A video measuring machine from Nikon Metrology has taken over the quality control assessment of printed circuit boards at Stevenage Circuits Ltd (SCL), after the graphics card failed in a piece of similar — albeit decade-old — equipment from another supplier.

The new iNexiv VMA4540 is used virtually continuously for 2-D optical co-ordinate measurement of machined features and copper tracks. It underpins not only first-article inspection of boards and printed reports for customers, but also process control in the drilling, etching and routing departments (www.nikonmetrology.com).

SCL evaluated alternatives to a replacement video measuring machine, including table-top varieties of measuring cameras — and co-ordinate measuring machines.

The company even considered the possibility of sub-contracting first-article inspection to an external bureau. However, it decided to continue with established in-house procedures, so Robert Brown (owner and managing director of SCL) and James Pickett (quality engineer) set about sourcing the best equipment for their requirements — with both productivity and affordability in mind.

Three potential suppliers were considered, and Nikon Metrology was selected. Mr Pickett said: “Technical support was there from day one, from the initial enquiry right through to the installation and commissioning of the video measuring machine in January this year.

Even now, the area sales engineer calls in periodically to see how things are going. Moreover, the machine was almost half the price of the other main contender, so it was very affordable — and the return on investment will be rapid.

We were particularly impressed by the class and the clarity of the iNexiv’s optics; we could easily see the difference between this machine and a competitor’s during trials that we carried out at both companies. Another positive factor was the VMA4540’s adaptable software.”

Competent company

Established in 1971, SCL is the only European manufacturer with an in-house capability for the design and production of all four principal types of PCB.

Single- and multi-layer boards are manufactured to a minimum standard of IPC class 2 and are supplied in high volumes, small batches and prototype quantities to PCB assembly houses that subsequently mount the electronic components (heat sinks are the only additions to the boards before they leave the Stevenage factory, to be sent to customers in over 40 countries).

Radio frequency boards account for the largest percentage of output, with about a third of total production destined for the medical industry. PCBs for the space and military sectors are also supplied.

SCL prides itself on the supply of PCBs in short lead times, with three- to five-day turn-round times being the norm from order to delivery.

Around half a dozen new jobs are progressed through the factory on a typical day, with up to 15 boards requiring in-process checking — keeping the company’s quality control department busy from 7am until 7pm, five days a week — and sometimes on Saturdays.

This level of activity means that the Nikon video measuring machine is required to work almost non-stop; moreover, it is used not only by Mr Pickett, who programs it, but also by machine operators in the various production departments.

The VMA4540 is used repeatedly during the life cycle of each new job; but because 95% of SCL products only need positional and dimensional measurement of profiles — such as board periphery, cut-outs and drilled holes, as well as copper tracks and pads — the machine is used exclusively in 2-D mode.

Its ability to accept a laser or touch probe is not used, and neither is its 3-D measuring capability (derived from repeated focusing down sloping feature).

Moreover, the reliability of first-article inspection at each stage of a board’s manufacture is crucial, as the results are used to control each process. Inaccurate or inconclusive readings have a negative impact on production.

Repeated useAfter the required number of layers of a PCB have been bonded together to form a board, the first operation is CNC drilling, using data sent from SCL’s CAD/CAM department; this data takes account of stretching or shrinkage during subsequent stages of manufacture.

At this stage, the first-off from each batch (typically 25) is checked on the video measuring machine against the corresponding Gerber file — the standard vector format for 2-D images used in PCB-industry software.

That said, if there is time, Mr Pickett converts the data to DXF, as it is easier to manage.

If any discrepancy is found — such as a hole not being present due to a broken drill — the measuring machine stops and flags up an error.

An out-of-tolerance hole diameter or position (these typically need to be within ±50µm and ±100µm respectively), results in a similar error message.

Usually, PCBs pass inspection, and a system report is prepared (this is not normally printed out, as it is rarely required by the customer). That said, the historical data can be interrogated for trouble-shooting purposes, if a problem subsequently occurs with the board.

The next stage of production is the etching of tracks using ‘laser direct imaging’. Track widths are important, especially on radio-frequency boards; typically, these have to be within 50-100µm (±10µm).

A sample scan is made by the VMA4540 across an area of the first-off and compared with the CAD/CAM data to ensure that track measurements — including their positions — are within tolerance.

After processing at the “solder resist and plating departments”, the PCBs progress to the routing shop, where CNC machines trim the boards to their final size and produce internal cut-outs, according to the CAD drawing. Again, the first-off is checked on the VMA4540 against the Gerber/DXF file.

The finished product is next inspected in its entirety; and it is this dimensioned first-article inspection report that SCL’s customers normally require.

The results are entered automatically by the Nikon Metrology software into a test-report template that populates an Excel spreadsheet.

This report is linked to the relevant CAD drawing and given to the customer; it is also archived for future reference.

Some 10-20% of final boards are evaluated in this way on the video measuring machine. In the case of smaller PCBs, up to a dozen or more can be on the table at the same time, and the software steps and repeats to check the entire panel in a single cycle — very rapidly.