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PCB Specification: What an Engineer Really Needs to Cover
Last Updated : 2011/7/20

"It really is inadequate to simply ask your supplier to take your design Gerber files and drilling drawing and to provide a green-coloured FR4 PCB." This statement was the issue debated during SMART Group's Webinar which focused on how to define a meaningful PCB specification and communicate it to the fabricator.

Introduced and moderated by SMART Group Technical Director Bob Willis, the Webinar included presentations by Technical Committee Chair Sue Knight and Vice-Chair Nigel Burtt, engineers with many years of practical experience in high-reliability electronic assembly and the measures necessary to control and assess quality, purchasing, design and production issues associated with PCBs.

Is it what I ordered? Nigel Burtt asked as he summarised the considerations that determine whether a PCB is fit-for-purpose in respect of materials, dimensions and tolerances, compatibility with assembly technology and end-product working environment, compliance with international standards, design and manufacturing constraints, cost constraints and cosmetic considerations.

Burtt was an enthusiastic advocate of designers and assembly engineers establishing good working relationships with PCB fabricators and taking the opportunity to visit PCB shops to appreciate the complexity of the manufacturing process and its capabilities and limitations, as well as understanding what information the fabricator needs to manufacture a design to properly fulfill the requirement, and how to communicate it effectively.

Beginning with the data package, Burtt explained how plot files in Gerber RS-274X format, supported by read-me files to explain details, were generally used to convey image information for copper layers, solder resist, legend, drilling and profiling, and how reference net-list information could be provided in IPC-365A format. But the Gerber files alone did not tell the full story, and needed to be supported by a fabrication drawing and associated notes along with a generic or specific purchase specification.

The fabrication drawing was necessary to provide the PCB manufacturer with a clear description of the mechanical requirements the design--dimensions, tolerances, profile, etc. and fabrication notes detailing the requirements and the limitations of the design. It would normally include a table of finished hole sizes and tolerances and a cross-section drawing showing the positions of all the layers, copper weights and dielectric spacing, overall thickness and tolerance.

The fabrication drawing would also define material specifications: Laminate classification and finished copper weight, solder resist type and colour, legend type and colour and solderable finish. Additionally, a typical fabrication drawing would indicate the preferred location for the manufacturer's identification, UL marking and date code and information concerning revision and internal change tracking, with particular reference to whether the drawing or the Gerber files constituted master data.

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