A METHOD FOR APPLYING A BARCODE

TECHNICAL FIELD

The present invention relates primarily to a method of applying and constructing a bar code on a surface of a printed circuit board. By printed circuit board is meant a board which includes "printed" circuits but which has not yet been loaded with necessary electronic devices. Such loaded printed circuit boards are referred to as loaded PCB's.

Bar codes are normally used to distinguish one printed circuit board of one particular structure from another printed circuit board of another particular structure, such as differences in the number of conductive layers or conduc¬ tive fields and in the conductor patterns chosen for each conductor pattern layer.

According to the present invention, the bar code can be applied to an outer surface of a printed circuit board and structured so as to be easily read and detected by light receiving means adapted to receive from light transmitting means sections of light reflected by bar-code sections of high luminosity that are delimited laterally by bar-code sections of low luminosity.

According to the present invention, the printed circuit board may include a base unit having a number of electrically conductive layers and a number of electrically insulating layers in mutually superimposed relationship.

The present invention may also be applied on EMC-protected printed circuit boards (EMC = Electro Magnetic Compatibili- ty).

DESCRIPTION OF THE BACKGROUND ART

Known to the art are EMC-protected printed circuit boards which are comprised of a base unit which is provided on each side with a thin electrically conductive layer or metal layer. These layers may be connected together electrically by edge-related and earth-potential related sections.

It is also known with regard to such EMC-protected printed circuit boards to thicken a first metal layer with a second metal layer and to mount on these coordinated metal layers a soldering mask having selected surface portions which expose corresponding surface portions on the free surface of said second metal layer, and to cover said exposed surface portions with a third metal layer on to which discrete components can be soldered.

When producing loaded printed circuit boards or circuit board assemblies, it is necessary that a board of predetermined structure can be provided with a number of discrete compo¬ nents which are soldered to the board through the medium of connection pads.

This requires each printed circuit board to include some form of identification, preferably an individual identification number and a category number.

Although it is indicated in the aforegoing that the printed circuit board used may be either an EMC-protected circuit board or a circuit board which is not EMC-protected, the invention is described in the following primarily with regard to the former type of board.

It is also known that "free" metal surfaces give rise to EMC- radiation at high frequencies, such as frequencies in the GHz-range, and can receive EMC-radiation. Consequently, exposed connection pads and bonding wires normally require the presence of metallic enclosures.

It is not unusual in the manufacture of different EMC- protected circuit boards to use one and the same kind of EMC- protected circuit board to produce a number of such circuit board assemblies.

Because the rational manufacture of circuit boards requires the use of a large number of printed circuit boards, it is usual to provide each printed circuit board with a separate identity code, a bar code.

It must be possible to readily read the code during the manufacturing process with the aid of a light transmitting device and a light receiving device, preferably combined in a single unit, so as to be able to produce a circuit board of specific structure with the aid of the code and other process-related parameters.

Naturally, when producing printed circuit board assemblies which do not need to be EMC-protected, printed circuit boards that are not EMC-protected may be used.

However, even in this case there must be provided a means of identifying the individual printed circuit boards used, preferably an individual identification means and a group- membership identification means.

With these low requirements there is known a method for identifying printed circuit boards used in different process stages, this method being described and illustrated in US Patent Specification 4,947,335, in which printed circuit- boards are identified through the medium of holes drilled in specific patterns along an edge of respective boards.

This patent specification also teaches a method and a system for producing loaded circuit board modules, by combining and soldering components on printed circuit boards and by feeding specified printed circuit boards from a store of printed

circuit boards. The method and the system are based on reading the identification code on respective printed circuit boards and therewith obtain information relating thereto, so that the printed circuit board can be transported through selected treatment stations in accordance with said identifi¬ cation information and there provided with selected compo¬ nents, so as to gradually form a complete loaded printed circuit board.

With regard to the features of the present invention, it can be mentioned that the US Patent Specification 4,782,219 teaches the use of a light generating source 2 which directs a laser beam 4 onto a bar code, the bars of which are etched into a highly reflective bar-code carrying material 6 (column 5, lines 14-16).

This patent specification discloses the possibility of allowing the laser beam to be reflected directly onto a projection surface from the bright or highly luminous sections of the bar code, and of reading the bar code on said surface with the aid of a light receiving device or a detector.

After having established the identity of the printed circuit board, it is also known to apply to the exposable surface thereof a sticker or label which exhibits a bar code corre¬ sponding to the group identity of the printed circuit board, with the position of the sticker being chosen so as to enable the bar code to be easily read by a combined laser transmit- ter/receiver unit during the process, this task of affixing the sticker being effected manually in an additional working step.

The publication "IBM Technical Disclosure Bulletin Vol. 29 No. 7 (1986)" pages 2980 - 2981 teaches a marking of the solder mask through Infra Red laser beams.

The publication teaches a number of various methods of marking a printed circuit board. One is to generate an identification in the same wa as the circuit where the

other, wherefore great precautions are required in order to not generate electrical and magnetical disturbances.

An other described method is to apply the identification through not electric conductive screen printing.

A method to identify every semi conductor wafer is presented in the publication EP-A3-0 311 087. The identifying code is here applied on or across a front surface and a light beam is directed towards an opposite bottom surface, whereby the light beam is to pass through the wafer.

In the Abstract on JP62-69377 it is described how the code is generated through a solder application on a solder resistant layer.

The publication DE-A1-4 308 890 teaches a system intended to bring an identification to the top surface of a printed circuit board by defining the identifying symbols to correspond to recesses in the solder mask and to thereby achieve a good contrast between the illumination of the top surface of the base plate and the solder mask.

In the Abstract on JP62-69377 it is described how the code is engraved in the surface (5) of a metal part and how the light beam is reflected in the code towards a screen where the light intensity is detected.

A system where the identifying code is detected through a reflecting surface is also described in the patent publication EP-A2-0 326 796.

SUMMARY OF THE INVENTION

TECHNICAL PROBLEMS

When considering the present state of the art as described above it will be seen that a technical problem resides in

providing conditions which will enable a printed circuit board to be provided with a bar code significant in identify¬ ing the group-membership and/or the individual identity of the printed circuit board in a simple manner without the need of additional working steps, and wherein the bar code is so constructed on the printed circuit board as to make it impossible to remove the bar code from the printed circuit board without destroying the surface structure of said board.

It will also be seen that a technical problem resides in realizing the provisions required to provide an EMC-protected printed circuit board with an easily read bar code without influencing the EMC-protection in a deleterious manner.

A technical problem also resides in realizing the necessity of and the advantages afforded by allowing each EMC-protect- ing metal layer to be intact and to form the bar-code printed circuit initially through the medium of the solder mask layer and thereafter washing away the solder mask layer to form a printed circuit-board identifying bar-code, and therewith provide conditions for obtaining bright or highly luminous bar-code lines and bar-code lines of low luminosity related to dark solder mask layers, wherein said bright lines can be created either by working (organic passivation) an exposed metal surface or coating said surface with a time-durable metal, where the surface structure will not be changed deleteriously with time to any appreciable extent, i.e. will not oxidize.

Another technical problem is to choose in this stage of manufacture a development path which utilizes a bright surface structure for the bar-code sections of high luminosi¬ ty, and therewith be forced to use a special detection or sensing method when the bar code is to be detectable in a transmitter/receiver unit.

It will also be seen that a technical problem is one of

creating simple conditions which enable a bar code belonging to the printed circuit board to be produced at the same time as a further metal layer adapted for soldering discrete components is applied to an existing metal layer, such as an EMC-protecting metal layer.

It will also be seen that another technical problem is one of realizing those advantages that are afforded when the dark or lowly luminous marking lines of the bar code that are mounted on the metal-layer are chosen to represent parallel lines in the soldering mask, while the highly luminous marking lines of the bar code are chosen to represent lines formed via said further metal layer or the like, wherein each of said highly luminous marking lines will be represented by very bright metal lines.

SOLUTION With the intention of providing a solution to one or more of the aforesaid technical problems, the present invention is based on a method of applying and constructing a bar code with the provisions given in the introduction.

The inventive method is based on choosing the lowly luminous or dark marking lines of the bar code to represent lines in a solder mask, selecting the highly luminous marking lines of the bar code to represent metal lines such as lines formed when applying a further metal laycir or the like such as a further metal layer on said EMC-protecting metal layer wherein said highly luminous marking lines are represented by bright metal surfaces or metal lineε all of which is achieved while leaving an EMC-protected metal layer intact.

In accordance with proposed embodiments that lie within the scope of the inventive concept it is proposed that a solder mask layer is applied to the upper surface and hardened and/or dried so as to fasten to said surface to form dark lines of the bar code in a bar-code section, and that the remainder of the solder mask layer in said section is then removed in a known manner.

Said further metal layer is then applied to the exposed surface in said section of the bar code.

ADVANTAGES

Those advantages primarily significant to the present invention reside in the creation of conditions which enable a bar code to be provided on an upper surface of a printed circuit board without additional working operations, where the sections of low luminosity of th bar code can be included as part of a solder mask, so that the printed circuit-board bar code and identification are effected simultaneously with the forming of chosen surface parts that are intended to expose corresponding surface parts on a metal layer, and that said exposed surface parts shall be treated, such as by organic passivation, or covered by a further metal

layer on to which discrete components can be soldered.

The highly luminous marking lines of the bar code are chosen to represent bright lines, for instance formed when applying said further metal layer.

The primary characteristic features of the inventive method are set forth in the characterizing clause of the following Claim 1,

BRIEF DESCRIPTION OF THE DRAWTNG

The present invention will now be described with reference to an exemplifying embodiment thereof at present preferred and having features significant of the invention, and also with reference to the accompanying drawing, in which

Figure 1 is a perspective sectional view of an EMC-protected printed circuit board on which a bar code which has been provided in accordance with the present invention, and which shows an appropriate beam path; and

Figure 2 is a sectional view of a printed circuit board according to Figure 1 provided with the bar code.

BEST MODE OF CARRYING OUT THE INVENTION

Figure 2 is a sectional view of an EMC-protected printed circuit board, and shows a bar code A which can be read and detected by a light receiving device.

The printed circuit board 1 includes a base unit 10 having a number of electrically conductive layers and a number of

electrically insulating layers, arranged in superimposed relationship.

For the sake of clarity these layers have not been shown in Figure 1 , although it can be mentioned that in Figure 2 the thickness of the base unit 10 has been reduced considerably in relation to the thickness of a first metal layer, such as a copper layer 11, applied to the upper surface 10a of the base unit.

The first metal layer 11 is normally given a thickness far smaller than 0.5 μm and a second meal layer 12 is applied to the surface 11a with the intention of thickening the metal or copper layer.

The combined thickness of the two metal layers 11 and 12 may be 5 μm or thicker, to create a satisfactory EMC-protection.

The copper layer is comprised of two EMC-protective layers 11 and 12 which are applied on each side of the printed circuit-board base unit 10 (only one copper layer (11, 12) is shown) .

For the purpose of creating connection islands and the like, there is applied to the second copper layer 12 a further, a third, copper layer 15 whose thickness exceeds the thickness of the first layer 11.

The surface 15a will then comprise a highly luminous code-bar section in the bar code A and the surfaces 13a, 14a will comprise adjacent dark or lowly luminous bars 13, 14 related to the solder mask.

In the Figure 2 embodiment, it is assumed that the metal surface 12a between the solder-mask related sections 13, 14 comprises a bright line-code related bar.

In order to ensure that the surface will resist oxidation for a long period of time, it is proposed that the surface is subjected to an organic passivation process.

Alternatively, there may be applied between the bar codes 13 and 14 a metal layer 15 ^' whose surface 15a ^' is resistant to oxidation.

Several metals and metal alloys are available in this regard.

It will be evident that normal light generating devices and light receiving devices combined in a single unit cannot be used to read the bar code since directly reflected light will "dazzle" the receiver.

It will be understood that the EMC-protective metal layer 11 and 12 may consist of a metal other than copper, and that the further metal layer 15 may also consist of a metal other than copper.

It is normal to apply to the uppermost surface 12a of the metal layer 12 a thin Sn/Pb-metal layer 15 adapted for requisite solder connections.

The metallic layer may consist of a metal other than tin/- lead, such as nickel/gold. The uppermost surface 15a (15a ^' ) of the metal layer 15 may be subjected to an organic passiva¬ tion process.

In order to provide satisfactory EMC-protection, the metal layers (11, 12) will preferably have a total thickness of about 5 μm. In practice, there can be accepted a thickness of up to 200-300 μm, although preferably between 10 and 30 μm. The printed circuit-board base unit 10 may have a thick- ness between 0.2-10 mm, normally between 0.4-7 mm.

The present invention fully satisfies the desire to construct

a surface orientated printed circuit-board identification in a simple manner, without deleteriously affecting the EMC- protection, by ensuring that the metal layers 11 and 12 or the necessary bar code remain completely intact.

This is achieved by allowing the lowly luminous marking lines 13a, 14a of the bar code A to be represented by lines in the solder mask, by allowing the highly luminous marking lines 15 of the bar code to be represented by bright lines, treated by means of organic passivation or formed when applying said further metal layer 15.

The aforesaid highly luminous marking lines are able to be represented by bright metal lines with measurement accuracy, while leaving the EMC-protective first and second metal layers 11, 12 intact.

The invention also relates to a system according to Figure 1. In order to be able to identify a bar code 2 on a carrier substrate when the dark or lowly luminous marking lines 2b on the bar code are comprised of solder-mask related lines and when the highly luminous marking lines 2a of the bar code are comprised of bright lines, there is used a light generat¬ ing device 15 whose light beam is reflected directly on said bright lines 2a and with which a mirror image is projected onto a projection surface 4 that has a dull surface structure 4a.

The directly reflected bright lines 2a are projected onto the projection surface 4, 4a and a light receiving device 7 detects the thus projected diffuse image 2a ^' of the bar code.

This detection of the bar code can be effected in accordance with the proposals given in the aforesaid US Patent Specifi- cation 4,782,219. Alternatively, there can be used a signal detection method and a signal detection arrangement of the kind described in a patent application filed on the same day

as the present application.

It will be understood that the invention is not restricted to the aforedescribed and illustrated exemplifying embodiment thereof and that modifications can be made within the scope of the following claims.