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Inner layer separation is a separation of the plated through hole wall from an internal conductor. This condition may be due to poor hole cleaning or etchback, problems with the electroless plating, or other direct metallization and related process problems. These defects may be exacerbated by thermal excursions, such as the thermal stress (solder float) test and rework simulation test, as well as by soldering processes. The condition is often more severe on layers near the outside of the board (layers 2 and N-1). When observed in microsections, these separations may indicate serious reliability issues with the related boards. Since the holes microsectioned from coupons are a very small statistical sample of the total number of holes on the panel, even slight defects may indicate more serious problems on other areas of the board or on different hole sizes.

Internal Layer Separation -- References:
IPC-6012B, paragraph 3.6.2.1
IPC-6013A, paragraph 3.7.8
MIL-PRF-55110G, paragraph A.3.6.9.4
MIL-PRF-31032/1B, paragraph 3.6.9

Good Innerlayer Connections

 

Negative Etchback  -- References: 

IPC-6012B, paragraph 3.6.2.6
IPC-6013A, paragraph 3.7.7
MIL-PRF-55110G, paragraph A.3.6.6
MIL-PRF-31032/1B, paragraph 3.6.7

When required, etchback is a process that removes base material from the hole wall after drilling.  This process leaves a portion of the internal layer extending into the hole, allowing a "3-point" connection between the internal layer and the plated copper.  Negative etchback is a condition where the internal copper foil is recessed relative to the surrounding base material.  Separation may occur with any etchback condition.

Negative Etchback without separation

Resin Smear  -- References: 

IPC-6012B, paragraphs 3.6.2.1 and 3.6.2.7
IPC-6013A, paragraph 3.7.6
MIL-PRF-55110G, paragraphs A.3.6.9.4 and A.3.6.6.2
MIL-PRF-31032/1B, paragraphs 3.6.7 and 3.6.8

Poor cleaning or etchback processes may leave a layer of resin covering the internal layer and prevent a good electrical connection between the internal layer and the plated through hole wall.  In an optical cross section this condition looks similar to interplane separation, except it is as likely to be observed on any or all layers and is not usually exacerbated by thermal excursions.  Confirmation of this condition is usually accomplished with SEM/EDX.

Nail Heading  -- References: 
MIL-PRF-55110G, paragraph A.3.6.13

Inefficient hole-drilling can cause the copper on an inner conductive layer to flare. This condition is called nailheading, and could be an indicator of other problems also associated with poor hole-drilling.

Corner Cracks  -- References: 

IPC-6012B, paragraph 3.6.2.1
IPC-6013A, paragraph 3.7.8
MIL-PRF-55110G, paragraph A.3.6.12
MIL-PRF-31032/1B, paragraph 3.6.5

Internal Foil Cracks  -- References: 

IPC-6012B, paragraph 3.6.2.1
IPC-6013A, Table 3-9
MIL-PRF-55110G, paragraph A.3.6.12
MIL-PRF-31032/1B, paragraph 3.6.5

Internal foil cracks are cracks of the internal copper layer, usually near the plated through hole.  These cracks usually develop during thermal excursions when the ductility of the copper in the internal layers is insufficient for the stress levels that occur.  These defects may be accompanied by resin voids or recession that increases the local stresses.

Nodules and Plating Folds  -- References: 

IPC-6012B, paragraph 3.6.2.1
IPC-6013A, Table 3-9
MIL-PRF-55110G, paragraph A.3.6.11
MIL-PRF-31032/1B, paragraph 3.6.4.2

Nodules are areas of the copper plated hole wall that have extended into the hole.  This can be caused by contaminants in the plating process.  This defect may cause difficulties with component insertion during through hole assembly.  They are not allowed to decrease the diameter of the hole below the minimum requirement. 

Plating folds occur as a result of rough drilled hole walls or excessive etchback.  Folds in the plating may not reduce the copper plating thickness below the allowable minimum.

Laminate Void Outside the Thermal Zone -- References: 

IPC-6012B, paragraph 3.6.2.3
IPC-6013A, paragraph 3.7.4
MIL-PRF-55110G, paragraph A.3.6.7
MIL-PRF-31032/1B, paragraph 3.6.10

 

Resin Recession -- References: 

IPC-6012B,  Table 3-6
MIL-PRF-55110G, paragraph A.3.6.14

Hole wall dielectric / plated barrel separation.

(Acceptable after Thermal Stress or Rework Simulation)

Wicking -- References:  

IPC-6012B, Table 3-6
IPC-6013A, Table 3-9
MIL-PRF-55110G, paragraph A.3.6.6.3
MIL-PRF-31032/1B, paragraph 3.6.4.3

Wicking is the capillary absorption of the hole wall copper plating along the fibers of the base material.

Internal Annular Ring Breakout -- References: 

IPC-6012B, paragraph 3.6.2.9
IPC-6013A, paragraph 3.7.10
MIL-PRF-55110G, paragraph A.3.6.1
MIL-PRF-31032/1B, paragraph 3.6.1

Internal annular ring breakout is a condition in which a hole is not completely surrounded by the land. Possible causes of annular ring breakout include misalignment of the layers during lamination and/or mislocation of the drills during hole drilling.

Unsupported Overhang

Assorted Microsection Photos

Along with printed circuit boards, many other products can be cross-sectioned and examined using microsection analysis.

Some examples are:

Ceramic Circuit Boards

Composite Panels

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