WO2005118212A1

WO2005118212A1 - Solder mask enhancement and method of inspecting printed circuit board assemblies

Info

Publication number: WO2005118212A1
Application number: PCT/CA2005/000828
Authority: WO
Inventors: Michael Feld; Mohamed Yamlahi; Peter Conlon
Original assignee: Original Solutions Inc
Current assignee: Original Solutions Inc
Priority date: 2004-06-01
Filing date: 2005-05-31
Publication date: 2005-12-15
Anticipated expiration: 2006-12-01

Abstract

A solder mask for a printed circuit board (PCB) assembly exhibits florescent characteristics when the PCB assembly is introduced under invisible light. The solder mask shows a light colour which provides a high contrast with components that show a dark colour to facilitate inspection of the PCB assembly.

Description

Solder Mask Enhancement and Method of Inspecting Printed Circuit Board Assemblies

This invention relates to a solder mask and method of inspecting printed circuit board (PCB) assemblies, and more particularly to a method of detecting assembly defects.

BACKGROUND OF THE INVENTION Printed circuit board (PCB) assemblies are increasing in complexity and speed of operation while the components are shrinking in size. Manufacturers must inspect PCB assemblies to ensure that no defects persist prior to shipping products to their customers. A PCB assembly comprises a single layer PCB or multi-layer PCB and components mounted on the PCB. A typical multi-layer PCB has a core layer called "laminate", one or more dielectric layers and conductive layers alternately deposited on the both sides of the laminate, and top and bottom conductive layers. The conductive layers are typically made of copper etched into desired patterns. A solder mask is generally used to protect the copper on the top and bottom layers. There are two means of applying the solder mask: one is a spray on process, and the other is a film lamination by which a film of solder mask is adheres to the surface of the board. The solder mask is then etched, using a photo imaging process, in order to expose portions of the copper layer to provide areas for solder pads. Components are then mounted on the PCB using the solder pads to form a PCB assembly. Inspection of PCB assemblies has been carried out by human inspectors. Inspectors manually check for and identify defects using microscopes or magnifying glasses. Such manual inspection is error prone and slow. Automated Inspection systems are available, which use optical characteristic recognition, using visible light sources, to store and to compare images to determine the differences thereby showing the probable defects. While automated inspection is faster, it requires the placement of large, complex automated inspection machines on the production line. Also, existing inspection machines require constant support and are often error prone. Both manual inspection and automated inspection are error prone due that dark coloured components are mounted on dark coloured solder masks. United States Patent No. 6, 272,018 B1 issued to Feld et al. on August 7,

2001 and United States Patent No. 6, 480,394 B1 issued to Feld et al. on November 12, 2002_.pjpjDpse embedding UV markers Lnt _a_PCB_to Lrnproye the insp_ectio_n_ process. The PCB of Feld et al uses an epoxy resin dielectric layer that can be energized by and reflect a UV light. A copper layer of a desired pattern is etched onto the epoxy resin layer and further a solder mask is laid over the copper layer and the epoxy resin layer. UV markers are created by etching the solder mask in areas where there is no copper layer underneath the solder mask to expose the epoxy resin layer. When the PCB is placed into a UV environment, the exposedareas of the epoxy resin layer are energized by the UV light through the etchings in the circuit board and reflect the UV light. These reflective areas are then used to isolate missing, bridged or shifted components in an assembled PCB. While the embedded UV markers improve the inspection process, this approach may not be suitable in some situations. UV markers are seen only in the areas of the etchings. In order to provide UV markers, the solder mask design needs to be modified to expose the epoxy resin layer, which is not desirable for some manufactures. It is therefore desirable to provide an inspection process without involving modification of the product design. SUMMARY OF THE INVENTION It is an object of the invention to provide a novel solder mask and method of detecting defects in PCBAs that obviates or mitigates at least one of the disadvantages of existing systems. The present invention uses a solder mask that exhibits florescent characteristics under invisible light. In accordance with an aspect of the invention, there is provided a solder mask material comprising a solder mask base material for forming a solder mask, and a florescent agent added to the solder mask base material to provide florescent characteristics to the solder mask. In accordance with another aspect of the invention, there is provided a solder mask comprising a solder mask body formed of a solder mask base material and a florescent agent added to the solder mask base material, and a surface that exhibits florescent characteristics when the solder mask is introduced under invisible light. In accoπdarLce

printed circuit board (PCB) comprising a dielectric layer and a conductive layer provided on the dielectric layer. The conductive layer has a predetermined pattern. The PCB further comprises a solder mask provided on the conductive layer and the dielectric layer where the conductive layer is absent. The solder mask exhibits florescent characteristics when the solder mask is introduced under invisible light. In accordance with another aspect of the invention, there is provided a method of inspecting a printed circuit board (PCB) assembly. The method comprises the steps of obtaining information of predetermined inspection areas on a PCB assembly; introducing the PCB assembly under invisible light; and detecting reflection of the invisible light at the inspection areas on the PCB assembly. In accordance with another aspect of the invention, the method comprises the steps of introducing a bare PCB under invisible light; detecting a first pattern of reflection of the invisible light by the bare PCB; introducing a PCB assembly under invisible light, the PCB assembly comprises the PCB with components mounted thereon; detecting a second pattern of reflection of the invisible light by the PCB assembly; and comparing the first pattern and second pattern of the reflection of the invisible light. Other aspects and features of the present invention will be readily apparent to those skilled in the art from a review of the following detailed description of preferred embodiments in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be further understood from the following description with reference to the drawings in which: Figure 1A is a schematic top plan view of an example of a PCB in accordance with an embodiment of the present invention; Figure 1B is a cross sectional view of the example; Figure 1 C is a cross sectional view of the example; Figure 2A is a schematic top plan view of the example under UV light; Figure 2B is a cross sectional view of the example shown in Figure 2A; and Fjgure_2C _.is a cros _.sectional view of the example shown jn Figure_.2A;

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A solder mask in accordance with an embodiment of the present invention is formed of a solder mask material that includes a mask base material and a florescent agent. The solder mask material may further include a pigment and/or other materials suitable for formation of a solder mask. The florescent agent is added to the mask base material such that the resultant solder mask exhibits florescent characteristics when the solder mask is introduced into an invisible light environment, such as under ultraviolet (UV) light or infrared (IR) light. The surface of the solder mask rejects and reflects the wavelength of the UV or IR light, and displays a visible and recognizable surface. When the solder mask is under normal ambient lighting conditions, the solder mask has no colour when no pigment is used. When a pigment is used, the solder mask shows the colour of the pigment. A pigment is typically used in a solder mask in order to protect the materials underneath the solder mask from degradation by ambient lights. To this end, pigments used for solder masks typically provide pigmentation of dark colour, such as green colour. To form a PCB assembly, a PCB is mounted with components, such as integrated circuits (ICs) and surface mount technology (SMT) components. Components come in different colors but under UV light most of these show up black. Under ambient light however this tremendous array of components becomes quite difficult to see against the green back ground and creates difficulties in determining the correct assembly of the components on the PCB. The solder mask with a florescent agent allows that, when the PCB assembly is brought under the influence of the UV or IR light, the surface of the solder mask exhibits fluorescent characteristics and shows white or a light colour. This provides a light colour background while the components assembled on the PCB show black under the influence of the UV or IR light. Thus, the contrast between the components and the area that has no components is very high, which facilitate efficient and_. accurate Lnsp_ectjo_n_of the PCB assembly. The solder mask that exhibits florescent characteristics may be used to cover the entire surface of the PCB and then etched areas for solder pads in accordance with a desired PCB design. There is no need to modify the PCB design to provide special markers or signatures. Thus, the solder mask is applicable to a broader base of products. An example of a PCB 10 that exhibits florescent characteristics is described referring to Figures 1 and 2. Figures 1 A, 1B and 1 C show a top plan view and cross sectional views of the

PCB 10 under normal ambient lighting conditions. The PCB 10 has a solder mask 20 that is provided on the top of a conductive layer of a preformed pattern. In the cross sectional views of Figures 1B and 1C, solder pads 30 and 32 are schematically illustrated to represent a conductive layer and solder pads formed on the conductive layer through holes in the solder mask 20. The next layer is a dielectric layer 40, e.g., a laminate or a prepreg layer. A layer 50 is shown to schematically represent further layers if any. The solder mask 20 is formed of a solder mask material as described above. In this example, the solder mask 20 contains a green pigment like typical existing solder masks. As shown in Figure 1A, under normal ambient lighting conditions, visual elements of the PCB 10 from the top are the green solder mask 20, a series of solder pads 30 for mounting an IC and pairs of solder pads 32 for mounting SMT components. The solder mask 20 is seen green due to the pigment. Solder pads 30 and 32 are made of copper. As shown in Figures 2A-2C, when the PCB 10 is introduced under the influence of the UV light, the surface of the solder mask 20 reflects the UV light and becomes white. Thus, the green solder mask 10 is capable of acting like a white mask under the UV light. Examples of florescent agents that may be suitably used for solder mask jnclude_^ but_n t limited_.to^a_hjgh_moJecular weight optical briαjitenerpf the thiophenediyl benzoxazole class, such as Uvitex (trademark of Ciba) OB, that is suitable for the optical brightening of polymers. Florescent agents are typically mixed in a suitable solder mask base material as fillers. For a typical PCB, about 5,000 to 50,000 parts per million by weight of a florescent agent are mixed in a solder mask abase material. For a different PCB, more or less florescent agent may be used. In an example, in an FR 4 material with light green solder mask a filler like Uvitex, OB is used as a florescent agent. 5,000 parts/million of the florescent agent was mixed into a solder mask base material with a green pigment. The mixture was applied on a copper layer to form a solder mask. The solder mask was then etched to provide holes to expose the copper layer, and solder pads are provided in the holes to form a PCB. The PCB showed green under normal ambient lighting conditions. When it is introduced in a UV environment, the surface of the solder mask turned to white. Solder masks having florescent characteristics are suitably used for automated inspection. An inspection system is provided with information regarding predetermined inspection areas on a PCB assembly where the inspection should be performed. When a PCB assembly is introduced to the inspection system, the system illuminates the PCB assembly with a UV light, and inspects the pre-selected inspection areas on the PCB assembly. The inspection system determines existence or absence of defects depending on the UV light reflection at the inspection areas. The inspection areas may be determined such that correct mounting of components on a PCB will totally cover the inspection areas. In that case, if reflection of the UV light at an inspection area is detected, the inspection system determines that there is a defect. By selecting multiple inspection areas for a component or selecting specific shapes of inspection areas, the system can determine the nature of the defect, such as missing components, misaligned components, bridging of solder pads and the like. lnihe above. erabOdirjrιen.t,_the. automated inspection.was performed. using the information regarding predetermined inspection areas on the PCB assembly. In a different embodiment, automated inspection may be performed by detecting patterns of UV reflection by the PCB surface prior to mounting components and after mounting components, and comparing the patterns to determine any assembly defects. In the above embodiment, the automated inspection uses UV light. In a different embodiment, automated inspection may use IR light. While particular embodiments of the present invention have been shown and described, changes and modifications may be made to such embodiments without departing from the true scope of the invention.

Claims

What is claimed is:

1. A solder mask material comprising: a solder mask base material for forming a solder mask; and a florescent agent added to the solder mask base material to provide florescent characteristics to the solder mask.

2. A solder mask comprising: a solder mask body formed of a solder mask base material and a florescent agent added to the solder mask base material; and a surface that exhibits florescent characteristics when the solder mask is introduced under invisible light.

3. The solder mask as claimed in claim 2, wherein the solder mask further comprises a pigment such that the surface shows a colour of the pigment under ambient lighting, and the surface shows white under invisible light.

4. The solder mask as claimed in claim 2, wherein the invisible light is UV light or IR light.

5. A printed circuit board (PCB) comprising: a dielectric layer; a conductive layer provided on the dielectric layer, the conductive layer having a predetermined pattern; and a solder mask provided on the conductive layer and the dielectric layer where the conductive layer is absent, the solder mask exhibiting florescent characteristics when the solder mask is introduced under invisible light.

6. A printed circuit board (PCB) assembly comprising: a PCB as claimed in claim 5, and components assembled on the PCB.

7. A method of inspecting a printed circuit board (PCB) assembly, the method comprising the steps of: obtaining information of predetermined inspection areas on a PCB assembly; introducing the PCB assembly under invisible light; and detecting reflection of the invisible light at the inspection areas on the PCB assembly.

8. The method as claimed in claim 7, further comprising the step of: determining a defect based on existence or absence of reflection of the invisible light at an inspection area.

9. The method as claimed in claim 7, further comprising the step of: determining a defect based on the shape of reflection of the invisible light at an inspection area.

10. A method of inspecting a printed circuit board (PCB) assembly, the method comprising the steps of: introducing a bare PCB under invisible light; detecting a first pattern of reflection of the invisible light by the bare PCB; introducing a PCB assembly under invisible light, the PCB assembly comprises the PCB with components mounted thereon; detecting a second pattern of reflection of the invisible light by the PCB assembly; and comparing the first pattern and second pattern of the reflection of the invisible light.