by Blaine Partee
With increased board density, the use of alternate surface finishes and the need for high product yield board qualification has become a crucial step in the quality control process. A number of circuit board assemblers and manufacturers have recently reported the use of substandard or unspecified materials in the fabrication of substrates, conductors, laminates, and surface finishes. Changes in laminate and substrate manufacturing have resulted in boards that have increased moisture sensitivity, decreased flexural rigidity, poor adhesive properties, and dielectric properties that are below specifications. Failure analysis laboratory testing at ACI’s EMPF has revealed an increase in the number of cases involving substandard PWB manufacturing. Those are just some of the reasons that qualification of new and preexisting PWBs has become crucial to quality control for assemblers and their clients.

There are two main questions that must be asked when examining the quality of printed circuit boards:

1. Does the delivered product meet the fabrication requirements specified in the assembly drawings?

2. Does the printed circuit board exhibit good quality and demonstrate good fabrication process control?

To ensure that the PWB meets the drawing specifications and exhibits good quality, qualification tests are performed. The results of these tests are compared to industry-accepted specifications or guidelines. Although most of the qualification tests are performed by the board supplier, the ultimate responsibility for quality control falls on the assembler. This is the reason that many industry leaders now opt for third party board qualification. Qualifications can be performed efficiently and at low cost in external laboratories that have board qualification programs.

The correct approach to outside PWB qualification is to perform the quality control testing on incoming board lots before board related issues arise. Often, manufacturers find themselves scrambling for PWB failure analysis after assembly, burn-in testing, or after experiencing field returns. One recent EMPF customer requested cross-sections of multiple BGA solder joint locations to determine the root cause of an open circuit from a field return. After transmission X-ray imaging and cross-sectioning confirmed continuity of the solder joint to the board, it was suggested that bare boards from the same lot be investigated. Although the bare board manufacturer approved the lot of boards for use, the test samples sent to the EMPF exhibited nodules and areas with thin plating in the barrel. These areas were below specification and proved unreliable.

One of the most common specifications used to assess the quality of rigid circuit boards is IPC-6012A with Amendment 1. This specification produces generic guidelines for performance and quality specifications for rigid circuit boards. J-STD-003A describes not only the test methods but also provides guidelines for assessing bare circuit board solderability. IPC-A-600F is almost always used in conjunction with 6012A and JSTD-003A because it presents a visual representation of the specifications. There are also numerous other industry-accepted guidelines and specifications that pertain to explicit quality areas such as board design, plating, solder mask, metal foil, dielectric films, support drawings and composite materials. The EMPF utilizes IPC 6012A, 600F, J-STD-003, and TM-650 when qualifying rigid printed boards unless other specifications are applicable.

The first and one of the most essential steps in bare board qualification is visual inspection. This inspection is performed with an unaided eye and a low power (5X-90X) optical microscope. Surface imperfections such as burrs, voids, nicks, scratches, and gouges are quickly identified and compared to the standard. Inspection of the solder mask (solder resist) material involves investigating registration, blisters, delamination, bubbles, and thickness. Some subsurface imperfections such as foreign inclusions, measling/crazing, voids and delamination can be detected from the external visual inspection. For example, Figure 1 shows delamination of the composite substrate just beneath the surface of a printed circuit card.
Many of the characteristics of the plated through holes are also assessed during external visual inspection. Misregistration, plating or coating defects, diameters and foreign materials are identified and recorded.

Digital imaging enables measurement and assessment of dimensional characteristics such as board thickness, hole size, pattern accuracy, conductor width and spacing, annular ring and registration. These measurements are critical to assessing board quality. For instance, characterizing the annular ring of both supported and unsupported through-holes requires measurement of the hole offset. For high reliability (Class 3) applications, the hole need not be centered in the lands but at least 0.050 mm [0.0020 in] of conductor material must remain between the hole and the edge of the land. Digital imaging is also used before and after adhesion tests are performed on solder resist, surface finishes and metal conductors.

Cross-sections of the circuit board are performed to assess internally observable characteristics. Much of the information about the quality of the fabrication of the circuit board can come from a cross-section of the area in and around a through-hole. The laminate material immediately surrounding the through-hole is divided into zones. Each zone is permitted a certain type of defect such as voids, blisters, or resin recession. Low magnification views of the through-hole cross-section provide information regarding internal through-hole dimensions and foil thickness. These dimensions are compared to the assembly drawings and the industry guidelines.

Plated through-hole cross-sections also yield an abundance of information regarding the bare board manufacturer's process controls. The EMPF examines cross-sections using both low magnification optical microscopy and high magnification scanning electron microcopy (SEM). The target condition for plated through-hole is plating that is uniform and meets the minimum thickness requirements. Quite often some small defects are observed and used as process indicators. These defects could be small nodules, slightly non-uniform plating, nailheading, or burrs that pass acceptability guidelines but indicate improvements may be warranted in the drilling, cleaning, or plating processes. Defects and anomalies such as cracks, lifted lands, or separations are not allowed and indicate poor product quality and reliability concerns. Figure 2 shows a separation between a trace and barrel of a through-hole that would not only fail visual inspection but would also fail electrical continuity tests. Cross-sections should also be performed on bare PWBs after thermal stress. This thermal stress may include thermal cycling or solder pot exposure if specified.

Bare boards must exhibit acceptable solderability. Circuit board solderability assessment is performed to ensure that fabrication and storage processes do not inhibit solder joint quality. J-STD-003 specifies five specific tests with established accept/reject criteria. Of these the EMPF utilizes Test A (Edge Dip Test), Test C (Solder Float Test), and Test E (Surface Mount Process Simulation Test). Samples used for solder float testing can also be evaluated for plated through-hole integrity after stress. The wetting balance test for bare boards does not have established accept/reject criteria but when coupled with an optical assessment after testing (dip and look) reliable conclusions about board solderability can be made.

PWB cleanliness is a commonly tested quality indicator. When ionic cleanliness is assessed by Resistivity of Solvent Extract (ROSE) methods, the IPC recommends a maximum of 1.56µg of NaCl /cm2. Ion Chromatography (IC) is often coupled with ROSE techniques to help identify individual ionic species.

There are also many tests that are not commonly performed including outgassing, thermal expansion measurement, organic contamination, fungus resistance, and impedance. These tests may be requested by the PWB assembler but are generally reserved for special circumstances. For quality assurance purposes, solderability testing, construction integrity analysis, electrical measurements, environmental testing and visual inspection provide a comprehensive appraisal of the quality of incoming bare printed circuit boards. A program that includes these tests should be an integral part of a good quality assurance plan. A full table of IPC recommended guidelines for rigid circuit boards is available as Appendix A of IPC-6012A.