WO2004051244A1

WO2004051244A1 - Method and device for detecting defective pc blanks

Info

Publication number: WO2004051244A1
Application number: PCT/EP2002/013676
Authority: WO
Inventors: Peter Klugkist; Bodo Nemeth
Original assignee: Solectron Gmbh
Priority date: 2002-12-03
Filing date: 2002-12-03
Publication date: 2004-06-17

Abstract

The invention relates to a method and a device for detecting subsurface defects (18) in PC blanks (10), which occur in an intermediate layer (14) below a cover layer finishing the PC blank (10) off towards a component side (28). The invention is characterized by generating a temperature gradient which is preferably oriented perpendicularly to the intermediate layer (14) in at least one subsurface area of the intermediate layer (14) and detecting the temperature distribution in the cover layer (16) for example by using an infrared camera.

Description

Method and device for the detection of defective circuit board blanks

The invention relates to a method and a device for detecting near-surface defects in printed circuit board blanks which occur in an intermediate layer below a cover layer which closes the printed circuit board blank to an assembly side.

Printed circuit board blanks have one or two component sides, which are provided with structured electrical conductor tracks and are equipped with passive or active electrical components during the production of electrical circuit boards. In particular, the so-called SMT method (SMT = surface mounted technology) is widespread, in which so-called SMD components (SMD = surface mounted device) specially designed for this method are soldered directly onto the surface of the printed circuit boards. This technology does away with the otherwise required holes in the circuit board blanks for receiving connecting wires of the components.

Printed circuit board blanks consist of a carrier, glass and plastics such as polyimides or epoxy resins being predominantly used as carrier materials. The conductor tracks usually consist of a base layer made of copper, which is often covered by a thin intermediate layer and a covering layer arranged above it, which closes off the printed circuit board blank on the component side. The intermediate layer has the task of protecting the underlying base layer from corrosion, while the cover layer facilitates the soldering of electrical components. With so-called ENIG circuit boards (ENIG = e- lectroless nickel imπiersion gold), the intermediate layer consists of nickel and the top layer of gold.

It has been shown that the intermediate layer occasionally has local defects in printed circuit board blanks constructed in this way. These defects are usually traces of corrosion products, with nickel intermediate layers e.g. Nickel phosphide or nickel oxide, which accumulate on the surface of the intermediate layer facing the cover layer. The circumstances that lead to these corrosion phenomena have not yet been clarified, so that no measures are known with which the occurrence of these defects could be prevented. If electrical components are soldered onto the conductor tracks in the area of such defects, reliable soldering often does not take place, since the solder used does not bond or does not fully bond with the corrosion products. These defective areas are also referred to as "black pads". As a result, electrical malfunctions can occur or the electrical components can even fall off the circuit board.

The difficulty now lies in the fact that these defects are not recognizable from the outside due to the overlying layer. Only after the cover layer has been destroyed can these defects on the surface of the intermediate layer be discovered from the outside. At most, in the case of a functional circuit board test following its manufacture, these defects can be determined; however, malfunctions often only occur later, so that faulty circuit boards leave production unrecognized. It is therefore an object of the invention to improve a method and a device for the detection of defects near the surface in printed circuit board blanks of the type mentioned at the outset in such a way that the detection can be carried out without destruction.

With regard to the method, this task is solved by the following steps:

a) generating a temperature gradient, preferably oriented perpendicular to the intermediate layer, in at least one region of the intermediate layer near the surface;

b) Detecting the temperature distribution in the cover layer.

With regard to the device, the object is achieved by:

a) a heating or cooling medium for generating a temperature gradient, which is preferably oriented perpendicularly to the intermediate layer, in at least one region of the intermediate layer near the surface;

b) a temperature meter for recording the temperature distribution in the cover layer.

The invention is based on the knowledge that the thermal conductivity of the intermediate layer is locally reduced in the area of the defects. This local reduction in thermal conductivity is used according to the invention to detect the defects through the cover layer. For this purpose, the intermediate layer can be over the base layer, but preferably over the cover layer. layer, either heated or cooled, in order in this way to obtain a temperature gradient within the region of the intermediate layer near the surface in which the defects usually occur. Since heat is then transported in the intermediate layer, the defects lead to a local change in the spatial temperature distribution not only in the intermediate layer, but also in the cover layer. If, for example, the at least one near-surface area of the intermediate layer is heated via the overlying cover layer, the cover layer has a higher temperature in areas over defects, since the heat is dissipated more slowly over the underlying defect areas of the intermediate layer. When the cover layer cools, however, the areas of the cover layer lying over defects are cooler, since heat flows there more slowly from the intermediate layer.

In this way, defects on the surface of the intermediate layer can thus be identified non-destructively, so that all printed circuit board blanks can be reliably checked immediately after they have been produced or before they are fitted with electronic components.

The temperature gradient will preferably have to be aligned at least approximately perpendicular to the intermediate layer. In principle, however, it is also possible to generate a temperature gradient aligned parallel to the intermediate layer, for example, in the case of small circuit board blanks, heat is supplied not from the base or cover layer, but from the side. The cover layer can be cooled, for example, by briefly wetting the blank of the printed circuit board with liquid nitrogen. The top layer is briefly cooled by a few ° C so that the effects mentioned occur and can be demonstrated. The covering layer can be heated, for example, with the aid of a high current, which heats the conductor tracks on account of the ohmic heat generated. Due to the slightly different electrical resistances of the individual layers from which the conductor tracks are built, a temperature gradient then likewise arises within the intermediate layer.

In a preferred embodiment of the invention, however, to produce a temperature gradient in the at least one region of the intermediate layer near the surface, the cover layer is heated by irradiation with light, preferably infrared light.

Heating with light has the advantage that a brief rise in temperature in the cover layer can be caused by brief high-energy radiation, which leads to a correspondingly pronounced temperature gradient in the intermediate layer. The changes in the temperature distribution on the cover layer caused by the defects are then greater than is the case with slow heating or cooling.

It is therefore further preferred if a flash lamp is used as the light source for irradiating the cover layer. In this way, short-term and spatially largely homogeneous heating of the printed circuit board blank can be achieved with simple means.

In an advantageous further development of this embodiment, a plurality of flash lamps, in particular halogen tubes, are used as the light source.

In this way, even in the case of larger circuit board blanks, approximately identical starting conditions can be created over their entire assembly area before the temperature distribution on the cover layer is detected and evaluated.

The temperature distribution in the cover layer can be e.g. with the help of a variety of temperature sensors that record their surface temperature directly above the conductor tracks.

However, the temperature distribution in the cover layer is preferably recorded by an infrared camera.

In this way, the temperature distribution of the cover layer can be determined particularly easily and with high spatial resolution. The temperature image of the cover layer recorded by the infrared camera can be displayed, for example, in multiple colors on a monitor, so that an operator can draw conclusions about possible defects by analyzing the temperature distribution. However, this analysis can also be carried out by a suitable evaluation device, for example a suitably programmed personal computer, by using image analysis known per se. draw conclusions about the presence of defects.

In another advantageous embodiment of the invention, the temperature distribution in the cover layer is recorded at at least two different times, in particular after the temperature gradient has been generated.

In this way, one obtains additional information on the temporal cooling behavior (or warming-up behavior, if the covering layer was previously cooled) of the covering layer. This additional information can be used to detect defects locally more precisely and, above all, more reliably. For side effects, e.g. In this way, dirt or scratches, changes due to the temperature distribution of the cover layer can be distinguished more easily from changes which are attributable to said defects.

In a particularly preferred development of this embodiment, the temperature distribution in the cover layer is periodically recorded after generation of the temperature gradient and evaluated by Fourier analysis.

If the periodically recorded temperature distributions are subjected to a discrete Fourier analysis, for which very fast calculation algorithms are available, additional information on the temporal behavior of the cover layer after temperature excitation can be obtained in this way. Changes in the temperature distribution due to defects are particularly evident in the Fourier spectrum in a certain frequency range. Alternatively or in addition to this, the temperature gradient can be generated periodically in the at least one region of the intermediate layer near the surface and the temperature distribution in the cover layer can be recorded with the same time offset for generation.

In this way, too, changes in the temperature distribution caused by defects are particularly evident. Frequencies that are suitable for the periodic temperature excitation of the intermediate layer are e.g. in the order of 30 Hz and can be achieved with the help of pulsed infrared lasers.

Furthermore, it can be advantageous to also record the temperature distribution in the cover layer before generating the temperature gradient after step a).

In this way, the temperature distribution determined after the heating or cooling of the cover layer can be compared with a previously determined temperature distribution. This allows a differential image to be derived in which only the temperature changes emerge. In this difference image, changes due to defects can be recorded even more easily, since side effects are averaged out.

When determining differential distributions, at least one temperature distribution that was recorded in another printed circuit board blank can also be taken into account.

For example, a whole series of similar printed circuit board blanks can be checked for defects in a simple manner by first at room temperature with a printed circuit board blank the temperature distribution of the cover layer is detected. Subsequently, only the temperature distributions after heating or cooling of the cover layer are then recorded for all further printed circuit board blanks. A differential distribution is then determined for each printed circuit board blank, using the temperature distribution recorded once at room temperature and the temperature distribution recorded in each case after heating or cooling.

It goes without saying that the features mentioned above and those yet to be explained below can be used not only in the combination specified in each case, but also in other combinations or on their own without departing from the scope of the present invention.

Further advantages and features of the invention result from the description of the following exemplary embodiment with reference to the drawing. In it show:

1 shows a detail from a printed circuit board blank in a lateral section, which has a defect in a region of the intermediate layer near the surface;

2 shows a device according to the invention for the detection of such a defect in a schematic representation;

3 shows a graph in which the temperature is plotted over time for two different locations on the cover layer. 1 shows a section of an ENIG circuit board blank 10 in a lateral section in a representation that is not to scale. A carrier 11 is indicated, which can consist, for example, of a polyimide. A conductor track 13 is applied thereon, which essentially consists of a base layer 12 made of copper. A thin intermediate layer 14 made of nickel is deposited thereon, which in turn is covered by an even thinner cover layer 16 made of gold. In typical printed circuit board blanks of this type, the thickness of the intermediate layer 14 is, for example, 5 μm and the thickness of the cover layer 16 is approximately 70 nm. The printed circuit board blank 10 is assembled from the component side 28 for the production of printed circuit boards by electronic components through the cover layer 16 with the intermediate layer 12 are soldered. The gold of the cover layer 16 diffuses into the solder used, which wets the intermediate layer 14 made of nickel.

The intermediate layer 14 has a defect 18 which, as can be seen in the enlarged illustration, e.g. can consist of a small recess created by corrosion and filled with nickel phosphide. If an attempt is now made to solder an electrical component on the conductor track in the region of the defect 18, the defect 18 can result in the component not being reliably soldered to the intermediate layer 12 and possibly even falling off the printed circuit board.

2 shows an exemplary embodiment of a detector device 20 according to the invention in a schematic illustration, with which such a defect 18 can be detected reliably and without destruction. Two are shown as halogen tubes trained flash lamps 22 and 24, which are controlled by a control device 26. The flash lamps 22 and 24 are arranged so spaced from the circuit board blank 10 that the component side 28 of the circuit board blank 10 is illuminated in parallel and approximately uniformly by the flash lamps 22 and 24.

The detector device 20 according to the invention also has an infrared camera 30, the optics of which can record a partial area or the entire component side 28 of the printed circuit board blank 10. In the exemplary embodiment shown, the infrared camera 30 and the control device 26 are connected to a control and evaluation device 32, which is e.g. can be a suitably programmed personal computer. The control and evaluation device 32 comprises a monitor 33 on which the temperature images recorded by the infrared camera 30 can preferably be displayed in multiple colors. Furthermore, the control and evaluation device 32 has the task of switching on the flash lamps 22 and 24 with a suitable flash duration via the control device 26 and synchronizing this in time with the recordings made by the infrared camera 30. Experiments have shown that flash times between 3/100 and 6/100 seconds lead to particularly good results.

The light emitted by the flash lamps 22 and 24 strikes the assembly side 28 of the printed circuit board blank 10 formed by the cover layer 16 and leads to heating there. This heat is dissipated by heat conduction into the intermediate layer 14 underneath and finally into the base layer 12 of the printed circuit board blank 10. Wherever in the Layer defects 18 are present, the heat conduction is hindered. The overlying small area of the cover layer 16 therefore has a slightly higher temperature than the surrounding areas. This temperature deviation is detected by the infrared camera 30 and displayed on the monitor. An operator can recognize this temperature deviation and reject the circuit board blank in question. If necessary, this can also be carried out automatically if the control and evaluation device 32 has appropriate image analysis capabilities. The rejection can then be carried out, for example, by an ejector or a robot arm.

In order to be able to reliably detect the relatively small temperature differences on the component side 28 of the printed circuit board blank 10 and to distinguish them from noise effects, the infrared camera 30 preferably takes several temperature images of the component side 28, each of which corresponds to a specific temperature distribution. The additional information obtained in this way can be used in particular to distinguish deviations in the temperature distribution that are not caused by defects 18 but, for example, by defects in the cover layer 16 itself that are harmless per se, from defects 18 in the intermediate layer 14.

FIG. 3 shows a graph in which the temperature T is plotted against the time t for a specific point on the component side 28 of the printed circuit board blank 10. With t ₀ is the time at which the flash lamps 22 and 24 are actuated. The energy supplied as infrared light leads to a sudden heating of the assembly side 28 on this sem point until a maximum temperature T _{max is} reached. Thereafter, the component side 28 gradually cools down again at this point, as shown in FIG. 3 by a first decay curve 34.

If, however, there is a defect 18 at the relevant point on the component side 28, the heat dissipation into layers of the printed circuit board blank 10 underneath is impeded. As a result, the temperature at such locations on the component side 28 is slightly higher than in the vicinity thereof. This case is shown in FIG. 3 by a second decay curve 36, shown in broken lines.

If the temperature distribution of the cover layer 16 is now _detected at a plurality of times t _lf t ₂ and t ₃ , then reliable defects 18 can be detected. If a temperature distribution is detected only at one point in time, it would be difficult to distinguish the relatively small temperature difference that exists between a location on the component side 28 above a defect 18 and outside such a defect 18 from a general signal noise. However, if, as shown in FIG. 3, the temperature distribution is recorded at several times, then significant temperature deviations can be reliably identified by statistical evaluation.

Although defects 18 can definitely be seen with the naked eye on the monitor 33 of the control and evaluation device 32, a fully automatic evaluation of several recorded temperature distributions is preferred within the scope of the present invention. With such a fully automatic evaluation can further noise minimization, differential distributions are also generated, in which a temperature distribution recorded before the exposure at time t _{v is} subtracted from temperature distributions recorded, for example, at times t _ιr t ₂ or t ₃ . In this way, background effects can be at least partially averaged out.

With identical circuit board blanks, it is also possible to record the temperature distribution at time t _v only once for all circuit board blanks with the aid of the infrared camera 30. This temperature distribution is then subtracted from the temperature distributions recorded after heating for all other printed circuit board blanks which are also to be checked. In this way, a very large number of circuit board blanks can be checked for defects reliably and non-destructively, since the individual circuit boards are only exposed briefly and the decay curves need to be recorded by the infrared camera 30 with the aid of several recordings.

The control and evaluation device 32 can furthermore have suitable program means by means of which the recorded temperature distributions can be subjected to a discrete Fourier analysis. In the Fourier spectrum, the changes in the temperature distributions caused by defects 18 appear even more clearly. In this case, several temperature distributions can either be recorded at periodic intervals or exposures can be carried out at periodic intervals, each of which a temperature distribution is recorded at. Instead of the flash lamps 22 and 24, an infrared laser that can be operated in pulsed mode is used as the light source. costume, the beam of which strikes the printed circuit board blank 10 via suitable scattering optics.

Claims

claims

Method for the detection of defects (18) near the surface in printed circuit board blanks (10) which occur in an intermediate layer (14) below a cover layer (16) which closes the printed circuit board blank (10) towards an assembly side (28), characterized by the following steps:

a) generating a temperature gradient, preferably perpendicular to the intermediate layer (14), in at least a region of the intermediate layer (14) near the surface;

b) detecting the temperature distribution in the cover layer (16).

A method according to claim 1, characterized in that to generate a temperature gradient in the at least one region of the intermediate layer (14) near the surface, the cover layer (16) is heated by irradiation with light, preferably infrared light.

Method according to Claim 2, characterized in that a flash lamp (22, 24) is used as the light source for irradiating the cover layer.

4. The method according to claim 3, characterized in that a plurality of flash lamps, in particular halogen tubes (22, 24), are used as the light source.

5. The method according to any one of the preceding claims, characterized in that the temperature distribution in the cover layer (16) is detected by an infrared camera (30).

6. The method according to any one of the preceding claims, characterized in that the temperature distribution in the cover layer (16) at least two different times (t _{ι r} t ₂ , t ₃ , t _v ) is detected.

7. The method according to claim 6, characterized in that the temperature distribution in the cover layer (16) is also detected before generating the temperature gradient after step a).

8. The method according to claim 6 or 7, characterized in that the temperature distribution in the cover layer (16) at least two different times (t _{ι r} t ₂ , t ₃ ) is detected after generating the temperature gradient.

9. The method according to claim 8, characterized in that the temperature distribution in the cover layer (16) after generating the temperature gradient periodically detected and evaluated by Fourier analysis.

10. The method according to any one of claims 7 to 9, characterized in that from temperature distributions to un- different times have been recorded, differential distributions are formed.

11. The method according to claim 10, characterized in that at least one temperature distribution, which was detected in another printed circuit board ling (10), is taken into account for determining differential distributions.

12. The method according to any one of the preceding claims, characterized in that the temperature gradient in the at least one near-surface region of the intermediate layer (14) periodically generated and the temperature distribution in the cover layer (16) is recorded with the same time offset for generation.

13. The method according to any one of the preceding claims, characterized in that the intermediate layer (14) consists of nickel and the cover layer (16) consists of gold.

14.Device for the detection of near-surface defects (18) in circuit board blanks (10) which occur in an intermediate layer (14) below a cover layer (16) which closes the circuit board blank (10) towards an assembly side (28), characterized by:

a) a heating or cooling means (22, 24) for generating a temperature gradient, preferably perpendicular to the intermediate layer (14), in at least one region of the intermediate layer near the surface; b) a temperature meter (30) for detecting the temperature distribution in the cover layer (16).

15. The apparatus according to claim 14, characterized in that the heating means is a light source, preferably an infrared light source (22, 24).

16. The apparatus according to claim 15, characterized in that the light source is a flash lamp (22, 24).

17. The apparatus according to claim 16, characterized in that the light source has a plurality of flash lamps (22, 24), in particular halogen tubes.

18. Device according to one of claims 14 to 17, characterized in that the temperature meter is an infrared camera (30).

19. The apparatus according to claim 18, characterized in that the infrared camera (30) can take several images per second.

20 Device according to one of claims 14 to 19, characterized by an evaluation device (32) for evaluating the temperature distributions determined by the temperature meter (30).

21. The apparatus according to claim 20, characterized in that the evaluation device (32) has a comparator in which different temperature distributions are comparable to one another by forming a difference.