The EMPF Helpline has recently received several requests to inspect printed circuit boards (PCBs) to IPC-A-600 Revision G (Acceptability of Printed Boards) and IPC-6012B (Qualification and Performance Specification for Rigid Printed Boards). These requests have come from customers requiring help with incoming inspection as well as from customers looking for an independent third party evaluation of their boards.

Together, both IPC-A-600G and IPC-6012B represent the core IPC documents for describing the acceptable and nonconforming conditions that are either externally or internally visible on finished printed boards. IPC-A-600G relies on IPC-6012 for the minimum acceptability requirements for PCBs and the acceptance testing frequencies required for PCB manufacture and procurement. IPC-6012 relies on IPC-A-600G for visual support and interpretation of those requirements.

At the EMPF, PCBs are inspected to either Class 1, 2, or 3 (high reliability) depending on the customer’s requirements. Some of the typical procedures used by the EMPF to inspect PCBs include the following:

• Visual Inspection
  
• X-Ray Inspection
  
• X-Ray Fluorescence (XRF)
  
• Micro-Sectioning

Visual Inspection

Typically, the first step in a PCB qualification is to perform visual inspection with a low magnification microscope or by the unaided eye. Surface defects --- such as nicks, scratches, voids, gouges, cut fibers, weave exposures, and burrs --- can be seen and compared to the IPC-A-600 Revision G standard. Some sub-surface defects can also be observed, such as measling, laminate voids, and foreign inclusions. Hole characteristics are also examined during the visual inspection. Hole defects include misregistration, plating imperfections, and dimensional errors. Solder mask (or solder resist) defects can also be detected, including delamination, bubbles, misregistration, and blistering.

Test Method IPC-TM-650 2.4.22C covers procedures using visual inspection to determine the bow and twist percentage calculation while Test Method IPC-TM-650 2.4.22.1C covers procedures used to determine maximum allowable vertical displacement of an unrestrained PCB.

The adhesion/tape test is another procedure used in PCB inspection. Test Method IPC-TM-650 2.4.1 outlines the procedure for evaluating the adhesion quality of plating, marking inks or paints, and other materials used in conjunction with PCBs.

X-Ray Inspection

X-ray inspection is a non-destructive method of evaluation used to inspect bare boards for any anomalies prior to micro-sectioning. Typically, x-ray is used to inspect plated through holes, blind and buried vias, as well as other sub-surface metallic features.

X-Ray Fluorescence (XRF)

XRF is a non-destructive process that identifies the composition of and measures the thickness of surface finishes on the PCB. During PCB inspections, XRF can be used to measure the thickness of the copper traces and pads, as well as surface finishes including HASL, immersion tin, white tin, Electroless Nickel/Immersion Gold (ENIG), Organic Surface Protectant (OSP), and other finishes.

Micro-Sectioning

Micro-sectioning is a destructive process which is used to examine internal characteristics of the PCB and is covered in Test Method IPC-TM-650 2.1.1E. After removing a section of the PCB in the area that is to be inspected, the specimen is mounted in an epoxy resin and is then ground and polished. Optical microscopy and high magnification scanning electron microscopy (SEM) is used to examine the cross section. Micro-sectioning is a good technique to obtain information about the quality of the PCB fabrication. Typically, cross sections are taken around the area of plated through holes allowing examination for voids, blisters, or resin recession. Low magnification inspection of the plated through hole cross section provides information regarding the dimensions of the through hole. The plating can be inspected to ensure uniformity and that it meets the minimum thickness requirements. Figure 2-1 shows a typical plated through hole with plating thickness measurements. Defects or anomalies that can be observed from the cross section include non-uniform plating, dimensional issues, nailheading, plating voids, nodules, cracks, resin smear, inadequate or excessive etchback, wicking, and inner layer separation. Figure 2-2 shows an angular ring misregistration. Internal conductor anomalies can also be detected by cross section inspections. These defects include over or under etching, conductor cracks or voids, and improper foil thicknesses.

The EMPF is able to provide a full range of testing to qualify PCBs for incoming inspection. In addition to the techniques above, the EMPF can perform: solderability testing using wetting balance and sequential electrochemical reduction analysis (SERA); cleanliness testing using resistivity of solvent extract (dynamic ROSE in an Ionograph) and ion chromatography; testing for surface organic contamination using FTIR spectroscopy; as well as various electrical tests. The EMPF can assist customers with performing these tests and visual inspections to ensure a high quality PCB manufacturing process. For more information, please contact Ken Friedman at 610.362.1200, extension 279 or via email at kfriedman@aciusa.org.