A method and a system for an automatic classification of the solder printer tooling in the preparation of a printed circuit board

Abstract

The present invention discloses a method and a system for an automatic classification of the solder printer tooling in the preparation of a printed circuit board, the method comprising: - providing the printed circuit board and placing a stencil layer on the printed circuit board thereby covering the printed circuit board with a stencil layer; said stencil layer comprising a variety of openings that fit to positions of the printed circuit board where a solder past pad has to be deposited; - bringing the solder past onto the stencil layer and wiping the squeegee over the stencil layer wherein the squeegee is used to bring the solder past into the openings of the stencil layer by exposing a predetermined pressure to the solder past and the stencil layer in a direction substantially perpendicular to the plane of the printed circuit board; - positioning in alignment with the squeegee an array of distance sensors in proximity to the squeegee and measuring the distance between the distance sensors and the stencil layer while wiping with the squeegee over the stencil layer; and - evaluating the measured distances with respect to a predetermined threshold for the distance between the stencil layer and the distance sensors.

Description

A method and a system for an automatic classification of the solder printer tooling in the preparation of a printed circuit board

[0001] TECHNICAL FIELD

[0002] The present disclosure is directed, in general, to computer-aided systems for Printed Circuit Board ("PCB") and Surface-Mount Technology ("SMT") manufacturing and board quality inspection and analysis. More in particular, the disclosure is directed to a method and a system for an automatic classification of the solder printer tooling in the preparation of a printed circuit board .

[0003] BACKGROUND OF THE DISCLOSURE

[0004] Electronic devices contain Printed CircuitBoards (PCB) that makes electric connects from component lead to another component's lead. An electric connect is made with thin copper pathways on the PCB. In both ends of the copper pathway, there are wider copper areas where the component lead is electrically connected to the copper pathway. This wider copper area is called as "pad" or "land". In surface mount technology (SMT) components are laid on PCB so that component leads are matching the pads and are displaced on top of them. The component lead and the pad are electrically connected together by soldering. In SMT, the soldering is done by adding solder paste on top of a pad before the component is assembled.

[0005] There are many different kind of solder pastes in the market that works differently in different processes. Solder paste contains small tin alloy balls and some chemicals. When the PCB has solder paste added and all the components are assembled, the PCB goes through a reflowoven where the solder paste melts and solders the component lead and the pad together.

[0006] As shown in Figure 1, the solder paste is added on the pads by screen printing method. A steel foil (stencil 2) that has holes (apertures 6) in the same places where pads 4 exist on the PCB is placed on top of the PCB and the solder paste 8 is sweeped (printed) by a squeegee 10 over all apertures. During the sweeping of the squeegee 10, the solder paste 8 is forced to fill the apertures 6 in the stencil 2. Usually, the solder paste is sticky and stays on the copper pads 4 when the stencil is lifted up. The properties of the solder paste, in particular its viscosity, the thickness of the stencil, the shape of the apertures and the printing parameters, such as pressure, speed, stencil removal speed, are the key characteristics of how the solder paste pads 12 are added on top of the copper pads 4.

[0007] Estimately, about 70% of the defects in the assembly of the PCB using Surface Mount Technology (SMT) are originated from the process of the solder printing. Possible problems are that there is too much or not enough solder paste deposited on the pads and / or the solder paste gets to places where it shouldn't go and / or the component lead and the pad don't get correctly soldered and / or the component gets misaligned in the oven when the solder paste melts .

[0008] Reliable printing processes require that all possible variations in printing process are being eliminated on the one hand side, but that the variations are still possible on the other hand side. Printing parameters, like the speed of the squeegee, the force, thespeed of the stencil release, must be matching to the parameters of the used squeegee and the used solder paste.

[0009] Further, the printed circuit board also needs to be supported in a proper way in order to prevent warpage, which causes undesired variations in the process, such as paste misalignment and paste leaking issues between board and stencil foil.

[0010] The support of the PCB is known to be an important factor in the solder paste printing process (see Figures 2 and 3) . Many different techniques have been invented to reduce board support originated issues in this printing process. Variations in the solder paste apertures can be detected using a Solder Paste Inspection machine (SPI) . It requires many samples to statistically prove the variation and the root cause can be related to tools in use, printing parameters or mechanical support of board.

[0011] ASM company has developed a system that measures squeegee Z-axis motor movement from the point where the squeegee is attached to the solder paste printing machine. When maintaining a constant force to the squeegee the z-axis system measures if the squeegee moves up or down (see Figure 3) . Variations in Z-axis movements may indicate that the support of the PCB is not good enough. The squeegee is normally made of steel. Thus, it distributes the force in the z-direction into a wide area of the PCB. The PCB may still bend in some areas under the squeegee (in Y-direction) , but it is not detected if there is no movement in Z-axis that holds the squeegee. In general, there aren't any solutions to know if the support of the PCB during the solder paste printing is good enough.

[0012] Therefore, it is the objective of the present invention to provide a method and a system for an automatic classification of the solder printer tooling in the preparation of a printed circuit board that allow to determine whether the solder paste printing process has been executed under a smooth and equal distribution of the pressure effected by the squeegee to the PCB .

[0013] SUMMARY OF THE DISCLOSUREVarious disclosed embodiments include methods and systems for an automatic classification of the solder printer tooling in the preparation of a printed circuit board, the method comprising the following steps:A printed circuit board is provided and a stencil layer is placed on the printed circuit board thereby covering the printed circuit board with a stencil layer; said stencil layer comprises a variety of openings that fit to positions of the printed circuit board where a solder past pad has to be deposited. Then, the solder paste is brought onto the stencil layer and the squeegee is wiped over the stencil layer wherein the squeegee is used to bring the solder past into the openings of the stencil layer by exposing a predetermined pressure to the solder paste and the stencil layer in a direction substantially perpendicular to the plane of the printed circuit board. Further, in alignment with the squeegee an array of distance sensors is positioned in proximity to the squeegee thereby measuring the distance between the distance sensors and the stencil layer while wiping with the squeegee over the stencil layer. Eventually, the measured distances are evaluated with respect to a predetermined threshold for the distance between the stencil layer and the distance sensors. Thus, this evaluation can be used to identify areas of the printed circuit board and / or the stencil layer where thisdistance goes beyond the predetermined threshold thereby simultaneously identifying areas where defects in the solder paste printing process may have occurred.

[0014] Va rious disclosed embodiments include methods and systems wherein the array of distance sensors can be designed as a line array being oriented parallel to the squeegee .

[0015] Various disclosed embodiments include methods and systems wherein the array of distance sensors is placed upstream and / or downstream of the squeegee as seen in the direction of the movement of the squeegee.

[0016] Various disclosed embodiments include methods and systems wherein in case that the evaluation delivers some results having a distance above the threshold, the method of the solder past printing is repeated at an increased predetermined pressure and / or at an improved support of the printed circuit board for areas where the results occurred.

[0017] The foregoing has outlined rather broadly the features and technical advantages of the present disclosure so that those skilled in the art may better understand the detailed description that follows. Additional features and advantages of the disclosure will be described hereinafter that form the subject of the claims. Those skilled in the art will appreciate that they may readily use the conception and the specific embodiment disclosed as a basis for modifying or designing other structures for carrying out the same purposes of the present disclosure. Those skilled in the art will also realize that such equivalentconstructions do not depart from the spirit and scope of the disclosure in its broadest form.

[0018] Before undertaking the DETAILED DESCRIPTION below, it may be advantageous to set forth definitions of certain words or phrases used throughout this patent document: the terms "include" and "comprise," as well as derivatives thereof, mean inclusion without limitation; the term "or" is inclusive, meaning and / or; the phrases "associated with" and "associated therewith," as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like; and the term "controller" means any device, system or part thereof that controls at least one operation, whether such a device is implemented in hardware, firmware, software or some combination of at least two of the same. It should be noted that the functionality associated with any particular controller may be centralized or distributed, whether locally or remotely. Definitions for certain words and phrases are provided throughout this patent document, and those of ordinary skill in the art will understand that such definitions apply in many, if not most, instances to prior as well as future uses of such defined words and phrases. While some terms may include a wide variety of embodiments, the appended claims may expressly limit these terms to specific embodiments.

[0019] BRIEF DESCRIPTION OF THE DRAWINGS

[0020] For a more complete understanding of the present disclosure, and the advantages thereof, reference is now made to the following descriptions taken inconjunction with the accompanying drawings, wherein like numbers designate like objects, and in which:

[0021] Figure 1 schematically illustrates the process of the solder paste printing at predetermined positions of copper pads located on a printed circuit board.

[0022] Figure 2 schematically illustrates a side view of the solder paste printing using a squeegee according to the prior art.

[0023] Figure 3 schematically illustrates a top view of the solder paste printing using a squeegee according to the prior art.

[0024] Figure 4 schematically illustrates a side view of the solder paste printing using a squeegee having aligned an array of distance sensor.

[0025] Figure 5 schematically illustrates a top view of the solder paste printing using a squeegee having aligned an array of distance sensor.

[0026] DETAILED DESCRIPTION

[0027] FIGURES 1 through 5, in particular Figures 4 and 5, discussed below, and the various embodiments used to describe the principles of the present disclosure in this patent document are by way of illustration only and should not be construed in any way to limit the scope of the disclosure. Those skilled in the art will understand that the principles of the present disclosure may be implemented in any suitably arranged device. The numerous innovative teachings of the present application will be described with reference to exemplary non-limiting embodiments.

[0028] Furthermore, in the following the solution according to the embodiments is described with respect to methods and systems for an automatic classification of thesolder printer tooling in the preparation of a printed circuit board.

[0029] Features, advantages, or alternative embodiments herein can be assigned to the other claimed objects and vice versa.

[0030] In other words, claims for methods and systems for an automatic classification of the solder printer tooling in the preparation of a printed circuit board that are intended to support the manufacturing process for a printed circuit board can be improved with features described or claimed in context of the methods and systems for determining an assembling risk for an electronic component during the preparative solder paste printing, said component being is intended to be mounted to a printed circuit board.

[0031] As used herein, the terms "PCB boards", "SMT boards" and simply "boards" denote electronic circuits composed of a plurality of electronic components being connected via their respective leads to the board by conductive wires or traces.

[0032] Typical examples of electronic components include, but are not limited by, resistors, transistors, capacitors, inductors, and diodes.

[0033] As used herein, the term "component" is meant in a broader sense to denote any element or feature or layer of the board, including for example the traces, which can be subject to the method for an automatic classification of the solder printer tooling in the preparation of a printed circuit board that is intended tosupport the manufacturing process for a printed circuit board .

[0034] As used herein, the term "for an automatic classification of the solder printer tooling in the preparation of a printed circuit board" broadly denotes for an automatic classification in the preparation of the PCB during the solder paste printing that is intended to facilitate the mounting of the components to the PCB at discrete copper pad locations.

[0035] Previous techniques did not enable an efficient method for an automatic classification of the solder printer tooling in the preparation of a printed circuit board, said classification being intended to support the manufacturing process for a printed circuit board during the solder paste printing process. The embodiments disclosed herein provide numerous technical benefits, including but not limited to the following examples .

[0036] Coming back to Figure 1, the upper, medium and lower illustrations schematically show the principle of the solder paste printing. A printed circuit board - hereinafter called PCB - comprises a plurality of copper pads 4 being connected to the lines and leads of the PCB. In order to mount a component 14 into the copper pad 4, the copper pads 4 require the deposition of a solder paste pad 12. In order to generate these solder pads 12, the PCB is covered with a stencil 2 that comprises openings 6 in alignment with the copper pads 4. Once the solder paste 8 is loaded onto the stencil 2, a squeegee 10 is wiping the solder paste 8 over the stencil 2 thereby depositing solder pads 8 in the openings 6 on the stencil 2 (see upper illustration of Figure 1) .

[0037] Once the wiping of the solder paste is completed, the stencil 2 can be removed and ideally an optimally formed solder paste pad 12 is deposited on the copper pads 4 (see illustration of Figure 1 in the middle) . Eventually, the component 14 is mounted on the PCB and its leads are located in the solder paste pads 12. While heating the PCB in an oven, the solder paste pads 12 melt and allow to connect the component 14 to the PCB.

[0038] Figure 2 schematically illustrates a side view of this wiping process. Figure 3 as mentioned already before shows a top view of the wiping process according to the prior art. As already mentioned in the introduction, the systems according to the prior art are able to measure squeegee movement from the point in z-axis where the squeegee 10 is attached to a solder paste printing machine (not shown in detail here) . When maintaining a constant force 16 to the squeegee 10 the z-axis system 18 measures if the squeegee 10 moves up or down (see Figure 3) . Variations in Z-axis movements may indicate that the support of the PCB is not good enough. The squeegee 10 is normally made of steel. Thus, it distributes the force 16 in the z-direction into a wide area of the PCB. The PCB may still bend in some areas under the squeegee 10 (in Y- direction) , but it is not detected if there is no movement in Z-axis that holds the squeegee 10. This defect is illustrated schematically in Figure 3 by an elliptic area 20 that represents an area of the PCB that has bend too much during the squeegee wiping step. Thus, in the prior art there aren't any solutions to know if the support of the PCB during the solder paste printing is good enough.

[0039] Figures 4 and 5 now schematically illustrate the method and the system for an automatic classification of the solder printer tooling in the preparation of the PCB according to the present invention. Without measuring where the PCB bends under the pressure of the squeegeelO during the solder paste wiping, it is difficult to track down when a defect in the printing process is caused by a bad board support and which areas of the PCB need more support.

[0040] In order to measure the distance between the squeegee 10 and the stencil 2, the squeegee 10 is aligned with a number of distance sensors 20a to 20g. During the wiping of the solder paste 8 with the squeegee 10, the distance sensors 20a to 20g each measure a distance profil along the direction of movement of the squeegee 10 during the wiping of the solder paste 8. Having the distance sensors 20a to 20g attached right after (downstream of) the squeegee 10 and measuring from these multiple distance sensor points along the surface of the stencil 2, it is possible to know how much the PCB and the stencil 2 bend under the pressure of the squeegee 10 during the solder paste wiping. Thus, these distance values of the plurality of distance sensors 20a to 20g are now compared to a predetermined threshold for a tolerable distance. This comparison enables now to reliably say when a non-tolerable bending happens and in which exact spot this bending happens (X, Y location) and how much bending happens (Z- direction) .

[0041] Combining this information, it is possible to say if the support of the PCB is good enough for a reliable printing of the solder paste pads 12 or which areas of the PCB are problematic and require a better support of the PCB during the solder paste printing. The system and the methodthat measure the board bending with an array of separate sensor (line array shown in Figure 5 with the distance sensors 20a to 20g) , the solder paste printing does not suffer the fact that the squeegee 10 distributes force or has only one measurement point which only measures the movement of the squeegee along the z-axis. The distance sensors 20a to 20g can be any type of sensors being able to measure the distance between the distance sensor and the surface of the stencil. Distance sensor (s) can be laser, inductive, ultrasonic or any type that can accurately measure distance from the sensor to the stencil surface.

[0042] Although an exemplary embodiment of the present disclosure has been described in detail, those skilled in the art will understand that various changes, substitutions, variations, and improvements disclosed herein may be made without departing from the spirit and scope of the disclosure in its broadest form.

[0043] None of the description in the present application should be read as implying that any particular element, step, or function is an essential element which must be included in the claim scope: the scope of patented subject matter is defined only by the allowed claims.

Claims

WHAT IS CLAIMED IS:

1. A method for an automatic classification of the solder printer tooling in the preparation of a printed circuit board, the method comprising:- providing the printed circuit board and placing a stencil layer on the printed circuit board thereby covering the printed circuit board with a stencil layer; said stencil layer comprising a variety of openings that fit to positions of the printed circuit board where a solder past pad has to be deposited;- bringing the solder past onto the stencil layer and wiping the squeegee over the stencil layer wherein the squeegee is used to bring the solder past into the openings of the stencil layer by exposing a predetermined pressure to the solder past and the stencil layer in a direction substantially perpendicular to the plane of the printed circuit board;- positioning in alignment with the squeegee an array of distance sensors in proximity to the squeegee and measuring the distance between the distance sensors and the stencil layer while wiping with the squeegee over the stencil layer; and- evaluating the measured distances with respect to a predetermined threshold for the distance between the stencil layer and the distance sensors.

2. The method of claim 1, wherein the array of distance sensors is designed as a line array being oriented parallel to the squeegee.

3. The method according to claim 1 or 2, wherein the array of distance sensors is placed upstream and / ordownstream of the squeegee as seen in the direction of the movement of the squeegee.

4. The method according to any of the preceding claims, wherein in case that the evaluation delivers some results having a distance above the threshold, the method of the solder past printing is repeated at an increased predetermined pressure and / or at an improved support of the printed circuit board for areas where the results occurred.

5. A system for an automatic classification of the solder printer tooling in the preparation of a printed circuit board, said system comprising:- a first mechanism to hold the printed circuit board and place a stencil layer on the printed circuit board thereby covering the printed circuit board with a stencil layer; wherein said stencil layer comprises a variety of openings that fit to positions of the printed circuit board where a solder past pad has to be deposited;- a second mechanism to bring the solder past onto the stencil layer and wipe the squeegee over the stencil layer wherein the squeegee is used to bring the solder past into the openings of the stencil layer by exposing a predetermined pressure to the solder past and the stencil layer in a direction substantially perpendicular to the plane of the printed circuit board;- an array of distance sensors being positioned in alignment with the squeegee in proximity to the squeegee thereby measuring the distance between the distance sensors and the stencil layer while wiping with the squeegee over the stencil layer; and- an evaluation unit to compare the measured distances to a predetermined threshold for the distance between the stencil layer and the distance sensors.

6. The data processing system of claim 5, wherein the array of distance sensors is designed as a line array being oriented parallel to the squeegee.

7. The data processing system of claim 5 or 6, wherein the array of distance sensors is placed upstream and / or downstream of the squeegee as seen in the direction of the movement of the squeegee.

8. The data processing system according to any of the preceding claims 5 to 7, wherein in case that the evaluation unit delivers some results having a distance above the threshold, the systems is enabled to repeat the solder printing method at an increased predetermined pressure and / or at an improved support of the printed circuit board for areas where the results occurred.

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