Cause Analysis and Countermeasures for Plating Defects in Metallized Holes of Multilayer PCBs

The quality of plated-through holes is closely related to the quality and reliability of multilayer printed circuit boards. This article analyzes the main defects found in metallized-hole plating and their causes, then presents methods for optimizing process parameters and strengthening process and production control to ensure reliable hole metallization.

Introduction

Metallized holes provide the electrical interconnection between the different circuit layers of a multilayer PCB. The quality of the copper plating on the hole wall is therefore a core quality factor. The plating must have suitable thickness, uniformity, and ductility. It must also withstand a 288 °C thermal shock for 10 seconds without cracking.

A fracture in the hole-wall copper plating during thermal shock is a critical defect. It can cause an open circuit between inner-layer circuits or between an inner layer and an outer-layer circuit. In less severe cases, intermittent conduction may occur; in severe cases, the multilayer board may have to be scrapped.

The most common defects in metallized-hole plating include voids in the copper deposit, nodular deposits, insufficient copper thickness inside the hole, pink rings, and poor connections between the hole wall and the inner-layer copper lands. Most of these defects can result in product rejection, significant economic losses, and delayed delivery.

Multilayer PCB Manufacturing Process

A typical multilayer PCB manufacturing flow is:

Material cutting → inner-layer fabrication → etching → black oxide treatment → lamination → drilling → resin smear removal and desmear/etchback → hole metallization → panel plating → outer-layer fabrication → pattern plating → stripping → etching → solder-mask screen printing → hot-air solder leveling → legend screen printing.

The main causes of metallized-hole defects are discussed below from the perspectives of drilling, hole-wall desmear and etchback, plating, and multilayer lamination.

1. Drilling

Most copper-plating voids are associated with hole-wall defects caused by poor drilling quality. Typical defects include burrs at the hole entrance, rough hole walls, substrate pits, and epoxy resin smear. These defects can lead to voids in the hole-wall copper, separation between the substrate and the copper deposit, or an uneven plated surface.

1.1 Formation and Removal of Hole-Entrance Burrs

Regardless of whether manual or CNC drilling is used, and regardless of the drill type or drilling parameters, some burr formation is unavoidable when drilling copper-clad laminate. The effect of entrance burrs on metallized-hole quality has often been underestimated, but it is significant for high-reliability PCBs.

First, a burr changes the effective hole diameter and reduces the opening at the entrance, which can interfere with component insertion. Second, copper burrs that protrude into or recess around the hole affect the distribution of electric field lines during plating. This can produce insufficient plating thickness at the hole entrance and create local stress concentration. During thermal shock, the hole-entrance copper may then crack under the axial tensile stress caused by thermal expansion of the substrate.

The traditional burr-removal method uses 200- or 400-grit abrasive paper for careful grinding. Later, mechanical brushing with silicon-carbide abrasive nylon brushes became common. As PCB technology developed and 9–18 μm ultra-thin copper foil became more widely used, deburring technology also evolved. Liquid abrasive blasting has been adopted in some applications to remove hole-entrance burrs.

For copper-clad laminates with copper foil thicker than 18 μm, mechanical brushing is generally effective. However, the silicon-carbide abrasive size and brush pressure must be controlled carefully. Excessive pressure or overly coarse abrasive can expose the substrate at the hole entrance. Silicon-carbide abrasive in the range of approximately 320–380 grit is commonly used for deburring brush rolls.

Modern double-sided deburring machines typically use four brush rolls, with two above and two below the panel, to remove burrs from both sides in a single pass. When treating the same panel surface, the two brush rolls rotate in opposite directions. One rotates clockwise and the other counterclockwise. Combined with axial oscillation of each brush roll, this applies brushing force uniformly in multiple directions across the panel surface and removes the burrs more thoroughly. The deburring machine should also include a high-pressure spray-rinse section.

In liquid abrasive blasting, silicon-carbide abrasive is carried by a water jet and directed onto the panel surface to remove burrs.

1.2 Effects of Rough Hole Walls and Substrate Pits

When the electroless copper process is in good condition, holes with poor drilling quality are still more likely to develop copper voids. A smooth hole wall allows a continuous electroless copper layer to form more easily. On a rough drilled surface, the continuity of the electroless deposit is reduced and pinholes may form.

The risk is particularly high when the hole wall contains drilling pits. Even if the electroless copper layer appears complete, the subsequent electrolytic copper-plating step may produce a folded or uneven deposit around the pit. The copper layer may therefore become very thin in the recessed area, or copper may fail to deposit there entirely, creating a void.

1.3 Causes of Epoxy Resin Smear

PCB drilling is a complex machining process. Under the cutting, shearing, compression, tearing, and friction forces generated by the drill, the laminate undergoes elastic deformation, plastic deformation, and material fracture before the hole is formed.

A significant portion of the mechanical energy is converted into heat. The temperature rises sharply during high-speed cutting, and the drill temperature can exceed 200 °C. The glass-transition temperature of the resin in the PCB laminate is much lower than this. The softened resin is dragged by the drill and smeared onto the exposed copper-foil edges along the hole wall.

Resin smear is difficult to remove. Once a sufficient amount has accumulated on the copper-foil edge, it can reduce or even destroy the electrical interconnection between the multilayer board’s inner layers and the plated hole.

1.4 Methods for Improving Drilling Quality

Hole-entrance burrs, rough hole walls, substrate pits, and epoxy resin smear can be removed or reduced by strengthening process and quality control in the following areas.

1.4.1 Drill Quality Control

Drill quality has a decisive effect on drilling quality. The tungsten-carbide material should have a very fine, submicron-grade grain structure. The carbide should be free of pores and highly wear-resistant. The diameter tolerance of the shank and cutting section should be within 0–0.005 mm, and the concentricity tolerance of the complete drill body, tip, and shank should also be within 0.005 mm. The drill geometry must be free of defects; when inspected under 40× magnification, no chips or cracks should be visible.

A high-quality drill must also be symmetrical. Both sides of the drill must be identical in size and shape. Poor symmetry accelerates cutting-edge wear. Drill suppliers should therefore be selected carefully, and incoming drills must be inspected. Nonconforming drills must not be released for production.

1.4.2 Drill Shape Selection

Common drill geometries include standard, tapered, shovel-type, and special designs. For small holes, particularly small holes in multilayer boards, tapered, shovel-type, or special drills are generally preferred. These designs have a shorter margin length, typically about 0.5 mm, which can significantly reduce drilling heat and resin smear.

1.4.3 Length of the Chip-Flute Section

The length of the drill flutes is critical to chip evacuation. If the flutes are too short, chips cannot escape smoothly. Drilling resistance increases, the drill is more likely to break, and debris can smear the hole wall.

In general, the flute length should be approximately 1.15 times the total thickness of the stacked panels, including the entry sheet, plus the depth to which the drill enters the backup board. At least 15% of the flute length should remain outside the panel stack during drilling.

1.4.4 Control of Drilling Parameters

Important drilling parameters include the number of panels in each stack and the drill-speed-to-feed ratio. These parameters must be matched to the board construction, hole size, drill geometry, and required hole-wall quality.

1.4.5 Drill Life Control

Drill life varies from factory to factory and must be determined through specific process trials. When any hole-quality indicator approaches its tolerance limit, the drill should be replaced, resharpened, or scrapped. Under general conditions, the maximum permitted number of holes for a multilayer-board drill is approximately 1,500. Drills used for multilayer boards are generally not resharpened.

1.4.6 Use of Entry and Backup Materials

1.4.6.1 Entry Sheet

An entry sheet can perform several functions during drilling:

1. Protect the panel surface from damage caused by the pressure foot.
2. Reduce burr formation at the hole entrance.
3. Improve hole-diameter accuracy.
4. Reduce the probability of drill breakage in small holes.

Without an entry sheet, after the drill penetrates the thin copper foil, one side of the drill may strike the glass fabric and experience a larger cutting force. The drill can tilt, reducing positional accuracy. After breakthrough, the unbalanced force acting on the returning drill also increases the risk of breakage.

A hard aluminum entry sheet with a thickness of approximately 0.2–0.4 mm may be used.

1.4.6.2 Backup Board

A backup board prevents the drill from striking the machine table and helps prevent burrs on the exit side of the panel. In addition to being flat, the backup board must have suitable hardness and must be free of oil contamination to prevent hole misalignment, burrs, and hole-wall contamination.

Corrugated backup boards have also been used. After the drill passes through a thin aluminum entry sheet, it rotates at high speed in the air during retraction. A Venturi effect produces airflow that helps cool the drill and significantly reduces drilling temperature.

1.4.7 Control of Drill Penetration into the Backup Board

The drill should enter the backup board to an appropriate depth. Excessive penetration leaves too little of the flute section outside the panel, which hinders chip evacuation and increases the risk of hole blockage and drill breakage. Excessive penetration may also damage the machine table.

Backup boards approximately 1.5–1.7 mm thick are often used twice, once on each side, to improve utilization and reduce cost. If the drill penetrates too deeply, it may pass through the backup board and generate burrs.

For drills larger than 0.6 mm in diameter, the recommended penetration depth into the backup board is 0.75 mm. For drills smaller than 0.6 mm but larger than 0.3 mm, the penetration depth should be 0.6 mm. For drills smaller than 0.3 mm, a sheet of paper can be placed on the backup board and a trial hole drilled to determine the appropriate depth. Once established, these values should be entered into the drilling program.

2. Hole-Wall Desmear and Etchback

Etchback and desmear are related but independent concepts and process steps.

Etchback is the controlled removal of nonmetallic material from the hole wall to a specified depth in order to fully expose the conductive surfaces of the inner layers.

Desmear is the removal of molten resin and drilling debris from the hole wall. Etchback also removes smear, but a desmear process does not necessarily produce an etchback effect.

Even when high-quality laminate is selected and lamination and drilling parameters are optimized, some epoxy smear on the hole wall is unavoidable. Multilayer boards must therefore undergo desmear before hole metallization. To further improve the connection reliability between the metallized hole and the inner-layer conductor, etchback should preferably be performed at the same time.

Etchback removes the epoxy-resin smear and causes the inner-layer conductor to project slightly into the hole. After metallization, this geometry provides a more reliable three-dimensional connection between the inner conductor and the plated hole wall, significantly improving multilayer-board reliability. The typical etchback depth is 5–10 μm.

Four general desmear methods are used: plasma, concentrated sulfuric acid, chromic acid, and potassium permanganate. Potassium-permanganate desmear has several advantages. It produces a slightly rough resin surface rather than the smooth surface produced by concentrated sulfuric acid. It is also less likely than chromic acid to over-etch the resin and leave glass fibers protruding from the hole wall, and it has a lower tendency to produce pink rings. For these reasons, potassium permanganate is widely used.

2.1 Typical Process Parameters

For an Atotech solution system, the following parameters are typical:

1. Sweller: Securiganth P, 450–550 mL/L; optimum, 500 mL/L. pH adjuster, 15–25 mL/L; optimum, 23 mL/L, or sodium hydroxide, 6–10 g/L; optimum, 10 g/L. Operating temperature: 60–80 °C, optimum 70 °C. Treatment time: 5 minutes 30 seconds.

2. Potassium permanganate: KMnO₄, 50–60 g/L; optimum, 60 g/L. Sodium hydroxide: 30–50 g/L; optimum, 40 g/L. Operating temperature: 60–80 °C, optimum 70 °C. Treatment time: 12 minutes.

3. Reducer: Securiganth P, 60–90 mL/L; optimum, 75 mL/L. Sulfuric acid, H₂SO₄, 55–92 g/L; optimum, 92 g/L. Glass etchant, 5–10 g/L; optimum, 7.5 g/L. Operating temperature: 50 °C. Treatment time: 5 minutes.

2.2 Process Maintenance

The concentrations of potassium permanganate, potassium manganate, and sodium hydroxide should be measured once a week. The KMnO₄ and NaOH concentrations should be adjusted when necessary.

Where possible, continuous electrolysis should be maintained so that potassium manganate, K₂MnO₄, is oxidized back to potassium permanganate, KMnO₄.

The color of the PCB surface after potassium-permanganate desmear can also be used as a process indicator. A purple-red surface generally indicates a normal solution condition. A green surface indicates that the K₂MnO₄ concentration is too high and that electrolytic regeneration should be strengthened.

3. Plating

3.1 Chemical Micro-Etching Before Plating

To ensure good adhesion between the electroless copper layer and the base copper foil, the copper surface must undergo micro-roughening before electroless copper deposition. This is generally performed by chemical micro-etching. The process creates a microscopically rough surface with increased surface activity, improving the bond between the copper foil and the deposited copper.

Common micro-etch systems include ammonium persulfate, hydrogen peroxide, and sodium persulfate. Ammonium-persulfate solutions are relatively unstable and decompose easily, making the etch rate difficult to maintain. The hydrogen-peroxide/sulfuric-acid system is convenient and can be automatically replenished to control concentration, but it requires hydrogen-peroxide stabilizers and wetting agents. It is also relatively expensive and normally provides a lower copper etch rate of approximately 0.5–0.6 μm/min.

The sodium-persulfate/sulfuric-acid system provides a higher and more stable copper etch rate and can produce a suitable microsurface roughness. This helps the electroless copper layer withstand thermal shock. To prevent cracking during a 288 °C, 10-second thermal shock, the copper etch rate should reach approximately 0.7–0.9 μm/min. The sodium-persulfate/sulfuric-acid system can achieve this range.

A typical process formulation is:

• Sodium persulfate: 60–80 g/L; optimum, 70 g/L.
• Sulfuric acid: 15 mL/L.
• Cu²⁺: 1–20 g/L.
• Operating temperature: 30–50 °C.
• Treatment time: 2 minutes.

Before production each day, analyze the sodium-persulfate concentration and adjust it when necessary. Also measure the etch rate daily. An 18 μm copper-foil test coupon can be immersed in the operating tank, and the time required to remove the foil can be recorded to obtain a quick and convenient estimate of the etch rate. Sodium persulfate should be replenished when required.

When the dissolved copper concentration exceeds 20 g/L, the solution should be replaced. The etch rate may be low when a fresh bath is started, in which case the sodium-persulfate concentration can be increased appropriately.

3.2 Optimization of Electroplating Parameters

For fine-line, high-density boards, high-layer-count boards with 14–20 layers, and small holes with a high aspect ratio of approximately 6:1–10:1, the main challenges are poor solution exchange inside the holes and poor plating uniformity and throwing power. Electroplating parameters must therefore be optimized.

1. Use a plating-bath composition with relatively low Cu²⁺ and high H₂SO₄ concentrations. The sulfuric-acid-to-Cu²⁺ concentration ratio should be at least 10:1. Typical values are Cu²⁺, 10–13 g/L; H₂SO₄, 190–220 g/L; Cl^-, 30–50 ppm; [H₂SO₄]:[Cu²⁺] = 17:1–20:1; and temperature, 22–26 °C.

2. Use a relatively low cathode current density and a longer plating time.

3. Maintain the three normal agitation methods—cathode movement, compressed-air agitation, and solution circulation—and install a vibration device on the operating rod.

With a vibration device, the cathode moves not only back and forth but also vertically. This promotes solution exchange inside small holes and improves throwing power, reducing the difference in copper thickness between the hole entrance and the center of the hole.

These measures improve plating uniformity and throwing power in small holes, reducing the risk of thin copper deposits or copper voids and improving the reliability of hole metallization.

4. Multilayer Lamination

The development of information technology has increased PCB layer counts, wiring density, structural complexity, and dimensional requirements. As a result, lamination has become a critical step in multilayer-board manufacturing.

The lamination process mainly includes inner-layer preparation and lamination itself. Two aspects are particularly important to the quality of metallized holes.

4.1 Ensure Complete Laminate Curing

The laminate materials considered here include individual inner-layer sheets and the multilayer PCB after lamination.

1. After inner-layer sheets are cut, the number of panels in each stack should be controlled according to sheet thickness. The stacks should be placed horizontally and pre-baked.
2. After lamination, the multilayer PCB should undergo post-bake curing before drilling. Post-bake curing should not be postponed until after drilling.

Without these two steps, the laminate may not be fully cured. This increases the risk of epoxy smear and degrades drilling quality. When an incompletely cured laminate is drilled, large amounts of highly viscous debris can fill the drill flutes. If the debris cannot be evacuated, the drill may break inside the hole.

4.2 Optimize Lamination to Reduce Pink Rings

A pink ring is the condition in which the oxide film at the interface between a hole wall and an inner-layer copper land changes color or is removed by chemical reaction, exposing the natural pink color of the copper.

As the number of PCB layers increases, the risk of delamination at the intersection between the inner-layer copper foil and the hole also increases. As the hole diameter decreases, cleaning the hole becomes more difficult, and chemicals are more likely to penetrate and attack the interfaces between layers along the hole wall. Therefore, pink rings become more likely as layer count increases and hole diameter decreases.

Pink rings are often discovered late in PCB production and can seriously affect product quality. They can reduce interlayer bonding strength. Solutions may penetrate along the direction of the glass fibers, reducing insulation resistance between closely spaced pads and, in severe cases, causing short circuits. In addition, because the copper-ring contact area is reduced, defects that might otherwise be tolerated in a metallized hole—such as blistering or voids—can increase hole resistance or even cause an open circuit.

Pink rings may originate during inner-layer surface treatment, lamination, curing, drilling, etchback, electroless copper deposition, or electrolytic copper plating. The key factors are the bonding strength between the black-oxide layer and the substrate and the chemical resistance of the black-oxide layer.

During PCB processing, the board experiences both vertical mechanical impact and horizontal chemical attack. The interlayer bond must therefore be strong enough to resist both effects. Low peel strength between layers and poor chemical resistance of the black-oxide layer are the primary causes of pink rings.

Two main countermeasures are used:

1. Increase the bonding strength between the black-oxide layer and the substrate. Black-oxide treatment forms an oxide layer on the copper surface, consisting of black copper oxide, red cuprous oxide, or a mixture of both. The layer increases the effective surface area and improves bonding between the copper foil and the laminate. The bonding performance depends on the black-oxide process, the crystal structure of the oxide, and the oxide-layer thickness, so the process parameters must be optimized.

2. Improve the corrosion resistance of the black-oxide layer. The oxide-layer thickness can be reduced mechanically or by chemical reduction. Possible reducing agents include formaldehyde/sodium hydroxide, sodium hypophosphite, and sodium borohydride. After reduction, the black-oxide layer has improved peel strength and greater resistance to acid attack.

5. Other Factors That Cause Metallized-Hole Defects

5.1 Voids Caused by Air or Gas Bubbles

Gas bubbles inside a hole can prevent the plating or activation solution from wetting and accumulating on part of the hole wall. This ultimately produces a void in the metallized hole. Bubbles may be introduced from outside or generated by chemical or electrochemical reactions.

5.1.1 How Bubbles Enter the Hole

External bubbles may enter through holes when the panel is lowered into a process tank or during vibration and oscillation.

Gas generated internally may result from a side reaction in the electroless copper bath:

2HCHO + 2Cu²⁺ + 4OH^- → Cu + 2HCOO^- + 2H₂O + H₂↑

Gas may also be generated in the electroplating bath. Hydrogen can form at the cathode, and oxygen can form at the anode:

Cathode side reaction: 2H⁺ + 2e^- → H₂

Anode side reaction: 2H₂O - 4e^- → O₂↑ + 4H⁺

5.1.2 Characteristics of Gas-Bubble Voids

Voids caused by bubbles are often located near the center of the hole. Metallographic cross-sections typically show a symmetrical distribution, with copper absent over similar widths on opposing sides of the hole wall.

5.1.3 Processes That May Produce Gas-Bubble Voids

Gas-bubble voids may occur during electroless copper deposition, panel plating, or pattern plating.

5.1.4 Methods for Preventing Bubbles from Entering Holes

Vibration and impact are effective methods for removing bubbles from holes. Increasing the spacing between panels and increasing the cathode travel distance are also important.

Air agitation in the electroless copper tank and impact or vibration in the activation tank are less effective for preventing bubbles from entering holes. Improving solution wettability during electroless copper processing and preventing bubbles in the pretreatment tanks are also important.

The surface energy of the plating solution affects the size of hydrogen bubbles before they escape from or collapse inside a hole. The objective is to remove the bubbles from the hole before they grow large enough to obstruct solution exchange and create a plating defect.

5.2 Voids Caused by Dry-Film Resist

5.2.1 Characteristics of Dry-Film-Related Voids

Voids caused by organic dry film are often located near the hole entrance, approximately 50–70 μm from the panel surface and extending over a width of about 50–70 μm. Edge voids may occur on one or both sides of the panel and can cause either a partial or complete open circuit.

5.2.2 Causes of Resist Entry into Holes

When a hole is covered by dry film, the air pressure inside the hole can be approximately 20% lower than atmospheric pressure. During lamination, the air inside the hole is heated. As it cools to room temperature, the pressure decreases. The resulting pressure difference gradually draws resist into the hole until development.

Three main factors determine the speed and depth of resist flow:

• Water or moisture remaining in the hole before dry-film lamination.
• Small holes with a high aspect ratio.
• A long interval between dry-film lamination and development.

Moisture trapped in a hole is a particularly important factor. Water can reduce the resist viscosity and allow it to flow into the hole more rapidly. Small, high-aspect-ratio holes are more susceptible because they are difficult to dry and the resist is more difficult to remove during development. A long delay before development also allows more resist to enter the hole.

5.2.3 Formation of Hole-Entrance Voids

When resist enters the hole and is not removed during development, it blocks copper and tin plating. After the resist is stripped, the copper layer beneath it is exposed. During etching, this exposed copper is removed, creating a plating void.

5.2.4 Preventive Measures

The simplest and most effective method for preventing hole-entrance voids is to improve drying after surface treatment. If the holes are thoroughly dry, hole-entrance voids should not occur. Extended storage time and poor development alone will not cause this defect when the holes are properly dry.

After improving drying, the interval between dry-film lamination and development should be kept as short as practical while maintaining process stability. Hole-entrance voids may appear after any of the following changes or conditions:

1. Installation of new surface-treatment or drying equipment.
2. Failure or degraded performance of the drying section in the surface-treatment equipment.
3. Production of boards with small, high-aspect-ratio holes.
4. A long delay between lamination and development.
5. A change in resist type or the use of thicker dry film.
6. A larger pressure differential in a vacuum laminator.

5.3 Voids Caused by Other Contaminants

Solid particles, such as dust or fibers, and organic contamination can also prevent the plating or activation solution from wetting the entire hole wall. These contaminants ultimately cause voids in the metallized-hole copper layer.

Conclusion

The causes of metallized-hole plating defects in multilayer PCBs can originate as early as drilling or may not become apparent until lead or tin plating. A single plating defect is often caused by several interacting process conditions. These conditions may act simultaneously or occur in sequence.

Careful analysis along the complete manufacturing flow is therefore essential. When necessary, metallographic cross-sectioning should be used to identify the root cause accurately. By optimizing process parameters and implementing strict process and production control, manufacturers can improve the quality and reliability of metallized holes in multilayer PCBs.