Pad Cratering and Laminate Damage in Assembly

Pad cratering is a crack in the laminate resin under a pad, usually caused by the mechanical load applied when a component is pulled from a board. The pad may remain attached, the copper may be intact and the joint may look undamaged, while the resin beneath has cracked and the layer underneath has separated. The distinction between pad cratering and a lifted pad matters for the diagnosis: a lifted pad usually indicates a bonding failure at the pad to resin interface, while cratering indicates that the resin itself has fractured and laminate damage extends below the surface. It is a repair induced defect that often appears only after the unit has been in service.

What Cratering Is

The pad, the resin and the copper beneath form a composite structure. The interface between the resin and the copper is the weakest part, and a tensile load perpendicular to the board pulls the resin away from the copper and initiates a crack. The crack can propagate parallel to the board between layers, or it can extend into the laminate and lift a section of resin with the pad.

The damage is defined by where it occurs rather than by how it looks. A pad that has been pulled with too much force often shows a slight ring of discolouration or a fine crack at the resin boundary, and the electrical result is unaffected until thermal cycling opens it.

How the Load Is Applied

The most common cause is desoldering. When a component is removed before all of its joints are fully molten, the operator applies a lifting force through the still solid joints, and the load is transferred to the pad and to the laminate. The force needed to lift a component with a few remaining solid joints is far larger than the force that the pad and resin can tolerate.

Depaneling is the second cause. Breaking a board from a panel, particularly by hand along a breakaway tab, bends the board and applies a stress to the pads near the break line. The load is applied once rather than cyclically, but it is often large enough to initiate a crack in a thin or brittle laminate.

Materials and Their Contribution

The laminate material determines the resistance to cratering. A brittle material with a high filler content resists deformation and cracks rather than yielding, while a tougher material deforms and absorbs the load. The difference is significant on thin boards and on boards with a small pad, because both reduce the area over which the load is distributed.

The board thickness and the layer structure matter as well. A thin board flexes more under the same load, which increases the strain in the resin under the pad, and a structure with a plane directly beneath the pad limits the depth over which the load can be distributed. Where the product will be reworked, the material choice is a reliability decision as much as a cost decision.

Design and Process Controls

The design can reduce the risk. A larger pad distributes the load over more area, a larger mask opening avoids loading the mask edge, and a pad that is not connected to a plane through a very narrow relief heats more evenly and lets the operator remove the component with less force. The pad and relief design should be reviewed for any component that is likely to be reworked.

The process controls the load. Rework should use a bottom side preheater so that all the joints melt at the same time, a nozzle that heats the whole component rather than part of it, and a lifting force that is applied only after the joints have melted. Where a component is lifted with a pull force, the force should be limited and the operation should be performed with a tool that can detect when the alloy has melted. The practice is described in rework station selection.

Depaneling and Handling Controls

The separation method determines the load applied to the board. Routing and laser cutting apply almost no bending load, while a manual break applies a controlled but significant one. Where a manual break is used, the break line should be far from any pad, the tab should be designed so that the break is clean and the operation should be performed with a fixture rather than by hand.

Handling after assembly is a related risk. A board that is flexed while it is being fitted into an enclosure loads the pads in the same way as a hand break, and the damage is invisible. The handling rules that reduce that exposure are the same ones described for PCB quality assessment during assembly.

Inspection and Detection

Cratering is difficult to detect non destructively. A visual inspection can reveal a discolouration or a crack line around a pad, and a sample cross section confirms the condition, but a routine inspection will miss most instances. Because the damage is concentrated at reworked locations and near break lines, those are the areas to inspect.

The useful control is therefore prevention and record keeping. Where a board has been reworked, the operation should be recorded, and any unit that fails in the field should be checked against its rework history. A pattern of failures at reworked positions indicates a cratering problem, and the corrective action is a change to the rework method rather than to the assembly process.

Rework Practice That Avoids Laminate Damage

Most laminate damage is created by rework, so the rework procedure is the most effective control. The component should be heated from both sides, with a bottom side preheater bringing the board to a temperature below the melting point so that the top side heat source only has to supply the remaining energy. The nozzle should cover the whole component so that all joints melt at the same time.

The lift should be gentle and slow rather than a sudden pull. A component that is fully released will lift with negligible force, so the operator should increase the force gradually and stop if any resistance remains. Where a component resists, the correct action is more heat and not more force, and the work instruction should state that explicitly because the instinct under time pressure is the opposite.

Test Methods for Material Selection

Where the material choice is critical, a test can measure the resistance to cratering. A pull test on a pad, a ball shear test or a drop test on a representative assembly all apply a load and show whether the laminate or the joint fails first. The result guides the material selection and the design of the pad and relief.

The gopcb engineering team uses that comparison when a product is expected to be reworked repeatedly or when the assembly will see mechanical shock in service. The test result is recorded with the material so that a substitution made for cost reasons can be compared against the data, and the pad geometry is fixed from the same test rather than from a general rule.

FAQ

Can a cratered pad be repaired? Usually not. The resin under the pad is damaged and the layer beneath may be separated. A repair involves removing the damage to a stable layer and rebuilding the connection, which is rarely justified.

How much force is safe when removing a component? The alloy should be fully molten before any lifting force is applied, at which point the component lifts with almost none.

Is cratering visible on X-ray? Not reliably. A cross section or a visual inspection at the pad edge is the practical method.