A walkthrough shows why you can’t “guess” success.

When a customer learns their much-needed PCB job has inexplicably gone into the dreaded “hold” basket, the instinctive response is indignation. Let’s take a moment to examine the possible reasons. After all, the fabricator doesn’t want an unhappy customer, nor a pause in work volume. Yet, a great deal of precise data is needed to build a printed circuit board. As layer count and complexity increase, so does the volume of correct information needed by the fabricator to properly manufacture the job. If some necessary data are missing, the CAM operator will hand the file back to sales to sort out the problems.

The one steadfast rule all PCB manufacturing facilities hold dear is “we don’t guess.” Never. Break that rule and the consequences will bite back hard. To ensure no one is guessing, every question must be answered. If you failed to specify a tolerance on a set of holes, the job will go on temporary hold until the CAM operator can get a suitable answer. If you have an electrical short between ground power layers due to a misplaced via, the job goes back to sales to sort out. When a job is on hold for a serious problem, the result can be days of delay. If there are one or two small issues, however, in many instances the CAM operator or sales will call and sort it out. They might be able to move a trace or two to prevent a short, for example, or change a pad size to correct a problem with an annual ring that is too small. However, the CAM operator must meet a quota of jobs each day to keep the manufacturing facility fully loaded. They do not have an abundance of time to fix a multitude of problems in an individual customer’s data. Other jobs are waiting! In that case, the CAM operator hands the file back to sales to reject the data. The customer can then fix it and resubmit it through the whole process of price quote, DRC (design rule check) and setup.

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As printed circuit requirements become more complex, missing information and technical items become even more important. A common customer error is to specify an impedance that cannot be met due to the laminate thickness or Dk.

What other kinds of mistakes are we talking about? What are some of the common mistakes that cause jobs to go on hold? Here is a short list:

• Missing data; e.g., a drill file
• Solder mask or nomenclature white marking file not in the CAD package (very common)
• Missing or wrong information on drawing of outside dimensions
• Missing specifications of laminate in hole plating and surface finishes (very common)  
• Shorts or space violations in trace spacing (difficult for the CAM engineer to fix easily)
• Hole size relative to pad size too small, violating clearance spacing between pads
• No specification for laminate (typically, nothing or just FR-4-specified, whereas a fabricator may carry literally hundreds of different types of laminates)
• Spacing violations of clearance of solder mask around pads (common)
• White identification nomenclature marking across pads (all the time)
• Improper sequence of drilled holes in a buried via multilayer
• Hole sizes too small or too large relative to pad size ring requirements (common)
• Through-via holes short out ground planes to power planes (a popular one)
• Specifying a square inside corner in final routing (the fabricator may only have round router bits).

These items, and many others, can put a job on hold. Then the emails fly! But in truth, this isn’t a rant about customer mistakes. (We love our customers!) Rather, it emphasizes the importance of working closely with the fabricator. Keep communication open and detail-oriented.

In an actual example, one customer seemed to always be in the “rejected-on hold” bin at the sales desk. After a stressful initial meeting, they asked why their boards were always on hold at our facility and yet another board shop just simply built them. Our engineering team walked through what was missing on the drawing package. One observation was PCBs bought from various manufacturers performed in different ways. Because their specification sheet was so lacking, the other printed circuit manufacturers gave them different laminates they had in excess in stock. Different plating thicknesses were produced because they were not specified, and different solder masks were used. These are small changes a printed circuit manufacturer can make if not specified. During the lengthy meeting, it was illustrated to the buyers and designers that a simple change in laminate, solder mask, and plating thickness could change the impedance by a considerable degree. They agreed to specify a laminate that had a stable supplier known for consistent quality on the manufacturing drawing. The CAD manager showed them how a change in specified plating thickness would affect their yield and future reliability, and if they specified a minimum plating thickness of 1.2 mils copper in the hole, it would greatly improve yields out of wave soldering and increase thermal cycling life. This alone, hopefully, would solve one of their major problems. As the meeting progressed, we asked for well over 20 additions and changes to the technical specifications of the build sheet and for them to add more technical data to their technical drawings to ensure a tighter set of specifications.

Our plant manager then took them on a tour of the facility and showed why we don’t guess. They were shown the lamination storage area, with hundreds of different laminate types, each with 20 different thicknesses and copper weights. During the show-and-tell tour of the rest of the facility, the engineers and buyers witnessed many ways to build the same product. It was an eye-opener. From that point, they understood why so many tight specifications are needed when a critical board is required, and the need to provide such a large volume of information. Subsequent orders never saw the dreaded hold basket. Communication is key, because, in the end, the customer and fabricator are really partners in achieving a successful build, solid product, and fostering a long-term mutually beneficial relationship.

Akber Roy ("Roy") is chief executive officer of Rush PCB Inc., a printed circuit design, fabrication and assembly company (rushpcb.com); roy@rushpcb.com.

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