WO2019224534A1

WO2019224534A1 - Apparatus for monitoring a coating

Info

Publication number: WO2019224534A1
Application number: PCT/GB2019/051404
Authority: WO
Inventors: Peter Richard CORCORAN; David Sinclair; David Hall
Original assignee: 3D Automated Metrology Inspection Limited
Priority date: 2018-05-21
Filing date: 2019-05-21
Publication date: 2019-11-28
Also published as: GB201907178D0; GB201808325D0; GB2575543A

Abstract

Apparatus (100) for monitoring thickness of a coating material on an electronic assembly comprises a measuring apparatus (20) configured to monitor height of a surface and a controller (10). The controller (10) is configured to cause the measuring apparatus (20) to determine a first height profile of an outer surface of the assembly before the coating material is applied by measuring height of the assembly and a second height profile of an outer surface of the assembly after the coating has been applied by measuring height of the assembly. The controller (10) is configured to determine a thickness of the coating based on the first height profile and the second height profile. The controller (10) is configured to compensate for an effect of optical properties of the coating material on the determination of the second height profile.

Description

APPARATUS FOR MONITORING A COATING

TECHNICAL FIELD

The present invention relates to apparatus and a method for monitoring a coating applied to an electronic assembly.

BACKGROUND

Manufacturers of electronic assemblies in some industries have the requirement to conformal coat the assemblies to protect them in harsh environments against foreign objects and water ingress. An electronic assembly typically comprises a printed circuit board (PCB) with conductive tracks and pads (typically formed of copper) and electrical and/or electronic components electrically mounted to the PCB, typically by soldering. A conformal coating material is applied over the PCB and components. Industries that would normally conformal coat PCBs include aerospace, medical, automotive, military, marine.

Conformal coatings are typically sprayed onto the PCB by automated dispensing machines at the end of the PCB assembly line and then cured in order to set the coating material. There are many types of conformal coating materials. Many of the conformal coating materials are optically transparent or partially transparent.

There is a requirement to inspect the coverage and depth of conformal coating in order to verify compliance with industry standards. The important measurement of coating coverage is on the components, their leads and solder joints.

One known method for inspecting a conformal coating uses ultraviolet (UV) light to illuminate a UV trace within the coating material. This method does allow a degree of coverage inspection but does not measure depth, and does not work on reflective surfaces such as component leads and solder joints.

Another known method for inspecting a conformal coating uses a laser light to illuminate the coating material and measure the amount of reflection. The amount of reflection is used as a scaling factor of depth. This method only samples a limited number of points on the PCB and so does not provide a full coverage inspection. Also, the method, cannot provide a reliable measurement of coating depth on components. Another known method for inspecting a conformal coating is to section (i.e. cut) a PCB batch sample to physically measure coating depth. This is a destructive method and also suffers from the drawback that it only measures the particular sample.

It is an aim of the present invention to address disadvantages associated with the prior art.

SUMMARY OF THE INVENTION

An aspect provides apparatus for monitoring thickness of a coating material on an electronic assembly comprising:

a measuring apparatus configured to monitor height of a surface;

a controller configured to:

cause the measuring apparatus to determine a first height profile of an outer surface of the assembly before the coating material is applied by measuring height of the assembly at a plurality of positions across the assembly;

cause the measuring apparatus to determine a second height profile of an outer surface of the assembly after the coating has been applied by measuring height of the assembly at a plurality of positions across the assembly;

determine a thickness of the coating based on the first height profile and the second height profile;

wherein the controller is configured to compensate for an effect of optical properties of the coating material on the determination of the second height profile.

Optionally, the controller is configured to compensate using an adjustment factor. The adjustment factor may compensate for at least one of: transparency of the coating material; colour of the coating material; reflective properties of the coating material.

Optionally, the controller is configured to determine a thickness of the coating by:

CCah = CCmh x CCmr,

where:

CCah = actual height of the coating;

CCmh = measured height of the coating based on the first height profile and the second height profile;

CCmr = adjustment factor.

Optionally, at least a first point on the electronic assembly remains uncoated when the second height profile is determined, and the controller configured to determine a thickness of the coating by aligning the first height profile and the second height profile using profile data of the first height profile and the second height profile at the first point.

Optionally, the first point on the electronic assembly which remains uncoated is at least one of: a fiducial marker, track around a fixing hole; a region around a connector, an earthing pad.

Optionally, the electronic assembly is a printed circuit board with components mounted to the printed circuit board via solder joints, wherein the coating material is a coating material applied over the printed circuit board, the components and the solder joints, and wherein the first height profile of an outer surface of the assembly is a height profile of the printed circuit board, the components and the solder joints.

Optionally, the coating material is a polymeric material.

Optionally, the measuring apparatus is a triangulation scanner or another measurement apparatus which uses a radiation source, such as an optical radiation source. The optical radiation source can be a laser.

Optionally, the controller is configured to compare the thickness of the coating with a threshold value and to output data indicative of at least one of:

(i) a region of the board having a coating thickness greater than the threshold value;

(ii) a region of the board having a coating thickness less than the threshold value.

Optionally, the controller is configured to:

compare the thickness of the coating with a plurality of threshold values to determine regions of the board having a coating thickness within a plurality of different thickness bands; and

output data indicative of a region of the board having each of the thickness bands.

Optionally, the measurement apparatus is configured to move in an x-y plane with respect to the assembly.

Optionally, the measuring apparatus is a triangulation sensor.

An aspect provides a method for monitoring thickness of a coating material on an electronic assembly comprising: determining a first height profile of an outer surface of the assembly before the coating material is applied by measuring height of the assembly at a plurality of positions across the assembly;

determining a second height profile of an outer surface of the assembly after the coating has been applied by measuring height of the assembly at a plurality of positions across the assembly;

determining a thickness of the coating based on the first height profile and the second height profile; and

compensating for an effect of optical properties of the coating material on the determination of the second height profile.

Optionally, the method comprises any of the steps performed by the controller.

The electronic assembly may comprise a PCB with components which are mounted on, and electrically connected to, the PCB.

An advantage of at least one example is that it is possible to measure a thickness of a coating across an electronic assembly, including positions on the components, their leads and solder joints.

An advantage of at least one example is that it is possible to measure a thickness of a coating in a non-destructive manner.

An advantage of at least one example is that it is possible to measure a thickness of a coating, with increased accuracy, in situations where a PCB is warped.

An advantage of at least one example is that it is possible to measure a thickness of a coating, with increased accuracy, in situations where components are mounted at different heights on a single board and/or where there is variation in height of a component at a particular mounting position on a board between different assemblies.

An advantage of at least one example is that it is possible to measure a thickness of a coating, with increased accuracy, where a coating material is transparent, or partially transparent.

The functionality described here can be implemented in hardware, software executed by a processing apparatus, or by a combination of hardware and software. The processing apparatus can comprise a computer, a processor, a state machine, a logic array or any other suitable processing apparatus. The processing apparatus can be a general-purpose processor which executes software to cause the general-purpose processor to perform the required tasks, or the processing apparatus can be dedicated to perform the required functions. Another aspect of the invention provides machine-readable instructions (software) which, when executed by a processor, perform any of the described methods. The machine- readable instructions may be stored on an electronic memory device, hard disk, optical disk or other machine-readable storage medium. The machine-readable medium can be a non- transitory machine-readable medium. The term“non-transitory machine-readable medium” comprises all machine-readable media except for a transitory, propagating signal. The machine-readable instructions can be downloaded to the storage medium via a network connection.

Within the scope of this application it is envisaged that the various aspects, embodiments, examples and alternatives, and in particular the individual features thereof, set out in the preceding paragraphs, in the claims and/or in the following description and drawings, may be taken independently or in any combination. For example features described in connection with one embodiment are applicable to all embodiments, unless such features are incompatible.

For the avoidance of doubt, it is to be understood that features described with respect to one aspect of the invention may be included within any other aspect of the invention, alone or in appropriate combination with one or more other features.

BRIEF DESCRIPTION OF THE DRAWINGS

One or more embodiments of the invention will now be described, by way of example only, with reference to the accompanying figures in which:

FIGURE 1 shows a system for monitoring a thickness of a coating on an electronic assembly;

FIGURE 2 shows cross-sections of an electronic assembly before and after a conformal coating has been applied;

FIGURE 3 shows a detailed cross-section through a component and a PCB of an electronic assembly;

FIGURE 4 shows a method of processing an electronic assembly;

FIGURE 5 shows an example display of coating thickness data which has been graded into bands; FIGURES 6A and 6B show a measuring apparatus and two different coating materials;

FIGURE 7 shows apparatus for a computer-based implementation.

DETAILED DESCRIPTION

Figure 1 shows apparatus 100 for monitoring a thickness of a coating on an electronic assembly 30. The apparatus comprises a measuring apparatus 20. The measuring apparatus 20 is configured to monitor distance between the measuring apparatus and an object. In this example, the object is an upper surface of an electronic assembly 30. The system 100 is configured to measure height of the assembly 30 at a plurality of positions across the assembly. This is a scan of the electronic assembly. Figure 1 shows a measuring apparatus 20 which can move relative to the electronic assembly 30, with the electronic assembly 30 held in a fixed position. In this example, the electronic assembly 30 has a planar PCB 31 and the measuring apparatus 20 can move relative to the electronic assembly 30 in an x-y plane. The measuring apparatus 20 is configured to monitor distance between the measuring apparatus and the electronic assembly 30. In the arrangement shown in Figure 1 the measured distance may be considered as a distance in the z direction which is perpendicular to the x-y plane of the PCB 31.

In other examples, the measuring apparatus 20 can remain stationary with the electronic assembly 30 being movable relative to the measuring apparatus 20. In other examples, the electronic assembly 30 can remain stationary and the major part of the measuring apparatus 20 can also remain stationary. The measuring apparatus 20 can scan a surface of the electronic assembly by an optical arrangement, such as a movable deflector or a movable mirror. In other examples, the electronic assembly 30 is non-planar.

The measuring apparatus 20 can be implemented in various ways. One possible form of the measuring apparatus 20 is a triangulation sensor. A triangulation sensor determines distance between the apparatus and a target object by illuminating the object with radiation and determining the position of backscattered radiation on an optical detector. A triangulation sensor may comprise a laser source which emits optical radiation at a particular wavelength. Another possible type of measuring apparatus is a galvo scanner.

The electronic assembly 30 comprises a PCB 31 with a substrate 32 and electrically conductive tracks or traces 33. Typically, the electrically conductive tracks 33 are copper. The electronic assembly 30 comprises a plurality of components 35 which are mounted on, and electrically connected to, the PCB 31. Various mounting technologies are known. In surface mount technology (SMT) electronic components 35 are mounted to electrically conductive pads on an upper surface of the PCB. Components may connect to the conductive pads via leads, flat contacts or an array of solder balls, such as a ball grid array (BGA). Another technology uses plated through-holes, with component leads inserted through plated holes through the PCB, and solder electrically connecting the leads to the conductive traces around the through-holes. Other mounting technologies are possible.

Figure 2 shows the electronic assembly 30 before and after a conformal coating 40 has been applied. The coating material 40 is applied over the PCB substrate 32, conductive tracks and pads 33, components 35 and component leads 36. A PCB 31 typically has at least two fiducial markers 39.

A fiducial marker 39 is a marker which is used by machines during the assembly process. The fiducial marker may be used as a reference point. For example, an automated pick- and-place machine may use fiducial marker 39 as a reference point on the PCB to help to accurately locate a mounting position for a component relative to the fiducial marker. A fiducial marker 39 is typically a circular copper element, but other shapes are possible. In some electronic assemblies, there may be other regions of a PCB which are not coated. For example, uncoated regions can comprise one or more of: bare track around fixing holes; regions around connectors; earthing pads. Additionally, or alternatively, the PCB 31 may comprise uncoated regions which are purposely provided for measurement purposes.

While the PCB 31 is nominally flat, it has an undulating profile when viewed at a magnified scale. Figure 2 shows, in box 80, a magnified section of the PCB. The extent of the undulation has been exaggerated for illustrative purposes. An extent of the undulation (i.e. difference between highest point and lowest point on a top surface) may be >1mm, but typically is in the range of 200-500pm.

Figure 3 shows a detailed cross-section through a component 35 and the PCB 32, 33. The component 35 is surface-mounted to conductive pads 33 on the upper surface of the PCB. The component 35 has leads 36 extending from ends of the component. The leads 36 are soldered 37 to the conductive pads 33. Figure 3 shows the component after a coating material 40 has been applied. It can be seen that the coating thickness varies at different points around the component. Some points are particularly prone to poor coverage. For example, coverage may be poor at corners 50 of a component 35.

The coating material 40 can vary in thickness due to one or more of the following factors: • quantity of coating material applied to the assembly;

• factors of the curing process for the coating, such as cure temperature, cure time;

• board shape (e.g. warp above or below a nominal“planar” position can cause pooling of coating material;

• physical dimensions of a component (e.g. these can vary due to manufacturing tolerances);

• height of a conductive track/pad 33;

• dimensions and/or shape of leads 36;

• amount of solder 37;

• gravity causing the coating material to run down the leads and sides of components before curing;

• via holes (i.e. holes which extend through a PCB) can act as plug holes and transfer coating material to the reverse side of the PCB, thereby reducing coating thickness in the region of the via hole.

• fine pitch devices can be difficult to coat between the leads.

The system 100 of Figure 1 has a controller 10 connected to the measuring apparatus 20. Figure 4 shows a method of processing an electronic assembly. Steps of this method are performed by the controller 10. Step 101 is performed on an electronic assembly 30 before coating material is applied. The electronic assembly has the state shown in Figure 2(A). The controller 10 is configured to cause the measuring apparatus 20 to determine a first height profile of an outer surface of the electronic assembly 30. The first height profile is a height profile of the upper surface of a combination of the printed circuit board (substrate 32 and conductive tracks/pads 33), the components 35, component leads 36 and the solder joints 37. The first height profile is obtained for a plurality of positions across the electronic assembly 30. The first height profile is a set of data representing height (z direction) of positions in the x and y plane across the entire assembly. This is a first scan of the electronic assembly. The scan is performed at a high resolution. An example value of resolution in the z direction is 1-5pm. An example value of resolution in the x-y plane is 10- 40pm. That is, the measuring apparatus acquires measurements at locations which are spaced apart in the x-y plane by 10-40pm. The acquired measurements can be called a 3D cloud of points. For each scan position, the measuring apparatus acquires a positional co ordinate x, y, z.

At step 102, a coating material is applied to the electronic assembly 30. The coating material is cured by heating, ultraviolet (UV) radiation or any other suitable technique. Step 103 is performed on an electronic assembly 30 after the coating material has been applied. The electronic assembly has the state shown in Figure 2(B). The controller 10 is configured to cause the measuring apparatus 20 to determine a second height profile of an outer surface of the electronic assembly 30. The second height profile is a height profile of the upper surface of a combination of the printed circuit board (substrate 32 and conductive tracks/pads 33), the components 35, component leads 36 and the solder joints 37 plus the coating material 40. As described above, some isolated regions of the assembly will remain uncoated, such as fiducials 39. The second height profile is obtained for a plurality of positions across the electronic assembly 30. The second height profile is a set of data representing height (z direction) of positions in the x and y plane across the entire assembly. This is a second scan of the electronic assembly. The controller 10 now has two sets of height profile information: (i) a first height profile of an outer surface of the electronic assembly 30 before coating; (ii) a second height profile of an outer surface of the electronic assembly 30 after coating.

A difference between (i) the first height profile before coating and (ii) the second height profile after coating is indicative of thickness of coating material 40, subject to any adjustment due to optical properties of the coating. At step 104, controller 10 is configured to determine a thickness of the coating based on the first height profile and the second height profile. Some regions of the PCB are uncoated at step 102. The height of the uncoated regions is not expected to change between step 101 (first profile) and step 103 (second profile). Therefore, the uncoated regions can be used as reference points to align the first and second height profiles.

At step 105, the first and second height profiles are aligned using measurements obtained at the uncoated regions as reference points. The controller is configured to compensate for an effect of optical properties of the coating material on the determination of the second height profile. Step 104 determines coating thickness data for points across the entire electronic assembly. Step 105 can use measurements from as many uncoated regions as possible.

At step 106, the coating thickness data is processed to provide a useful output to an operator. Controller 10 is configured to compare the determined thickness data with one or more threshold values. A threshold value can represent a thickness value which is determined as unacceptable. A possible threshold value for an unacceptable coating thickness = 25pm. A threshold value can represent a thickness value which is determined as acceptable. A possible threshold value for an acceptable coating thickness = 100pm. Coating thicknesses falling in the range 25pm > coating thickness > 100pm can be determined marginal. It will be understood the threshold values can be set to any desired values. In other examples, the threshold value for an acceptable coating thickness may be a value in the range 25-75pm. The controller 10 is configured to output coating thickness data and/or graded thickness data (e.g. graded into bands of pass/fail or into the bands: pass, marginal, fail). At step 107 the coating thickness data is output to a display, for presentation to an operator. The display can allow an operator to judge whether the coating applied to the electronic assembly is acceptable.

Figure 5 shows an example display of coating thickness data which has been graded into the bands: pass, marginal, fail. The graded bands are displayed, superimposed upon the assembly, to identify parts of the assembly which fall within each of the graded bands. The electronic assembly shown in Figure 5 has two failed regions 301 , 302 and three marginal bands 303, 304, 305. The remainder of the board has passed. The output data can be displayed in a format which clearly identifies each grading band, such as a colour-coded scheme, e.g. pass = green; marginal = amber; fail = red.

Additionally, or alternatively, at step 106 the controller 10 may be configured to output coating thickness data and/or graded thickness data to a handling machine which directs the electronic assembly to an appropriate store or equipment for subsequent processing. For example, electronic assemblies which have a coating of an acceptable thickness are directed to a store for subsequent processing, packaging or distribution. Electronic assemblies which have a coating of an unacceptable thickness are directed to a store for waste products. The handling machine can use the coating thickness data and/or graded thickness data to automatically direct electronic assemblies without a need for operator decisions.

Figures 6A and 6B show a measuring apparatus 20 and two different coating materials 40. Figure 6A shows an opaque material. Figure 6B shows a partially transparent material. The measuring apparatus 20 is a triangulation sensor. An illuminating radiation source (e.g. a laser) emits a beam 22 which illuminates a target. An optical sensor 24 receives a beam 23 of backscattered radiation. Distance 25 between the measuring apparatus 20 and the target surface results is determined from a position 26 of the beam 23 on the optical sensor 24. As distance 25 varies between the measuring apparatus 20 and the target surface, the position 26 of the beam 23 varies on the optical sensor 24. In Figure 6A, the opaque material causes the optical sensor 24 to receive a beam 23 from the top surface 41 of coating 40. In this example, the measuring apparatus 20 can accurately determine the position of the top surface 41 of the coating material 40. In Figure 6B, the illuminating beam 22 partially penetrates the partially transparent material. The optical sensor 24 receives a beam 23 from a depth which is beneath the top surface of coating 40. In this example, the measuring apparatus 20 determines the height of the top surface of the coating material 40 as being at position 45, which is lower than the actual height of the top surface 41.

As described above in Fig. 4, at step 104 the controller is configured to compensate for an effect of optical properties of the coating material on the determination of the second height profile. One way of compensating is:

CCah = CCmh x CCmr

where:

CCah = Conformal Coating actual height;

CCmh = Conformal Coating measured height;

CCmr = Conformal Coating measurement ratio.

The Conformal Coating measurement ratio CCmr may also be referred to as a compensation factor CCmr or an adjustment factor CCmr. It can be determined by experiment or by measurement (e.g. using a confocal sensor or other) and will vary for different coating materials. For example, the CCmr may have a value in the range of 1 for a fully opaque coating material. The CCmr may have a value >1 for a material which is partially transparent, and may have a higher value such as 1.5, 2.0, 2.5, or any other value which is required to compensate the accuracy of the post-coating measurement. The value of CCmr varies with optical transparency, with CCmr having a higher value for transparent coating materials.

In the method describe above, the first and second two profiles are compared to obtain a difference, and then a compensation factor CCmr is applied to the conformal coating measured height CCmh to achieve the actual height CCah.

In an alternative method, the second height profile is adjusted by a compensation factor to provide a compensated second height profile. Then, the method determines a difference between the first height profile and the compensated second height profile. Both methods achieve a similar end result, and compensate for optical properties of the coating material on the measurement of the second height profile. In this second method, the compensation factor may have a value <1 for partially transparent coating materials, as the actual distance between the measuring apparatus 20 and the top surface 41 of the coating material is less than the measured distance 45. The Conformal Coating measurement ratio CCmr allows an adjustment of the measured height. After applying the adjustment factor CCmr to the output of the measuring apparatus 20, the top surface of the coating material 40 is correctly determined.

The adjustment factor compensates for at least one of: colour of the coating material; transparency of the coating material; reflective properties of the coating material.

In another example, the system described above can comprise a confocal sensor in addition to another, different, type of measurement apparatus. The system may comprise a confocal sensor in addition to a triangulation sensor. The confocal sensor can be used to accurately determine coating thickness at one (or more) positions on an electronic assembly. The confocal sensor can provide an accurate measurement of coating thickness which can be used as a local reference. This can increase the local accuracy as not relying on having bare PCB or copper.

An example method of operating the above system with two types of sensor is:

(i) make before coating and after coating measurements using the triangulation sensor at a first position on a PCB, and determine coating thickness;

(ii) measure coating thickness accurately at the same point using the confocal sensor.

(iii) compare (i) and (ii) to determine a value for the compensation factor, and use that value of compensation factor for all other measurements across the PCB.

In order to address the issue of board warp the pre and post coating scans require being “electronically de-warped”, this is achieved in one of two ways.

a. Using the fiducials and/or bare track found around fixing holes that will not be coating during the coating process a track height and surrounding bare PCB will be measured. Multiple positions on the bare PCB are manually or automatically selected and electronically set to a height of zero as a reference, this has the effect of removing the warp. All component heights will automatically be electronically raised or lowered by the same amount.

b. Using the fiducials and/or bare track found around fixing holes that will not be coating during the coating process a track height and surrounding bare PCB will be measured. The PCBs Computer Aided Design (CAD) will be used to electronically set to a height of zero all the bare PCB material, this has the effect of removing the warp. All component heights will automatically be electronically raised or lowered by the same amount.

In order to address the issue of acceptable variability in component height the pre coating scan is used to measure the height of each component in relation to the“local” bare PCB. Once the post coating scan is“de-warped” the height of each component is then known and the extra height measured post coating will be the coating material.

Figure 7 shows an example of processing apparatus 500 which may be implemented as any form of a computing and/or electronic device, and in which embodiments of the system and methods described above may be implemented. Processing apparatus may implement all, or part of, any of the methods described above. Processing apparatus 500 comprises one or more processors 501 which may be microcontrollers, microprocessors, controllers or any other suitable type of processors for executing instructions to control the operation of the device. The processor 501 is connected to other components of the device via one or more buses 506. Processor-executable instructions 503 may be provided using any computer- readable media, such as memory 502. The processor-executable instructions 503 can comprise instructions for implementing the functionality of the described methods. The memory 502 is of any suitable type such as read-only memory (ROM), random access memory (RAM), a storage device of any type such as a magnetic or optical storage device. Additional memory 504 can be provided to store data 510-513 used by the processor 501. The data comprises the first height profile data and the second height profile data 510 acquired during scans of an electronic assembly. The data comprises coating thickness data 511 of an electronic assembly. The data comprises at least one compensation factor 512 for adjusting acquired second profile values. The data comprises one or more threshold values 513 for grading coating thickness data The processing apparatus 500 comprises input/output (I/O) interfaces 507. The I/O interfaces 507 can receive signals from the measuring apparatus 20. The I/O interfaces 507 can output signals to a display. The processing apparatus 500 comprises one or more network interfaces 508 for interfacing with other network entities.

Although embodiments have been described in connection with electronic assemblies, it is possible to use the apparatus with other types of object where there is a need to measure a thickness of a coating applied to the object.

Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of the words, for example“comprising” and“comprises”, means “including but not limited to”, and is not intended to (and does not) exclude other moieties, additives, components, integers or steps.

Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith.

Claims

1. Apparatus for monitoring thickness of a coating material on an electronic assembly comprising:

a measuring apparatus configured to monitor height of a surface;

a controller configured to:

2. Apparatus according to claim 1 wherein the controller is configured to compensate using an adjustment factor which compensates for at least one of: transparency of the coating material; colour of the coating material; reflective properties of the coating material.

3. Apparatus according to claim 1 or 2 wherein the controller is configured to determine a thickness of the coating by:

CCah = CCmh x CCmr,

where:

CCah = actual height of the coating;

CCmr = adjustment factor.

4. Apparatus according to any one of the preceding claims wherein the controller is configured to determine a thickness of the coating by aligning the first height profile and the second height profile using profile data of the first height profile and the second height profile at a first point, wherein the first point is a point on the electronic assembly that remains uncoated when the second height profile is determined.

5. Apparatus according to claim 4 wherein the first point on the electronic assembly which remains uncoated is at least one of: a fiducial marker, track around a fixing hole; a region around a connector, an earthing pad.

6. Apparatus according to any one of the preceding claims wherein the electronic assembly is a printed circuit board with components mounted to the printed circuit board via solder joints, wherein the coating material is a coating material applied over the printed circuit board, the components and the solder joints, and wherein the first height profile of an outer surface of the assembly is a height profile of the printed circuit board, the components and the solder joints.

7. Apparatus according to any one of the preceding claims wherein the coating material is a polymeric material.

8. Apparatus according to any one of the preceding claims wherein the measuring apparatus is a triangulation scanner.

9. Apparatus according to any one of the preceding claims wherein the controller is configured to compare the thickness of the coating with a threshold value and to output data indicative of at least one of:

(i) a region of the electronic assembly having a coating thickness greater than the threshold value;

(ii) a region of the electronic assembly having a coating thickness less than the threshold value.

10. Apparatus according to any one of the preceding claims wherein the controller is configured to:

compare the thickness of the coating with a plurality of threshold values to determine regions of the electronic assembly having a coating thickness within a plurality of different thickness bands; and

output data indicative of a region of the electronic assembly having each of the thickness bands.

11. Apparatus according to any one of the preceding claims wherein the measurement apparatus is configured to move in an x-y plane with respect to the assembly.

12. A method for monitoring thickness of a coating material on an electronic assembly comprising:

determining a first height profile of an outer surface of the assembly before the coating material is applied by measuring height of the assembly at a plurality of positions across the assembly;

13. A method according to claim 12 wherein the compensating uses an adjustment factor which compensates for at least one of: transparency of the coating material; colour of the coating material; reflective properties of the coating material.

14. A method according to claim 12 or 13 wherein the thickness of the coating is determined by:

CCah = CCmh x CCmr,

where:

CCah = actual height of the coating;

CCmr = adjustment factor.

15. A method according to any one of claims 12 to 14 wherein the thickness of the coating is determined by aligning the first height profile and the second height profile using profile data of the first height profile and the second height profile at a first point, wherein the first point is a point on the electronic assembly that remains uncoated when the second height profile is determined.

16. A computer program product comprising a machine-readable medium carrying instructions which, when executed by a processor, cause the processor to perform the method of any one of claims 12 to 15.