Copper Voids And Plating Quality In PCB Barrels

The copper that lines a plated hole is deposited atom by atom from a bath, and it is possible for the deposit to grow with a gap inside it. A copper void is a cavity in the plating, usually at the corner where the hole meets the pad or in the middle of a barrel that is deep and narrow, and it is one of the most serious defects a board can carry because it is invisible from outside and it breaks a connection that the board was tested against.

This article explains how a void forms, how the bath and the current contribute, how the defect is detected, and how it is prevented.

What A Void Is And Where It Forms

Copper voids are regions of the barrel where the copper is missing or is not continuous, and they have to be distinguished from a thin plating and from a crack. A thin area conducts but has less margin; a crack is a separation that appears after thermal stress; a void is a hole in the metal that exists from the moment the plating was deposited. All three can be present in the same barrel.

The typical locations are the inside corner of the hole and the pad, the middle of a long barrel where the electrolyte is slow to exchange, and the junction between the electroless copper and the electroplated copper. Each of those locations has a different cause, and a section that shows the position of the void is what identifies which of them applies.

Chemical Causes

The bath has to deposit copper evenly on a surface that is not uniformly accessible. The electrolyte contains copper, acid, chloride and organic additives, and the additives are what produce a bright, ductile deposit with good throwing power. When the balance between them drifts, the deposit becomes rough, brittle or non uniform, and a void can form where the deposit does not fully cover the surface before the layer above it closes over the gap.

Contamination is the other chemical cause. Organic material carried in on the boards, particles from the anode bags, or a breakdown product from the additives all interfere with the deposition locally. The chemistry of the bath is therefore analysed on a schedule rather than only when a problem appears, and the additives are dosed by a hull cell or a cyclic voltammetric stripping test rather than by the number of panels plated. The general subject is described under electroplating additives.

Electrical Causes

The current density in a hole is not uniform. It is highest at the surface and falls towards the middle of a long barrel, because the current has to travel through a narrow column of electrolyte and the resistance of that column limits what reaches the middle. The result is a plating that is thicker at the ends of the hole than in the middle, and in the extreme case the middle is not covered at all.

The throwing power of the bath is the property that describes how well it compensates for that geometric effect, and it is controlled by the additive system and by the acid to copper ratio. A board with a high aspect ratio needs a bath with a better throwing power and a lower current density, which means a longer plating time. The interaction between the aspect ratio and the plating is described under electroplating and via filling in HDI.

Mechanical And Hole Wall Causes

The condition of the hole wall decides whether the plating has something to bond to. A wall with smear, with a rough, glass rich surface or with residue from the drilling and desmear steps will not be wetted uniformly, and the plating will start unevenly. A void that follows the surface of the wall rather than sitting in the middle of the copper is a symptom of a preparation problem rather than a bath problem.

The panel handling contributes as well. A board that has been touched with bare hands, left wet between steps or dried unevenly carries a film that interferes with the deposition. The interval between the activation step and the plating is one of the variables that is easiest to lose control of in a busy shop, and it shows up as a random distribution of voids across the panel rather than as a systematic pattern.

Detection

The definitive method is a microsection taken through the barrel, prepared and examined at magnification. The void appears as a gap in the copper, and the section also shows the thickness at the ends and the middle of the barrel, which is the information needed to judge whether the bath is working. Sections are normally taken from a coupon rather than from a product board, and from a position that represents the worst case on the panel.

X-ray inspection can reveal a void in some circumstances, and it is used as a non destructive check on a production lot. Its sensitivity depends on the size of the void and on the surrounding copper, so a small void in a barrel with thick plating may not be visible. Where a lot has to be released without destroying boards, the combination of a coupon section and an X-ray sample is the usual evidence. The wider class of defects that the same process can produce is described under copper plating defects prevention.

Prevention And Control

The controls that matter most are the bath analysis, the current density and the agitation. The bath is analysed for copper, acid, chloride and each additive, and the results are trended rather than compared with a limit alone, because a drift that stays inside the specification can still produce a change in the deposit. The current density is calculated from the actual surface area of the load, which is a figure that has to be measured rather than estimated.

Agitation is what keeps the electrolyte moving inside the hole. Air sparging, panel movement or a combination of both is used, and the effectiveness falls when the load is too dense or when the sparger is blocked. The plating time is set from the required thickness at the worst position on the panel rather than at the surface, since the surface is the easiest place to plate and the middle is the hardest. Where a design has a very high aspect ratio, the answer is often a thinner board or a different via structure rather than a longer plating time.

FAQ

Is a void the same as a thin plating? No, though both are thickness problems. A thin area is continuous and conductive, while a void is a gap. A void in a signal via is an open circuit, while a thin area may pass every test and fail later.

Can a void be created by thermal stress? A crack can be, and the two are often confused. A void is present as the copper is deposited, while a crack follows a thermal cycle and usually runs along the plating or at the interface with the wall.

Does a higher current density plate faster and better? It plates faster, and it produces a rougher, more brittle deposit with worse throwing power, which is the opposite of what a deep hole needs. The current density and the time are chosen together for the aspect ratio.