Browse Prior Art Database

PC6 LAYER BUILD CHECK

IP.com Disclosure Number: IPCOM000008368D
Original Publication Date: 1997-Dec-01
Included in the Prior Art Database: 2002-Jun-11
Document File: 3 page(s) / 132K

Publishing Venue

Motorola

Related People

P. S. Collis: AUTHOR [+2]

Abstract

The design of printed circuit boards (PCBs) has become a completely computerised process. The final output that is used to manufacture a PCB is a particular data file format, namely a GERBER format. This data is then plotted on photographic film, typically ten to twenty sheets are used. Using a 6 layer PCB, as an example, we find that there are four sheets of tooling photography needed in the process to produce the inner layers. The selection of these four sheets is a manual process and inevitably the PCB layers can be swapped. Final boards with swapped internal PCB layers may still work and will pass simple continuity tests because no connec- tions have been broken.

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MOTOROLA Technical Developments

PC6 LAYER BUILD CHECK

by P. S. Collis and I. D. Cairns

INTRODUCTION

  The design of printed circuit boards (PCBs) has become a completely computerised process. The final output that is used to manufacture a PCB is a particular data file format, namely a GERBER format. This data is then plotted on photographic film, typically ten to twenty sheets are used. Using a 6 layer PCB, as an example, we find that there are four sheets of tooling photography needed in the process to produce the inner layers. The selection of these four sheets is a manual process and inevitably the PCB layers can be swapped. Final boards with swapped internal PCB layers may still work and will pass simple continuity tests because no connec- tions have been broken.

  Production PCBs go through a simultaneous bare board test after manufacture, but this does not find layer swap errors. Radio frequency (RF) PCBs, with swapped internal layers, may have poor perfor- mance or in some extreme cases no RF performance at all. Furthermore, swapped internal layers may give problems on PCBs intended for intrinsically safe products.

  This paper deals with a simple and inexpensive technique of checking the order of PCB internal layers.

PROBLEM(S) TO BE SOLVED

  If a six layer PCB is visually inspected it is very difficult to see if any layers are swapped. This is because the view of an internal layer is obscured by layers of laminate or by copper plating. It is not uncommon to find printed circuit boards with copper numbers added to internal layers, each num- ber corresponding to the layer on which it is placed, as shown in Figure 1. This approach may seem ideal but it is of little practical use. All that this gives is a row of numbers, swapped layers are not

obvious because the swapped layers are still visible, as shown in Figure 2. This technique is of some use if the colour (or clarity) of the number is assessed, as numbers of layers viewed through more layers of laminate tend to lose definition.

  Another flaw in this technique is that one side of internal copper is usually oxidised during manu- facture (to improve bonding), the oxidised side being much blacker than the bright copper side. This means that alternate layers change from a dark to a bright contrast, making the judgem...