁺.

The second stage is complexation and deposition: released Cu²⁺ combines with long-chain carboxylate anions to form insoluble copper soaps, or forms complexes with benzotriazole-type inhibitors.

The third stage is maturation of the adhesive bed: the copper soap slowly spreads under shear into a continuous waxy layer whose adhesiveness far exceeds that of the particles themselves, so any particle that contacts it is captured and cannot be stripped by the flow field.

3.2 Barrier Penetration and Pinhole Galvanic Corrosion

A clear industry position exists that direct exposure of wet-surface coatings to coolant is not recommended as primary protection, because pinholes in the coating form a large-cathode, small-anode galvanic couple. This objection is entirely correct for electroplated layers.

Electroplated coatings typically show 5% to 20% porosity, and copper exposed at a pinhole acts as a large cathode against a small anodic zone at the surrounding coating edge, producing area ratios of 25:1 or worse. Anodic current density then rises sharply, manifesting as rapidly expanding groove corrosion around the pinhole until the coating fails.

3.3 Alkaline Hydrolysis of the Organic-Inorganic Hybrid Layer and Functional Layer Failure

Taking a siloxane-based hybrid network as an example, hydrolytic cleavage of its siloxane bonds by hydroxyl attack in alkaline media is the central failure path of such functional layers in liquid cooling coolant.

The rate is highly pH-sensitive: above pH 11 the siloxane bridges break down and depolymerize, converting the network into soluble silicates and dissolving the film entirely; pH 8.5 is already well into the evident decomposition regime. A single functional layer therefore cannot simultaneously perform the barrier and anti-fouling functions.

Its character changes only when demoted to a purely functional layer: even if the functional layer is partially hydrolyzed, the dense barrier layer beneath remains a continuous barrier, so hydrolysis costs surface energy without costing protection.

4. Impact on Heat Transfer Performance and Online Monitoring Capability

The value of an anti-fouling scheme ultimately rests on two measurable outcomes: whether heat transfer efficiency is genuinely protected, and whether the customer can tell from operation that the coating is beginning to fail.

This chapter addresses both, clarifying the true cost and benefit of the coating in thermal resistance terms and stating plainly where the routine customer-side online monitoring tools reach their limit.

4.1 The Quantified Thermal Cost of the Coating

Coating thermal resistance cannot be judged qualitatively; it must be quantified.

Against a liquid-side convective thermal resistance of 3×10⁻⁴ m²·K/W, the proportions are: atomic layer deposited alumina at 100 nm, 0.001%; diamond-like carbon at 2 μm, 0.03%; parylene chemical vapor deposition at 2 μm, 3.33%; electrophoretic epoxy at 20 μm, 26.7%; PTFE electrostatic spraying at 50 μm, 66.7%; against the 25 μm copper-soap fouling layer it avoids, 20.8%.

The sequence also includes two processes commonly shortlisted outside Table 1, in order to show the dominant role of thickness in thermal resistance. With a layered structure of a dense barrier combined with a low-surface-energy functional layer, the thermal resistance contributed by the coating itself remains at an extremely low level within the same order of magnitude.

The conclusion is that the layered collaborative scheme carries essentially no additional thermal-resistance penalty, and the true dividing line is coating thickness rather than material conductivity.

4.2 Capability Limits of Customer-Side Online Monitoring

This is a limitation that must be stated honestly to customers. The mass of coating in a loop is extremely small relative to the total coolant, while elemental analysis of the coolant has a detection floor, and the mass involved when a coating suffers local pitting or local spalling is usually far below that floor. Coolant copper measurement therefore detects only large-area uniform failure, not local failure.

Likewise, the copper ion limit in T/CECS 1722-2024 is a loop safety red line rather than a coating state monitoring metric; it serves to identify loss of loop control but cannot substitute for physical sampling.

5. Hefei Huaqing Gaoke Technical Solution

In response to these industry limitations, the scheme moves from single-layer physical coverage to a layered collaborative scheme: the base layer isolates copper from direct contact with the coolant, while the functional layer lowers surface energy and suppresses particle adhesion.

The two long-standing problems, alkaline instability and inadequate anti-fouling, are thereby assigned to separate layers, and the failure of one no longer drags the other down. The overall approach can be summarized in five steps, corresponding to Sections 5.1 to 5.5.

l A dense, pore-free base layer cuts the reaction chain between copper and coolant.

l A low-surface-energy functional layer weakens particle adhesion.

l Stepwise film formation preserves film integrity and continuity.

l A crosslinking system compatible with the coolant chemistry ensures long-term stability.

l Compatibility with downstream assembly operations keeps the coating from becoming a weak link in final assembly.

5.1 A Dense and Pore-Free Barrier Base Layer

The base layer carries the entire barrier duty. It must be dense and pore-free, capable of conforming to complex internal passages, and able to withstand long-term immersion in mildly alkaline coolant.

The first selection criterion is not the corrosion ranking but the deposition mode: a process that grows layer by layer following the workpiece geometry to form a continuous pore-free film inherently removes the pinhole galvanic corrosion path common to electroplated layers, which is more fundamental than any supplementary inhibitor system.

The base layer must also balance hardness, erosion resistance and thermal conductivity so that it does not become the weak link on the heat transfer surface.

5.2 A Low-Surface-Energy Anti-Fouling Functional Layer

The functional layer carries the entire anti-fouling duty, and its defining metric is surface energy rather than thickness. The technical point is to build an organic-inorganic hybrid network that markedly lowers surface free energy and thereby weakens the adhesion of the copper-soap adhesive bed and of particulates.

At the same time this layer must form reliable chemical bonding with the base layer rather than mere physical adsorption, so that interlayer delamination does not occur over long service.

Publicly reported long-duration tests on heat exchanger pilots and under erosion conditions show that such low-surface-energy coatings retain their surface performance over coupon exposure periods, with deposited particles adhering only weakly.

5.3 Stepwise Film Formation and First-Article Calibration

Film formation follows two principles. First, the film should not be deposited in a single pass to final thickness, but built up in thin passes with intermediate drying so that interlayer crosslinking and isolation are achieved and the thickness accumulates gradually, because a single thick deposition necessarily develops internal stress and cracks.

Second, the forming parameters cannot be copied from any existing process sheet and must be calibrated on first articles: film formation is affected by bath ageing, solids drift and solvent evaporation, so the same parameter set is not equivalent across different batches. The first few pieces from every new bath must therefore be measured for film thickness and the parameters locked accordingly.

5.4 A Crosslinking System Compatible with Coolant Chemistry

Liquid cooling coolant generally contains biocides and inhibitors, and the coating crosslinking system must be compatible with them, otherwise two consequences follow: the biocide is consumed by coating components and deactivates, or the coating matrix is eroded in reverse.

The selection principle for the curing system is therefore to avoid introducing functional groups that readily react with active coolant species, and all components including additives must be brought into the compatibility review. Bringing coolant chemistry into coating design is one of the points that distinguishes liquid cooling service from surface treatment for conventional heat exchangers.

5.5 Compatibility with Downstream Assembly Operations

Cold plates still undergo sealing face assembly, thread tightening, brazing or compression fitting after coating, and these operations place additional demands on the film.

The strategy is to mask threads and sealing faces throughout the coating process so that no film remains on assembly surfaces, to schedule high-temperature joining operations before coating so that the functional layer never sees decomposition temperatures, and to measure contact thermal resistance of the interface separately so that it does not become the new bottleneck in heat transfer.

6. Verification Methods

Coating performance verification has two directions: whether the coating itself meets specification, covered by routine inspection items, and whether it remains effective over the long term in the customer system with the real coolant. Passing the former does not guarantee the latter.

This chapter lays out verification arrangements along the three-tier verification architecture. Its design premise is precisely the capability limit identified in Chapter 4: customer-side online monitoring cannot detect local failure, so state criteria must come from physical specimens.

6.1 Three-Tier Verification Architecture

The first tier is field coupon verification, implemented jointly by supplier and customer, deployed in step with the first-article and production batches, answering whether the coating is stable and effective in the customer coolant.

The second tier is prototype verification, completed by the customer before prototype assembly, comparing heat transfer performance and checking process side effects with coated parts in the real coolant.

The third tier is system-level monitoring, carried out by the customer after installation, tracking particle counts, pressure drop and heat transfer coefficient to judge whether contamination is accumulating and when intervention is needed.

6.2 First Tier: Field Coupon Verification

The coupon set must contain coated specimens together with several types of control, since an array of coated parts alone is invalid and attribution becomes impossible without controls. A uniform-format pure copper panel with sealed edges and a single exposed face is recommended, and holders must be stainless steel or fluoroplastic so that no extra copper source is introduced.

Deployment should prioritize the three points of highest flow velocity, temperature and particle concentration, with additional coupons in high-flow low-velocity dead zones to cover the most severe conditions.

Inspection relies mainly on routine methods such as weighing, contact angle, adhesion, impedance and cross-section morphology, among which corrosion product examination is the most decisive: the presence of copper-bearing corrosion products means the barrier has been breached and the coupon is failed, irrespective of any numerical metric.

6.3 Second Tier: Prototype Verification

Prototype verification is completed before customer prototype assembly, confirming that the coating does not impair heat transfer and introduces no process side effects, with criteria set around total heat transfer coefficient, heat transfer non-uniformity, pressure drop change and flow deviation.

Three groups must be tested together, the coated part, the uncoated bare copper part and a pre-fouled bare copper part, with the third serving as the reference baseline that quantifies how much thermal resistance the coating actually saved. Comparing only coated and uncoated parts can show that the coating is harmless, but never that it is valuable.

6.4 Third Tier: System-Level Monitoring

Three routine monitoring tools are used after installation: particle counting, pressure drop and heat transfer coefficient.

The value of particle counting lies in where the sensor is placed rather than in judging coating state, since particle sources in the system are numerous and a rising count cannot be attributed to the coating; the correct use is to place measuring points upstream and downstream of the coated cold plate and compare before and after installation, to determine whether the coating introduces any new particle source.

Long-term trends in pressure and heat transfer provide the direct basis for deciding when the system needs intervention.

6.5 Proper Positioning and Limits of Accelerated Testing

What accelerated testing can do: by raising temperature and alkali concentration to accelerate functional-layer hydrolysis, it compares the alkaline resistance of different functional-layer schemes within a few weeks, which is effective for scheme screening.

What it cannot do: extrapolate a few weeks of accelerated results into a ten-year service life, because Arrhenius extrapolation is unreliable once several failure mechanisms are crossed. Accelerated testing serves screening; the primary evidence must still come from immersion in the actual coolant together with electrochemical impedance testing, and the two cannot substitute for each other.

7. Typical Application Scenarios

7.1 Direct-to-Chip GPU Liquid Cooling Cold Plates in Data Centers

In scenarios above 40 kW per rack using CPU and GPU cold-plate liquid cooling, copper cold plate internal passages are sub-millimeter microchannels highly sensitive to particle deposition, requiring a coating that does not impair heat transfer, does not release metal ions, and needs no cleaning for ten years.

7.2 CDU Secondary-Side Heat Exchangers and Manifolds

Exchanger cores and copper piping remain immersed in coolant continuously, and because flow velocity here is low and residence time long, copper soap deposition is most severe and the coating delivers the most direct benefit.

7.3 Copper Components in Hybrid Cold-Plate and Immersion Architectures

Copper piping, quick disconnects and transition fittings are the sensitive points of galvanic corrosion in mixed-metal loops, where the coating both prevents continuous copper dissolution and slows accelerated corrosion of the aluminum anode.

7.4 Industrial and Energy Storage Liquid Cooling Heat Exchange Equipment

In closed ethylene glycol water loops running continuously where on-site cleaning conditions are limited, such as energy storage cold plates and industrial refrigeration heat exchangers, the economic case for ten-year cleaning-free maintenance is clear.

8. Conclusion

Surface treatment of the pure copper cold plate, though occupying a seemingly simple position on the heat transfer surface, directly determines the long-term reliability of the entire cooling loop. Existing technology routes each carry shortcomings such as exceeded thermal resistance, regulatory restriction, geometric inaccessibility, hydrolytic failure or insufficient contact angle.

The root causes are insufficient barrier density, interfaces relying predominantly on physical adsorption, hydrophilic and swelling-prone organic components, and conflation of the anti-fouling and barrier duties in one layer.

The layered collaborative scheme set out earlier, through coordinated design of a dense pore-free barrier, a low-surface-energy functional layer, a stable film-forming process, crosslinking chemistry compatible with the coolant, and compatibility with downstream assembly, promises to alleviate both the alkaline instability and the inadequate anti-fouling capability that have constrained the industry.

The verification method is the key to whether this scheme can deliver on its commitments. The central judgement is that a well-executed coating does not guarantee performance in the customer system.

Under most failure morphologies the routine customer-side online monitoring tools cannot detect coating state, so the genuine state criteria must come from physical sampling, teardown inspection and heat transfer performance, and a measured comparison of thermal resistance against heat transfer performance is the only quantitative metric a customer can directly associate with coating value.

This verification approach can be matched to a customer’s specific coolant formulation, operating conditions and system architecture, and iterated continuously against measured data.