A MANUFACTURING PROCESS TO ENHANCE SURFACE MOUNT SOLDER PAD JOINT FORMATION VIA A LASER SUBTRACTIVE METHOD

Abstract

The present invention relates to the field of electronic assembly for the manufacture of electronic products such as mobile phones, where electronic components such as resistors and capacitors and microprocessors are joined to a bare circuit board to form a complete electronics device and relates to the preparation of attachment locations on a bare circuit board

Claims

1. A method of producing a printed circuit board (PCB) without adding a protective layer to the areas to be soldered, the method comprising the steps of: preparing the PCB base material; etching a circuit board connection pattern in the resistance pattern to the base material; adding a solder mask to permanently protect areas of the PCB which are not to be soldered; allowing the PCB pads which are not protected by a solder mask to passivate naturally by the formation of a copper oxide; selective laser cleaning and surface structuring of the PCB to be soldered; screen printing pads with solder paste; placing components on the circuit board; and passing the circuit board through a reflow oven.

2. A method according to claim 1, wherein the method comprises the further step of changing the surface shape of the PCB with the laser.

3. A method according to claim 1, wherein the base material is made from copper or a copper alloy.

4. A method of removing contaminants and oxidisation by means of laser irradiation from a circuit board joint land to be connected to an electronic device, wherein the method increases the surface area of the connection pad to improve the integrity of the joint land.

5. A method according to claim 4, wherein the laser may also change the surface shape of the copper board land.

6. A method according to claim 4, wherein the surface of the circuit board land to be connected to an electronic device is exposed to laser radiation by means of a series of one or more laser emissions applied to the surface of the copper land.

7. A method according to claim 6, wherein the laser emissions have sufficient energy to ablate or remove any surface contaminants and copper oxidisation present on the circuit board land, thereby removing any contaminants and oxides.

8. A method according to claim 6, wherein the laser emissions also have sufficient energy to ablate small volumes of the copper material, such that the surface structure of the material is itself altered.

9. A method according to claim 6, wherein the surface of the copper land can be altered in a manner that increases the surface area of the copper, thus improving the release of solder paste from the printing stencil during the screen printing process.

10. A method according to claim 6, wherein the laser used is of any wavelength and is applied to the circuit board land at any level of energy in a combination that is capable of coupling with the circuit board land in a manner that changes the energy of the material, thus causing areas of the surface of the copper land on the circuit board to change state and thereby be moved or removed, resulting in a change of the topography of the copper surface.

11. An apparatus for carrying out a method according to claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Non-limiting example embodiments the present invention will now be described with reference to the accompanying drawings in which:

[0045] FIG. 1 shows the basic principle of laser structuring to alter surface topography;

[0046] FIG. 2 shows the principle of how this preparation technique may be applied in a production process;

[0047] FIG. 3 shows a surface mount production line, showing a board loader, a solder paste printer, chip mounters, and reflow soldering;

[0048] FIGS. 4-8 show a method of PCB production according to the prior art; and

[0049] FIGS. 9-12 show the method according to the present invention.

DETAILED DESCRIPTION

[0050] A description of the figures is given below.

[0051] FIG. 1 shows the basic principle of laser structuring. The laser beam is incident on the conductive pad to be soldered to. Usually this is a laser pulse of energy but could be any laser beam having sufficient energy to cause the conductor material, oxides of the conductor material, contaminants of the conductor material, or any chosen conductive material protection layer, to change state with sufficient energy to lift off the surface required to be soldered.

[0052] Usually the conductor material is copper. Additionally, the surface may have discrete individual volumes of material removed by each laser pulse or pause in movement such that the surface is no longer largely flat, but instead has a three-dimensional structure created in it. This three-dimensional structure has the effect of increasing the total surface area to be soldered to, thereby increasing the print efficiency of the screen printing process and increasing the mechanical integrity of the solder joint after soldering.

[0053] FIG. 2 shows the principle of how this preparation technique may be applied in a production process. The bare circuit board may be introduced to the circuit assembly line without any special storage or preparation. Immediately prior to the screen-printing process, the laser joint preparation process is carried out using a high-speed laser scan to remove oxides, structure, prepare and decontaminate the pads to be screen printed with solder paste and subsequently used to solder surface mount components to the bare board. The laser structuring of the next bare board can be carried out in the time taken to screen print the present board under manufacture in the screen printer.

[0054] FIG. 2 shows a Laser (1), a laser beam (2), a laser steering apparatus (3), a bare circuit board (4) and a screen printer (5).

[0055] An example of a suitable laser would be a pulsed fibre laser having a wavelength of 1064 nm focused to an area of 3×10.sup.−10 M.sup.2

[0056] FIG. 3 shows a surface mount production line, showing a board loader 1, a solder paste printer 2, chip mounters 3, and reflow soldering 4. These are standard processes in a production line for circuit boards.

[0057] The proposed invention is to be inserted between the solder paste printer 2 and the first of the chip mounters 3. There is therefore a fundamental change to the entire process required. The proposed location of the invention should not be construed to be limiting in this figure, but should be used as an example location. The invention may be used with other components also, other than the ones shown in FIG. 3.

[0058] The bare circuit boards must be manufactured differently, i.e. without specialist coatings on the surface mount pads. There is the cost saving and elimination of pollution from this step, however if these boards were to be used without the apparatus according to this invention then it is important to note that the process would no longer work as the joints would fail.

[0059] If normal coated pads were used on a circuit board then the apparatus according to the invention could not be used as we might expose surfaces that also could not easily be soldered to. However the apparatus according to the invention could be switched in to a pass through mode or marking mode - where it would be used to mark (an ancillary function) but no pad preparation.

[0060] FIGS. 4-8 shows a method of PCB production according to the prior art. FIG. 4 is a simple bare copper pad shown as part of an ENIG process. The copper is shown as 1 in the figure, along with the FR4 coated with a solder mask in 2, and the connection tracks shown with a solder mask in 3. FIG. 5 shows the same pad as in FIG. 4, however the pad is coated with Ni. FIG. 6 is the same pad as in FIG. 5, however the copper and Ni pad has now been coated with Au. As shown in FIG. 7, the pad is then printed with solder paste. Finally, FIG. 8 shows the surface mount component placed on the solder paste, where it is reflowed to form a solder joint. The SMT component placed on the solder paste is shown as ‘4’ in FIG. 8.

[0061] Turning back to the invention, FIG. 9 shows a simple copper pad which is ready for the new laser process according to the invention. FIG. 10 shows the step which takes place just prior to the screen print process, where the laser is used to remove oxides, clean and structure the copper pad. This is shown as the textured surface ‘1’ in FIG. 10. FIG. 11 shows the next step, where the solder paste is printed directly on to the laser cleaned and structured copper. The screen printed solder paste can be seen as item ‘4’ in FIG. 11. FIG. 12 shows the surface mount component ‘5’ placed on the solder paste, before it is reflowed to form the solder joint.

[0062] For the present invention to function, the current end-to-end production process must change fundamentally. It is a combination of a change to the circuit board manufacturing process to omit the protective coatings from connection pads, allowing copper to oxidise naturally forming a protective layer of copper oxide, and the laser structuring which is the basis of the invention.

[0063] According to the present invention, the copper oxide can be beneficial as a surface protection that is easily removed. Alternatively a low cost pad protection layer may be used provided it may be easily removed by laser.

[0064] Having a bare copper pad also enables the invention to structure the surface to improved solder paste printing. Without the simplified PCB—this part will also not work.

[0065] The laser emission may be transmitted to the surface of the pad to be soldered in any suitable manner well known to the laser industry. For example the connection pad to be treated may be stationary while the laser energy is applied in one or more bursts using either a moving laser or by the use of steering optics that deflect the laser path in such a way as to control the path of the laser. Alternatively the laser may be stationary whilst the screen is moved under the laser emission path.

[0066] Whilst specific embodiments of the present invention have been described above, it will be appreciated that departures from the described embodiments may still fall within the scope of the present invention.