How to Clean PCB Gold Fingers: Essential Tips and Methods

September 19, 2023

Table of Contents

Introduction

PCB gold fingers are the gold-plated pads on the edge of printed circuit boards (PCBs) that provide electrical and mechanical connectivity. Over time, these fingers can accumulate dirt, grime, and oxidation which can degrade signal integrity.

Cleaning PCB gold fingers properly is therefore essential for maintaining optimal performance. This comprehensive guide will provide key information on how to effectively clean PCB gold fingers.

What are PCB Gold Fingers?

PCB gold fingers, also known as edge connectors, refer to the gold-plated copper pads on the edge of a PCB. They are used to make electrical contact and facilitate communication between the PCB and mating connectors or slots in a larger system.

The gold plating serves several important functions:

Excellent conductivity for signals
Corrosion resistance to prevent oxidation
Hardness to withstand repeated insertion and removal

Common applications of PCB gold fingers include:

Memory cards
Graphics cards
Motherboard expansion slots
Header connectors

Gold fingers are plated using an electroplating process to deposit a layer of gold over nickel. This creates a hard gold finish around 30-50 microns thick with excellent wear resistance.

Why Clean PCB Gold Fingers?

Over time, PCB gold fingers will accumulate contaminants from handling and exposure to air. This includes:

Oils and grease from skin contact
Dust and particulates in the air
Oxidation of the gold surface

These contaminants can cause several issues:

Increased contact resistance leading to transmission errors
Intermittent connections from poor contact with mating connectors
Accelerated wear when inserting dirty gold fingers into slots
Degraded signal integrity

Regular cleaning is therefore essential for optimal performance and maximizing the lifetime of PCB gold fingers. Other benefits include:

Improved conductivity for faster data speeds
Preventing downtime from connection issues
Avoiding damage to mating connectors and slots

How to Clean PCB Gold Fingers

Cleaning PCB gold fingers involves three main steps:

Visual Inspection

Closely inspect the PCB gold fingers to check for any signs of damage, wear, or contaminants. Use magnification if required.
Look for discoloration, scratches, and indentations which may impact performance.
Pay special attention to the contact regions and chamfered edges that experience the most wear. Any issues here will directly affect connectivity.

Remove Surface Contaminants

Use high-purity isopropyl alcohol (IPA) and lint-free swabs to remove any oils, grease, or grime.
Gently rub the swab back and forth along each gold finger pad. Avoid any scrubbing or friction which could damage the gold plating.
For heavier contamination, multiple cleaning passes may be required. Allow the IPA to slightly dissolve the grime before swabbing again.
Use an air duster to blow away any remaining dust or particulates after cleaning with IPA.

Remove Oxidation

Check for any dull, dark areas on the gold fingers indicating oxidation. This naturally occurs over time with air exposure.
Use a pencil eraser to lightly rub off the oxidized surfaces. This works well to restore the original gold color and shine.
For heavier oxidation, a more abrasive cleaner may be required. Fiberglass scratch brushes specially made for PCBs can scrape off oxidation without damaging the gold plating.
Swab with IPA afterward to remove any eraser or cleaner residue.

Best Practices

Follow these tips and best practices when cleaning PCB gold fingers:

Use lint-free swabs instead of regular cotton swabs which can leave behind fibers and lint.
Select high-purity 99% isopropyl alcohol for cleaning. Lower concentrate alcohol contains more water residue.
Allow alcohol to fully evaporate before reinserting PCBs to prevent corrosion issues.
Only apply light pressure when scrubbing. Excessive force can damage plating.
Do not use acetone or other harsh solvents which may react with materials.
Work in a clean environment to avoid recontamination after cleaning.
Periodically clean mating connectors and slots that contact the gold fingers.
Visually inspect for wear every few cleaning cycles and replace heavily worn fingers.
Consider conformal coating PCBs after cleaning for added protection.

Cleaning Methods by Contaminant

The best cleaning method depends on the specific contaminant present on the PCB gold fingers. Each contaminant requires a tailored approach and cleaning agents to effectively remove it from the gold-plated surfaces.

Dust and Particulates

Dust and particulates settling out of the air are some of the most common PCB gold finger contaminants. Fine particles accumulate rapidly in most environments.

Several cleaning methods can tackle dust buildup:

Compressed Air – Blasts of compressed air provide quick, non-contact removal of dust and loose particulates. angling the air nozzle along the gold finger rows dislodges debris. Avoid excessive air pressure which could damage plating.
Isopropyl Alcohol – Lint-free swabs dampened with isopropyl alcohol lift and absorb settled dust when wiped across gold fingers. The alcohol helps grab and remove fine particles from the finger surfaces. Swab gently to avoid any abrasion.
Vacuum – A vacuum with a narrow nozzle can suck away dust and particulates. This is more thorough than compressed air. Ensure the nozzle does not contact the PCB surface which could cause abrasion. Maintain a distance of 1/4 inch or more.
Anti-static Brush – Special conductive/carbon fiber brushes designed for electronics use can sweep dust away. The antistatic properties prevent reattracting particles. Keep brush bristles soft and avoid applying force.
Adhesive Roller – Adhesive lint rollers effectively pick up dust and fibers. Roll gently across gold fingers to lift away debris without leaving any sticky residue. Use a fresh adhesive sheet for each PCB to prevent recontamination.

Any remaining dust should be blown off with an air duster after cleaning with alcohol or other methods to fully prepare the gold finger surfaces.

Grease and Skin Oils

Grease, oil and other residues transferred by hand contact are common on PCB gold fingers. The skin naturally secretes oils that will coat any touched surfaces.

Several solvents and cleaning agents can tackle oily residues:

Isopropyl Alcohol – The most common choice for cleaning oils and greases. The alcohol cuts through and dissolves oil residues when gently wiped on gold fingers using lint-free swabs. May require multiple passes to fully remove heavy buildup.
Acetone – A stronger and faster-evaporating solvent, acetone breaks down oils even quicker than IPA. However, avoid overuse as acetone can damage some plastics if left to soak. Rinse with IPA after cleaning with acetone.
Aqueous Cleaners – Water-based cleaners containing surfactants and detergents emulsify and lift away grimy oils when used with a brush. A final IPA wipe is recommended after using aqueous cleaners as they leave a film.
Burnishing – A fiberglass burnishing tool rubbed across gold fingers mechanically grinds away oil residue. This is more forceful but avoids chemicals. Use very gently to avoid removing the plating.

Repeated cleaning over time is required for frequent handling as oils quickly reaccumulate on gold finger surfaces. Use IPA routinely to prevent heavy buildup.

Hardened Grime

Over time, dust and oils can combine into a thicker grime layer coating gold fingers. This greasy film attracts more contaminants and is challenging to remove when heavily caked on.

Specialized methods are required for hardened grime:

Ultrasonic Cleaner – Powerful ultrasonic energy vibrates and cavitates grime-free when submerged. Combine with a heated aqueous cleaning solution loaded in the bath for best results. Rinse fingers afterward.
Heated Soak – Soaking gold fingers in a bath of hot isopropyl alcohol helps soften caked-on grime so it can be swabbed away gently. Apply fresh IPA and scrub softly with a nylon brush once softened.
Abrasive Erasers – Lightly rub a fiberglass or aluminum oxide foam-type eraser across gold fingers to mechanically grind away stubborn grimy deposits. The mild abrasives target grime only. Rinse afterward.
Gold Plating Brightener – These specialized chemicals combine mild acids, detergents, and emulsifiers to dissolve hardened contaminant layers and restore gold color. Rinse thoroughly with deionized water after soaking fingers.

Avoid using excessive force and abrasives when removing heavy grime to prevent damage to the gold plating layer underneath. Repeated soft wiping or soaking works better than harsh scrubbing.

Oxidation

The thin gold plating on PCB fingers will slowly oxidize over time with environmental exposure. This forms a dark tarnish layer on the gold surfaces. Oxidation increases contact resistance.

Effective techniques for removing this oxidation include:

Pencil Eraser – The abrasive material in a pencil eraser polishes away oxidation when gently rubbed across gold fingers. This restores the original shine and color. It provides mild abrasion to minimize gold plating removal.
Burnishing Tool – More effective than erasers, motorized burnishing tools use abrasive tips at high speeds to rapidly abrade oxidation off gold-plated surfaces. Use the lowest speed setting and with very light pressure when burnishing PCB fingers.
Mild Acid Solution – A dilute citric or acetic acid solution removes oxidation through a chemical process without abrasion. Rinse fingers thoroughly with deionized water afterward. Only use acids compatible with PCB materials.
Ultrasonic Cleaning – The cavitation energy generated by ultrasonic cleaners blasts oxidation right off gold-plated surfaces. Use with an aqueous cleaning agent, and rinse thoroughly afterward.

Avoid sandpaper, steel wool, or aggressive abrasives which can scratch gold fingers down to the copper underneath. This exposes them to corrosion and reduces conductivity.

Any cleaner residue should always be removed with isopropyl alcohol to avoid attracting contaminants back to cleaned gold fingers.

Solder and Flux

Soldering near gold fingers will splatter tiny hot droplets of solder. Flux fumes also deposit residues. This must be chemically removed.

Solder Remover – Specialized organic solvent blends dissolve and strip solder without reacting with gold plating. Soak fingers to dissolve solder then rinse with isopropyl alcohol.
Flux Remover – Alkaline aqueous cleaners are formulated to cut through tacky flux residues. Follow with IPA rinse afterward.
Desoldering Braid – Heat and wipe carefully with a desoldering braid to wick solder away without touching gold. Avoid overheating fingers.

Preventing solder splatter onto gold fingers is ideal by using masking, barriers, and disciplined soldering techniques. Remove any splatter immediately before oxidation occurs.

Adhesives and Conformal Coatings

Conformal coatings and adhesive residues require chemical removal – avoid scraping or abrasion which could damage plating.

Specialized Strippers – Formulated to dissolve and detach cured conformal coatings and epoxies. Ensure compatibility with PCB.
Plastic-Safe Solvents – Dimethyl carbonate-based cleaners soften many coatings for removal with soft plastic tools. Avoid chlorinated or acetone-based solvents.

Test chemicals on a small area first to validate they will not react with PCB materials or damage the gold plating underneath.

Corrosion and Tarnishing

Discoloration, pitting, and roughness on gold fingers indicate corrosion or tarnishing processes are actively damaging the gold plating layer.

Mild Abrasive – Very gently rub with a fiberglass or hard eraser to remove corrosion damage. Take care not to rub through the thin gold layer underneath.
Gold Plating Touch-up – Replenish corrosion-damaged areas with chemical gold plating solutions or hard gold electroplating to restore conductivity and color.
Conformal Coating – Prevent recurrence by coating PCB with a thin acrylic, urethane, or perylene protective film. Mask gold fingers during application.

Identify and address the root cause of corrosion whether chemical, humidity, contamination-driven, etc. to prevent rapid re-tarnishing after cleaning.

Etching Damage

Overly aggressive PCB etching or damaged solder masks can etch into gold finger plating. This appears as roughness, pitting, or missing gold, especially along mask edges.

Gold Plating Touch-up – Plate additional gold thickness to restore conductivity in etched areas. May also require nickel underplating first.
Solder Mask Repair – Reapply the solder mask if damaged to protect the underlying gold plating from further etching.

Adjust PCB etching processes to use gentler chemicals and conditions if the etchant is too strong. Inspect boards after etching.

Wear and Abrasion

Repeated insertion cycles gradually wear down gold plating thickness along the finger contact surfaces and edges. This accelerates with contamination present.

Manual Touch-up – Use a gold plating pen or selective brush plating to build up thickness on worn areas.
Electroplating – Re-plate the entire gold finger area to restore even thickness across all fingers.
Gold Finger Replacement – For severe wear, desolder and replace damaged gold finger connector strips.
Preventive Maintenance – Clean frequently and consider edge card guides or separators to prevent slot abrasion.

Inspect gold fingers under magnification at regular intervals for signs of plating abrasion to avoid wear-out in the field.

Scratches and Nicks

Mishandling and repeated insertion cycles can cause mechanical damage like scratches or nicks in gold plating.

Gold Pen Plating – Fill in scratches with gold plating pens specifically for PCB rework. Match original thickness.
Selective Plating – Use miniature brushes to selectively plate over scratches and nicks. Avoid bridging between fingers.
Immersion Gold – For slight scratches, apply immersion gold over the entire area to replenish thin gold.
Conformal Coating – Coat with plastic film to prevent scratched areas from oxidizing and spreading.

Solder Mask Damage

The solder mask insulation layer around gold fingers can crack or peel, risking plating damage.

Solder Mask Repair – Use a solder mask repair pen, syringe, or spray to patch damaged areas and restore insulation. Avoid contact with gold fingers.
Conformal Coating – Edge coats the PCB alongside gold fingers to essentially replace the solder mask function. Prevent coating contact.

Ensure the solder masking process leaves no thin spots or gaps that can easily crack from gold finger friction and wear over time.

Cleaning Kits

Specialized PCB cleaning kits provide all the supplies needed to properly clean gold fingers. Typical components include:

Lint-free swabs – The swabs used for applying isopropyl alcohol and cleaning agents must be lint-free. Standard cotton swabs can leave fibers and particles behind. Look for plastic-handled swabs with cleanroom-grade synthetic tips.
Isopropyl alcohol – Select 99% purity IPA without any additives or water dilution. Lower-purity IPA contains more water residue. Look for manufacturer-sealed containers to avoid moisture absorption.
Fiberglass eraser – For polishing away mild oxidation, pencil-style erasers with fiberglass abrasive fillers are ideal for gold fingers. Ensure the eraser material is non-conductive.
Cleaning brush – Small handled brushes with soft nylon, carbon fiber, or ESD-safe bristles allow scrubbing with alcohol and cleaning agents. Avoid any brushes that could abrade gold plating.
Anti-static brush – A conductive brush can sweep away loose particulates without generating static charges that re-attract contaminants. Look for <10^5 ohm/sq resistivity.
Lint-free wipes – Pre-cut wipes allow safe handling of PCBs and drying after cleaning. Look for wipes meeting aerospace cleanroom standards with low ions and particulates.
Adhesive roller – Microscopic adhesive rollers lift away dust and fibers. Avoid standard lint rollers with paper backing that can tear and leave debris.
Cleaning trays – Provide a controlled work area for cleaning. Look for anti-static plastic trays with raised edges to prevent spills and lost small parts.

Cleaning Machines

For high-volume production environments, automated PCB cleaning machines provide consistent and repeatable gold finger cleaning:

Aqueous Washers – Multi-stage systems using spray wash cycles with water-based detergent cleaning agents held at 140-200°F rinsed by DI water.
Solvent Washers – Circulate heated solvents like modified alcohol, terpenes, and glycol ethers to dissolve soils. May incorporate ultrasonic energy. Rinse and vacuum dry.
Plasma Etchers – Use ionized gas plasma discharge to oxidize organic contaminants into gaseous byproducts which are vacuumed away.
Laser Ablation – Precisely scan a focused laser across the PCB surface to vaporize and decompose contamination without contact.

Automated cleaning machines offer these advantages compared to manual techniques:

Speed and Volume – Can clean hundreds of PCBs per hour without operator involvement.
Repeatability – Ensure every board receives the same wash treatment for consistent results.
Fewer Consumables – Greatly reduce cleaning agents, wipes, and other supplies used.
Reduced Risks – Removes operator variables and prevents mishandling or brush damage.

The initial equipment investment may range from $25,000 to $200,000 based on the cleaning methods and throughput requirements.

Cleaning Frequency

The optimal cleaning frequency for PCB gold fingers depends on environmental conditions and level of use:

Office environments – Minimal airborne particulates or oils. Clean every 2-3 months.
Industrial environments – More contaminants than an office. Clean every 1-2 months.
Workshop floors – Very high exposure to dust, abrasives, and fluids Warrants cleaning every 2-4 weeks.
Frequent mating cycles – Gold fingers inserted and removed hundreds of times per day require cleaning every 100-500 cycles.
Rugged use conditions – Vibration, shock, wide temperature swings. Accelerates wear and contamination ingress requiring more frequent cleaning such as monthly.

Intermittent electrical issues – Any connectivity or data errors are an immediate indicator of the need for gold finger cleaning to restore performance.

Signs Requiring Cleaning

Watch for these signs of dirty gold fingers requiring cleaning:

Darkened gold color – Dull, darkened, or discolored gold plating indicates oxidation or contaminants coating the fingers. The naked eye quickly sees this warning sign.
Visible debris – Dust, fibers, or grime visibly built up on the gold plating signals a need for cleaning before further accumulation. Lint and particulates readily cling to any residue.
Increased mating force – Greater engagement force needed to mate the PCB suggests contamination buildup, increasing friction and interfering with smooth insertion.
Intermittent contacts – Any detection of transient disconnects or electrical connectivity issues points to dirty/oxidized gold finger contacts.
Accelerated wear – Check for rubbing or accelerated wear on the ends of gold fingers or the mating connector sockets—direct evidence of abrasive contaminants.

The visible appearance of gold fingers is usually the first early indicator of contamination requiring attention, before any functional or reliability impact.

Cleaning Damaged Gold Fingers

Gold fingers suffering from scratches, nicks, wear, or missing plating present challenges:

Avoid abrasives – Even mild abrasives can extend damage to scratched or nicked fingers. Use very light-pressure and non-abrasive attachments when cleaning localized defects.
Gold layer touch-up – Replace missing gold plating using selective brush plating or gold dispensing pens on worn or scratched areas to prevent copper exposure. Match the original plating thickness.
ENIG coating – For scratches or cracks, apply a thin immersion gold layer across all fingers to reinforce thin gold plating and prevent copper oxidation.
Hard gold replating – For extensive wear or missing plating, electroplating the entire gold finger area with nickel underplating and hard gold on top to restore thickness.
SMT finger replacement – Desolder damaged fingers and replace them with new SMT gold-plated connector strips for a permanent repair. Avoid repeated desoldering to prevent pad damage.
Conformal coating – Edge coat the PCB with a thin polymeric film for added insulation and corrosion prevention on damaged gold fingers. Take care to avoid contact with healthy areas.

Cleaning Other PCB Connectors

Many other PCB components beyond gold fingers need periodic cleaning:

Edge Connectors

The exposed copper pads along edge card and PCI connectors oxidize readily without the gold plating seen on fingers. More frequent cleaning is needed.

D-Sub Connectors

The D-shaped metal pins housed in insulating plastic are prone to grime and oxidation buildup. Clean gently to avoid bending pins.

SIM Card Sockets

Repeated SIM card insertion gradually accumulates oils and dirt inside these sockets which can affect electrical contact.

Memory Card Slots

Portable electronics with externally exposed memory card slots require occasional cleaning as dust ingress is common through the opening.

Board-to-Board Connectors

Direct PCB-to-PCB stacking connectors experience abrasion and shedding of oxidation particles during vibration and shock which requires periodic cleaning.

Take care when cleaning any PCB electrical contacts besides gold fingers. Ensure no lint or fibers are left behind which could compromise insulation or electrical integrity.

Can Gold Fingers be Soldered?

It is not recommended to solder PCB gold fingers. The gold plating is very thin and soldering will damage the gold layer or dissolve it entirely. This destroys the corrosion resistance and interconnect functionality.

However, SMT gold finger connector strips can be soldered to a PCB to salvage damaged pads or replace worn fingers. Use solder paste and reflow soldering for best results.

Conclusion

Cleaning PCB gold fingers is a simple but essential maintenance process for preserving signal integrity and connectivity. Contaminants inevitably accumulate on gold-plated contacts from handling and exposure during use.

Regular inspection and cleaning using isopropyl alcohol, erasers, and specialized tools will remove grime, oxidation, and other contaminants. This maintains optimal performance and maximizes the lifetime of expensive PCB assemblies.

Implementing a scheduled cleaning regimen and using the right techniques for each contaminant will keep PCB gold fingers in pristine condition.

Frequently Asked Questions (FAQs)

How often should I clean PCB gold fingers?

Clean every 2-3 months for office environments, 1-2 months for industrial environments, and every 2-4 weeks for harsh conditions. Frequent mating cycles may require cleaning every 100-500 insertions.

What are the best materials for cleaning gold fingers?

Use high-purity 99% isopropyl alcohol applied with lint-free swabs. Pencil erasers work well for removing oxidation. Avoid acetone or other harsh solvents.

How do I clean heavy oxidation on gold fingers?

For heavy oxidation, gently roll a fiberglass eraser across the contacts. Letting them soak in isopropyl alcohol first helps soften stubborn oxidation. Follow up with alcohol and swabs afterward.

What causes gold fingers to oxidize and darken?

Oxidation naturally occurs over time with air exposure. Temperature swings, humidity, contaminants, and lack of mating can accelerate oxidation buildup on the thin gold surfaces.

Can I use sandpaper or steel wool on gold fingers?

Avoid abrasives like sandpaper or steel wool. The gold plating on the fingers is very thin and this will damage the gold layer leading to corrosion and conductivity issues.

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