Cross Section Analysis: Inside a PCB
How can the internal quality of a printed circuit board be inspected and verified using cross section analysis?
Recently we published an article that talked about IPC Inspection guidelines that provides an overview of the standards used in the inspection process of printed circuits boards. One of the many inspection processes that measure the quality of the circuit board is called a Cross Section Analysis or Micro-section.
To complete the analysis a section of the circuit board is cut out (approximately one inch square) and is placed into a soft acrylic that hardens around it in the shape of a hockey puck. Thus the enclosed section of the board is called a “puck”. This allows the side view of the board to be easily viewable under a microscope for detailed inspection.
Cross section or micro-section analysis is a threefold inspection tool of internal makeup of a printed circuit board. It is effective for in-process verification as well as a failure analysis.
The sections can be taken from coupon sets produced with the production boards or directly from the boards themselves. The inspection criteria are normally set by the product specification city by the customer – MIL-PRF55110/31032, IPC 6012, or by the customer’s own requirements.
Coupon designs are defined in IPC’s 2220 series of specifications and are intended to reflect the board’s design where required.
The coupons are placed on the production panels alongside the boards and are processed along with the production panel. They will reflect the process effects that are made to the whole panel eliminating the need to sacrifice a production part to verify that the board meets the criteria.
Through hole coupons can be used to determine surface and hole wall plating thickness, separation of the inner layers, registration of the drill to the outer and inner layer images, inner layer connectivity and the condition of the laminated materials surrounding the holes.
Various others are used to evaluate solderability, peel strength of the copper, electrical characteristics of inner layer traces as well as solder mask and legend inks. Inspection of the prepared sections at various stages of production can give the manufacturer clues to possible process issues and the viability of the boards on a specific panel.
Markings on the coupons and the boards identify which panel of the lot they were fabricated on and assist in separating poor product from the remainder of the lot. The markings also become part of the process records and serve to identify the processing specifics for boards that may be involved in future failures.
The second role of micro sections is to serve as an effective failure analysis tool. Individual boards may be subjected to sectioning when there is evidence of some type of internal failure. The visual inspection of the section will readily identify problems associated with internal processing failures. Additionally cross sectional views of the surface trace work, pads and solder mask can reveal extraneous copper, over etch, poor solder mask developing and numerous other types of defects that have caused the part to be rejected.
Data collected during the sectioning of a single or numerous lots of boards can be used for process improvement and as a validation of the various processing steps used to fabricate the boards.
Maintaining a micro-sectioning laboratory, a well trained staff and effective and timely communication regarding section results is becoming a necessity. As hole sizes shrink, multiple plating cycles are used and layer counts rise the micro-section will serve a more important role in verifying the results of manufacturing boards.
Bay Area Circuits maintains an internal lab for the processing of cross sections for both customer reports as well as in process quality inspections. Let us know if we can provide a cross section report with your next order!