Technical field of the invention The present invention generally relates to the field of providing substrates with viscous medium. More spe- cifically, the invention relates to a method of providing a substrate with viscous medium, a corresponding appara- tus, an apparatus for correcting application errors in connection with the application of viscous medium on a substrate, and use of jetting means for said correcting.

Background of the invention In the field of electronics production, it is well known that possible sources of errors are the electronic circuit boards. Mainly, these errors originate from the application of viscous medium, e. g. solder paste, prior to the mounting of components on the board. Over the years, a number of different techniques have been pro- posed for detecting errors resulting from solder paste application. Originally, this detection was performed manually. Lately, however, the use of cameras in combina- tion with image processing have become the preferred choice for detecting said errors. An example of this can be found in the International Patent Publication WO 00/42381, which discloses a method and a device for contactless inspection of objects on a substrate.

When errors resulting from the application of solder paste have been detected, and preferably identified, ap- propriate action must ensue. Traditionally, a board con- taining errors as a result of the application of solder paste is simply taken out of the process. The removed board can then be discarded or, commonly, cleaned and subsequently reinstated at the beginning of the process.

Recently, the results of the solder paste application, i. e. critical parameters thereof, are monitored and er- rors are identified, and the monitored parameters are fed

back so that appropriate measures can be taken. This is common in state-of-the-art screen printing applications.

This feedback can, in the simplest case, be a warn- ing message when a parameter has crossed a certain pre- set limit. The operator can then decide which actions to take. On the other hand, an advanced system could incor- porate artificial intelligence to adjust application pa- rameters such as, in the case of screen printing, squee- gee pressure, speed and angle, amount of paste on the stencil, cleaning interval, etc. Thus, critical process parameters can be monitored and adjusted so that the ap- plication errors can be reduced. However, the specific board containing the detected errors is dealt with in the traditional manner described above, i. e. the faulty board is removed from the entire process.

When using conventional dispensing, i. e. contact dispensing, as opposed to screen printing for the appli- cation of solder paste, dispensing heads with integrated measurement capabilities, for measuring the result of the application of solder paste, are known within the art.

The measured parameters can be fed back for process con- trol in order to reduce future errors. Since the inte- grated measurement capabilities can measure the solder paste application of a single dot, e. g. immediately fol- lowing the dispensing of said dot, process parameters can be altered for the subsequent dispensing of dots on the same board. Thereby, small errors can be detected and process parameters adjusted so that, in the best case, major errors requiring the board to be removed from the process, can be avoided.

Summary of the invention Thus, an object of the present invention is to pro- vide a solution to the above-mentioned problems and to provide an improved production efficiency.

This and other objects are achieved according to the present invention by providing apparatus and a method

having the features defined in the independent claims.

Preferred embodiments are defined in the dependent claims.

In general terms, the invention is based on the in- sight of the advantageous results that can be obtained by using correction measures in relation to the application of viscous medium on a substrate.

According to a first aspect of the invention, there is provided a method of providing a substrate with vis- cous medium, comprising the steps of applying said vis- cous medium onto the substrate, inspecting the results of said application, determining errors of the application based on said inspection, and correcting at least some of said errors.

According to a second aspect of the invention there is provided a system or an apparatus for providing a sub- strate with a viscous medium, comprising application means for applying the viscous medium onto the substrate, inspection means for inspecting the results of said ap- plication, processing means for determining application errors based on said inspection, and correction means for correcting at least some of said errors.

According to a third aspect of the invention there is provided an apparatus for correcting application er- rors in connection with application of a viscous medium on a substrate, said apparatus comprising processing means adapted to receive information of said errors in the application of said viscous medium, and correction means for correcting at least some of said errors based on said information.

According to a fourth aspect of the present inven- tion there is provided the use of jetting means for the correction of application errors in regard to viscous me- dium provided on a substrate.

For the purposes of this application, it is to be noted that the term"viscous medium"should be inter- preted as solder paste, flux, adhesive, conductive adhe-

sive, or any other kind of medium used for fastening com- ponents on a substrate, or resistive paste; and that the term"substrate"should be interpreted as a printed cir- cuit board (PCB), a substrate for ball grid arrays (BGA), chip scale packages (CSP), quad flat packages (QFP), and flip-chips or the like. It is also to be noted that the term"jetting"should be interpreted as a non-contact dispensing process that utilises a fluid jet to form and shoot droplets of a viscous medium from a jet nozzle onto a substrate, as compared to a contact dispensing process, such as"fluid wetting", which is the act of the viscous medium leaving the dispense tip, contacting and clinging to the substrate and remaining on the substrate as the dispense tip pulls away.

Preferably, use is made of jetting and/or removing means for the correction of errors resulting from the ap- plication of a viscous medium on a substrate. Said cor- rection is performed after the application of said vis- cous medium, but prior to any soldering thereof. Prefera- bly, said correction is also performed prior to the mounting of components, at the locations on the substrate where correction is required. It must, however, be noted that the correction of said errors can be performed after mounting of components at other locations on the sub- strate. This is for instance the case when mounting com- ponents on both sides of a substrate. Then, correction of errors according to the present application can be per- formed at one side after components have been mounted and attached to the other side of the substrate.

Thus, the correction of viscous medium applied on a substrate according to the present invention must not be mistaken for the known correction of soldered connec- tions, which is performed late in the component mounting cycle after the mounting of components and hardening of the viscous medium, i. e. reflow of solder paste, used for mechanically and electrically connecting the component to the substrate. An advantage of correcting errors prior to

the mounting of components and hardening of the viscous medium is that it is much cheaper, as compared to making corrections late in the mounting process cycle. This is due to the fact that valuable production capacity is not occupied by a substrate that must be corrected. Further, correction performed prior to the mounting of components is easier and requires less correction steps.

According to the present invention, said application of viscous medium is not intended to be limited to a par- ticular method of applying viscous medium, i. e. applica- tion through e. g. screen printing, conventional dispens- ing, jetting, etc. On the contrary, the present invention is intended to be applicable for the correction of any substrate that is provided with a viscous medium, regard- less of how the viscous medium was originally applied.

According to one embodiment of the present inven- tion, following the application of viscous medium on a substrate, the result of said application is inspected.

This inspection can be performed after the completion of the application of viscous medium on the entire sub- strate, or can be performed during said application, e. g. at intermittent intervals or continuously, depending on the application method being used. When applying the vis- cous medium through screen printing, the inspection is preferably performed when the application is completed.

When applying viscous medium through contact dispensing or jetting, the inspection can be performed following the dispensing of viscous medium at each specific site or po- sition, or at certain predetermined intervals.

The inspection not only detects whether a site or position has been provided with viscous medium, but also detects whether the correct amount, i. e. volume, of vis- cous medium has been applied, whether the position of the applied medium is correct, or within a predetermined fault range, and has the correct shape and height at a given position. The term"correct"has here the meaning

of the measured parameter being within a fault range that is predetermined.

According to alternative embodiments of the inven- tion, the inspection could involve inspection of the en- tire substrate, all locations on the substrate where vis- cous medium is to be applied, or only specific locations on the substrate. The latter would preferably involve in- specting specific locations that are particularly error prone, e. g. locations where components are later to be placed which are particularly sensitive to errors in the applied viscous medium, such as components having leads with a very fine pitch, or components which are difficult to inspect after soldering, such as BGA components.

According to an embodiment of the invention, follow- ing the detection of an error, the characteristics of said detected error are determined and stored. Prefera- bly, the error characteristics are further analysed and evaluated to determine whether the error needs correct- ing. If so, the proper method of correcting said error is also determined. According to an embodiment, the time needed for performing said correction is estimated and stored.

According to an embodiment of the invention, the ap- plication of viscous medium is performed through screen printing. Following the completion of the screen print- ing, the result of the screen printing is inspected and any errors are detected, determined, and analysed. The results of said inspection can be fed back to the screen printer for enabling the screen printer to correct or ad- just parameters. Following the inspection and determina- tion of errors, correction of the determined errors en- sues.

According to another embodiment of the invention, the application of viscous medium is performed using con- ventional contact dispensing or jetting. This enables continuous, on-line inspection of the results of said ap- plication. Thus, the application of viscous medium at a

single location can be immediately inspected, e. g. dot by dot, or deposit by deposit, and the results can both be fed back for adjustment of dispensing or jetting parame- ters, or can be immediately transferred to means for cor- recting any detected errors. According to an alternative embodiment, the inspection is performed following the completion of the application of viscous medium on the entire substrate. Then, and as described above, the in- spection could involve inspecting the entire substrate, all locations on the substrate where viscous medium is to be applied, or predetermined specific locations on the substrate.

According to the present invention, the correction of determined errors on a substrate is performed by the use of jetting for applying additional viscous medium.

Not only is jetting faster, jetting is also more accurate and more flexible in comparison with conventional dis- pensing. The use of conventional dispensing often provide a bottle neck that increases the cycle time in the over- all component mounting process, as compared to the use of jetting. It has also been found that it is possible to achieve smaller dots when using jetting than when using conventional dispensing. Further, by using jetting, it is possible to apply dots of viscous medium having differing heights, which is difficult when using screen printing.

However, at positions where it has been determined that there has been applied too much viscous medium, the surplus viscous medium must be removed. This is performed according to an embodiment of the invention by the provi- sion of removing means capable of removing viscous medium from a specific location on the board. Said removal is preferably performed through suction of the viscous me- dium, preferably in combination with heating the viscous medium prior to the suction thereof. Due to the charac- teristics of the viscous medium, e. g. solder paste, the remains on the substrate of viscous medium at a location at which removal of the viscous medium has been performed

is generally minute. But, there may be some flux remains left at said locations. However, this is not a problem due to the fact that where cleaning of the substrate is performed following the soldering of components, in order to get rid of deleterious flux that is used when mounting the components, said flux remains will be removed during said cleaning; and in cases where it is not necessary to remove flux following said soldering, the flux remains will not be a problem. Thus, additional cleaning of the substrate following the removal of surplus viscous medium will not be necessary.

The combination of jetting means and removing means for correcting errors on a substrate provided with vis- cous medium enables correction of all types of applica- tion errors. These errors may for instance include having applied a surplus of viscous medium or insufficient vis- cous medium at a given position. Furthermore, the errors may for instance include applying viscous medium at incorrect positions, applying viscous medium where the result of the viscous medium application is misaligned, applying viscous medium having characteristics deficien- cies of some sort, etc.

According to an embodiment of the present invention, the correction of errors is performed in a separate ma- chine, i. e. the substrate is forwarded to the correction machine following the determination that correction of the result of viscous medium application is required. Ac- cording to an alternative embodiment, the correction ma- chine also comprises the inspection means described above. Then, all substrates are forwarded to the correc- tion machine for inspection and possible correction of detected errors. Preferably, the results of said inspec- tion are fed back to the machine for allowing adjustment of application parameters.

Said embodiment of having a stand alone correction machine is particularly useful when the initial applica- tion of viscous medium is performed by screen printing or

contact dispensing. Then, the advantages relating to screen printing and contact dispensing, as recognised by the man skilled in the art and not further described herein, can be combined with the above described advan- tages of correction of errors through jetting.

According to an alternative embodiment of the inven- tion, the correction means, i. e. jetting means for cor- rection and removing means, are integrated in the machine for performing the initial application of viscous medium onto the substrate. Thus, the inspection can be performed continuously and any correction of the substrate neces- sary can be done instantly.

Said embodiment of having an integrated machine for both the initial application of viscous medium and the ensuing inspection and correction is particularly useful when the initial application of viscous medium is per- formed by jetting. Thus, a single jetting means can be utilised for both the initial application and the correc- tion. This would facilitate the construction of the ma- chine and minimise the size of the machine and also would provide a small footprint, i. e. the overall floor space required for the machine. Alternatively, the correction is performed by a separate jetting means not used for the initial application of viscous medium. This would de- crease the overall time required for the application and the possible correction.

Generally, a separate machine for the correction, and possibly also for the inspection, would increase line length but decrease the overall cycle time for each sub- strate as compared to an integrated machine as described above incorporating both the initial application of vis- cous medium and the subsequent or concurrent correction.

Further, an integrated machine where the initial applica- tion is performed through screen printing or contact dis- pensing would have to overcome a number of constructional difficulties.

According to a specific embodiment of the present invention, all errors occurring on a single substrate following the completion of the application of viscous medium, e. g. following screen printing, are jointly ana- lysed and the total amount of corrections needed for the entire substrate is then determined. Thus, it can be de- termined whether the sum of errors are so great that cor- rection of the errors on the substrate are not worth- while. Preferably, it has been determined the time re- quired for error correction of each detected error re- quiring correction, then the overall correction time for the entire substrate is evaluated. If it is found that correction of the errors on the substrate if not worth- while, the substrate can be removed from the process and cleaned without any corrections being made.

Thus, the use of jetting for correcting errors re- sulting from the application of viscous medium onto a substrate enables not only error prevention, which is known through the feedback of inspection results, but also error correction. This guarantees that a low defect level can be maintained, which in turn results in the main source of failures in the electronics production is overcome, reducing the production costs considerably.

This is done without more than a fractional addition of time required for the overall application of viscous me- dium on a substrate.

The above mentioned and other aspects, advantages, and features of the invention will be more fully under- stood from the following description of exemplifying em- bodiments thereof.

Brief description of the drawings Exemplifying embodiments of the invention will be described below with reference to the accompanying draw- ings, in which:

Fig. 1 is a block diagram illustrating a typical prior art arrangement for handling errors resulting from application of viscous medium.

Figs 2-5 are block diagrams illustrating arrange- ments according to alternative embodiments of the present invention.

Figs 6-11 are flow charts illustrating alternative embodiments of the method according to the present inven- tion.

Detailed description of preferred embodiments In the block diagrams of Figs 1-5, the thick-lined arrows depict the movement of a substrate through the production line. The dashed boxes simply indicate that the means or apparatus depicted within the box can be in- corporated in a single machine.

With reference to Figs 2-11, there is illustrated exemplary embodiments of the present invention. As to the means used for applying solder paste, inspecting the re- sults of said application, removing solder paste, and component mounting, use can be made of means which by themselves are known within the art, and the construc- tional features of which are also known. Therefore, a constructional and functional description of each sepa- rate means has been omitted.

With reference to Fig. 1 there is shown a typical prior art arrangement for handling errors resulting from solder paste application. The solid arrows depict the transportation of a substrate through the arrangement.

The substrate is first brought to a machine 2 for appli- cation of the solder paste. Following the application of solder paste, the substrate is brought to an inspection means 3 where the result of the application is inspected.

When the application is performed through contact dis- pensing or jetting, the inspection could take place si- multaneously with the application. Then, of course, the application means 2 and the inspection means 3 are incor-

porated in a single machine 1. If the inspection shows that the result of the solder paste application is satis- factory, then the substrate is conveyed to a component mounting machine 5. If not, then the substrate is taken out of the production line, either discarded or cleaned in a cleaning machine 4 and reinstated in the production line prior to the application machine 2.

There will now be described with reference to Figs 2 and 6, a first embodiment of the present invention for correcting errors resulting from solder paste applica- tion. A substrate is first brought to an application and inspection machine 10, incorporating both application means 12 and inspection means 14. A conventional method of application is used and therefore not further de- scribed. As can be seen from the flow chart in Fig. 6, solder paste is applied to the substrate at 102. Then the inspection of the application result commences at 104 by indicating the starting point for the inspection. The solder paste application is inspected for a position N at 106. At 108, it is determined whether an error is de- tected for position N and, if so, the parameters of the detected error, e. g. size of solder paste dot, dot posi- tion, dot shape, etc., is recorded and, possibly, also fed back to the application means at 109. At 110, there is a check whether all positions to be inspected have been inspected. If not, the position parameter N is in- cremented and the next position is inspected at 106.

Following the inspection for all selected positions, the parameters are evaluated at 112 to determine whether correction of the substrate is required. If not, the sub- strate is transferred at 113 to a component mounting ma- chine 18. However, if correction is required, then the total amount of correction needed is calculated at 114.

The result of this calculation is defined as a correction value, which at 116 is compared to a threshold value for determining whether correction of the substrate is worth- while. That is, if there are vast correction actions re-

quired, then it might be more economical to simply dis- card the substrate. This is of course dependent on the type of substrate and the cost involved for the sub- strate. If correction is deemed worthwhile, then the sub- strate is transferred at 118 to a correction machine 16.

The correction machine 16 incorporates both means for re- moving surplus solder paste, as well as jetting means for jetting additional solder paste onto the substrate.

As is understood by the man skilled in the art, the above-mentioned steps of recording, evaluating, determin- ing and sending and retrieving information can be per- formed by the use of conventional processing means (not shown).

Turning now to figs 3 and 7, there is illustrated a second exemplary embodiment of the present invention. The invention according to this embodiment differs from the first embodiment in that the application means 22 is not incorporated with the inspection means 24. Instead, the inspection means 24 can be incorporated with correction means 26 in a combined inspection and correction machine 20. As is shown in the flow chart of Fig. 7, in this ex- emplary method 200 the substrate is transferred to the application means 22 at 202 and provided with solder paste. Then, the substrate is at 204 transferred to the inspection and correction machine 20. The inspection means 24 inspects at 208 the solder paste application re- sult for position N.

At 210 it is determined whether the inspection for position N reveals that an error that require correction is detected. If so, the error is corrected by the correc- tion means 26 at step 211. At 212 it is checked whether all positions have been inspected. If so, the substrate is transferred at 214 to the component mounting machine 18. For ease of description, it is indicated in Fig. 7 that the correction for position N is performed directly following the inspection for position N.

However, the inspection means 24 might perform an inspection faster than the correction means 26 can per- form the corresponding correction. In reality, the in- spection is performed as fast as possible, and the infor- mation as to which positions require correction, and which correction that is needed, is continuously trans- mitted to the correction means 26. Thus, the inspection means 24 does not wait at a specific position for the correction means to finish correcting at that position.

This applies equally for the embodiment of Figs 5 and 11, which will follow.

Turning now to Figs 4 and 5, there is shown two al- ternative embodiments where the initial application of solder paste is performed with jetting means 32,40. In the embodiment of Fig. 4, there are provided two separate jetting means, one for the initial solder paste applica- tion, and one included in the correction means 36 for the correctional jetting of additional solder paste. In the embodiment of Fig. 5, the same jetting means is used for both the initial solder paste application, and for the correctional jetting of additional solder paste. Thus, the application, inspection and correction means are all integrated in one machine 40.

The method 300 for the arrangement illustrated by Figs 4,8 and 9 differs from the methods of Figs 6 and 7 in that the application for each position can be essen- tially immediately inspected. Turning to figs 8 and 9, solder paste is applied through jetting means 32 for po- sition N at step 304. The result of the application is then inspected by the inspection means 34 at step 306. If correction is required, determined at 308, the correction parameters is relayed to the correction means 36 at step 309. The application and inspection then continues at 304 and 306 until solder paste have been applied and the in- spection have been performed for all positions, deter- mined at 310. Simultaneously with said application and inspection, the correction means 36 performs correction

of the detected errors at 322 based on information re- ceived at 320. After each correction has been completed, a signal indicative thereof is transmitted at step 324 to processing means (not shown). At step 310, it is also de- termined whether all corrections have been completed be- fore the substrate is transferred to the component place- ment machine 18 at step 312.

Turning finally to Figs 5,10 and 11, there are il- lustrated two alternative methods 400,500 according to alternative embodiments of the present invention. As de- scribed above, a single jetting means is used for both the initial application of solder paste and the subse- quent possible corrective jetting of additional solder paste. In Fig. 10 there is shown an alternative where the corrective jetting is performed following the completion of the initial application, whereas in Fig. 11 there is shown an alternative where the jetting correction is per- formed as soon as an error has been detected. As obvious to the man skilled in the art, any combination of these two alternatives is also possible.

The initial steps 402-410 of method 400 correspond closely to the initial steps 102-110 of method 100, as shown in Fig. 6. However, following the completion of the initial solder paste application and the inspection and recording of errors, correction of the detected errors is performed by the correction means, both where additional jetting is required as well as the removal of solder paste. According to Fig. 10, for ease of illustration, this is performed by resetting the position parameter N, at 412 and looping through all positions on the substrate and correcting them in order, at 414-420. However, any type of correction algorithm can be used. Finally, the substrate is transferred at 422 to the component mounting machine 18.

The method 500 shown in Fig. 11 correspond to the method 300 of Fig. 8 for all steps 502-512, except for step 509. In method 500 the required correction is per-

formed at step 509, whereas at step 309 of the method 300, parameters are transmitted to separate correction means. However, as stated above, the correction for a specific position does not necessarily take place immedi- ately following the inspection of that position.

Even though the invention has been described above using exemplifying embodiments thereof, altera- tions, modifications and combinations thereof, as under- stood by those skilled in the art, may be made within the scope of the invention, which is defined by the accompa- nying claims.