US20040231886A1 - PCB design and method for providing vented blind vias - Google Patents

Abstract

An apparatus and method for providing a vented blind via in pad of a printed circuit board (PCB). A vent in the blind via in pad to allow gases formed during reflow soldering to escape from the solder joint. In one embodiment, the vent extends from the outer edge of the pad to the blind via. In another embodiment, a method includes forming a blind via in pad having a vent.

Description

CROSS REFERENCE TO RELATED APPLICATIONS
• This application is a divisional of, and claims priority from, U.S. patent application Ser. No. 10/442,834, filed May 20, 2003, and currently pending.[0001]
BACKGROUND OF THE INVENTION
• 1. Field of the Invention[0002]
• The present invention relates generally to printed circuit boards (PCBs) and, more specifically, the present invention relates to a method an apparatus for a vented blind via in pad structure of a printed circuit board.[0003]
• 2. Background Information[0004]
• A PCB typically includes a number of insulation and metal layers selectively patterned to provide metal interconnect lines (referred to herein as “traces”), and a plurality of electronic components mounted on one or more surfaces of the PCB and functionally interconnected through the traces. The routing traces typically carry signals that are transmitted between the electronic components mounted on the PCB. Some PCBs have multiple layers of routing traces to accommodate all of the interconnections.[0005]
• Traces located within different layers are typically connected electrically by vias formed in the board. A via can be made by making a hole through some or all layers of a PCB and then coating or plating the interior hole surface with an electrically conductive material. A via that connects all layers of the PCB, including the outer layers, is called a “through via.” A via that connects one or more inner layers to an outer layer is a “blind via.”[0006]
• In order to fabricate PCBs in which electrical components are mounted in higher densities, a via in pad structure is often used. FIG. 1A is a perspective view diagram illustrating a prior art blind via in pad. A blind via[0007]12 is configured inpad10. The pad is formed on a multilayer PCB16. Blind via12 has an opening atpad10 and extends into PCB16. The walls of the blind via12 are an electrically conductive material, such as copper. A solder mask (not shown) surrounds thepad10.
• One of the conventional ways of mounting electrical components on a PCB is called surface mount technology (SMT). SMT components have terminations or leads (generally referred to as “contacts”) that are soldered directly to the surface of the PCB. The solder joint forms the physical and electrical connection between the component and the PCB. One conventional type of SMT component utilizes a ball grid array (BGA) to connect to the PCB. A BGA component has a plurality of solder balls on one surface, each of which represents an electrical contact.[0008]
• The electrical contacts of an SMT component, such as a BGA component, are coupled to corresponding metallized mounting or bonding pads (also referred to as “lands”) on the surface of the PCB. Ordinarily one pad is dedicated to one SMT electrical contact.[0009]
• Prior to mounting the SMT component on a PCB, the PCB is selectively coated with solder paste, using a mask (also referred to in the art as a stencil) that permits solder paste to coat just the pads. To mount an SMT component to a PCB, the component is carefully positioned or “registered” over the PCB so that its electrical contacts are aligned with the corresponding pads. Finally, the entire package is heated to a temperature that melts the solder paste (e.g., reflow soldering), to form a solder joint that is an electrical and physical connection.[0010]
• During the heating process, one or more gas pockets can form within the soldier joint. Such gas pockets can be formed by expanding air bubbles trapped within the solder paste and/or blind via (also referred to as “outgassing”). FIG. 1B is a cross-sectional view diagram illustrating a prior art blind via in pad structure coupled to an electrical component. Blind via[0011]12 is configured inpad10. The blind via12 extends into multilayer PCB16 and is electrically coupled to an innerconductive layer14. During the heating process to affixcontact20 of electrical component22 topad10 by way ofsolder joint24, agas pocket26 was formed by trapped air within the blind via12. A solder mask (not shown) surrounds thepad10.
• Such a gas pocket can cause the solder joint to expand to the point where it touches an adjacent solder joint and, thus, create a short circuit. In the case of BGA components, this phenomenon is referred to as “BGA bridging.” The gas pocket may also cause the solder joint to crack and break resulting in an open circuit between the electrical component and the PCB. Additionally, a solder joint with a gas pocket may experience long term reliability issues due to repeated heating and cooling during its operative life.[0012]
BRIEF DESCRIPTION OF THE DRAWINGS
• The present invention is illustrated by way of example and not limitation in the accompanying figures.[0013]
• FIGS. 1A and 1B are diagrams illustrating a prior art blind via in pad structure.[0014]
• FIG. 2A is a perspective view diagram illustrating one embodiment of a vented blind via in pad in accordance with the teachings of the present invention.[0015]
• FIG. 2B is a cross-sectional view diagram illustrating one embodiment of a vented blind via in pad in accordance with the teachings of the present invention.[0016]
• FIG. 3A is a perspective view diagram illustrating one embodiment of a vented blind via in pad in accordance with the teachings of the present invention.[0017]
• FIG. 3B is a cross-sectional view diagram illustrating one embodiment of a vented blind via in pad in accordance with the teachings of the present invention.[0018]
• FIGS. 4A-4H are cross-sectional diagrams illustrating one embodiment of constructing a vented blind via in pad structure in accordance with the teachings of the present invention.[0019]
• FIGS. 5A-5I are top view diagrams illustrating embodiments of vented blind via in pad structures in accordance with the teachings of the present invention.[0020]
• FIGS. 6A and 6B are perspective view diagrams illustrating embodiments of vented blind via in pad structures in accordance with the teachings of the present invention.[0021]
• FIG. 7 is a diagram illustrating a computer system for implementing an embodiment of the present invention.[0022]
DETAILED DESCRIPTION
• Methods and apparatuses for a providing a vented blind via in pad structure for a PCB are disclosed. In the following description numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, to one having ordinary skill in the art that the specific detail need not be employed to practice the present invention. In other instances, well-known materials or methods have not been described in detail in order to avoid obscuring the present invention.[0023]
• Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments. In addition, it is appreciated that the figures provided herewith are for explanation purposes to persons ordinarily skilled in the art and that the drawings are not necessarily drawn to scale.[0024]
• FIGS. 2A and 2B illustrate one embodiment of a blind via in pad having a vent. Referring to FIGS. 2A and 2B, a[0025]pad202 is positioned on a PCB201. The PCB201 is a multilayer PCB having a plurality of substrate layers and one or more inner conductive layers.
• A blind via[0026]206 is positioned inpad202. The blind via206 includes walls of conductive material. The blind via206 is electrically connected to an inner conductive layer210, such as, for example, a routing trace. The combination of blind via206 inpad202 is typically called a via in pad (or “microvia”), and is well known in the art.
• Surrounding the[0027]pad202 is asolder mask214. A contact222 of electrical component224 is positioned overpad202. In one embodiment, contact222 includes a solder ball of a BGA. The electrical component224 may be, but is not limited to, a microprocessor, a microcontroller, an application specific integrated circuit (ASIC), an amplifier, a filter, a clock circuit, or the like. The conductive material ofpad202, blind via206, contact222, and inner conductive layer210 may be, but is not limited to, copper, aluminum, gold, or the like. While FIG. 2B shows an electrical component attached to one outer layer of PCB201, in another embodiment, electrical components can be attached to additional outer layers of PCB201.
• It will be appreciated that blind via[0028]206 extends partially into PCB201. Generally, a blind via is a via that electrically couples one or more inner layers to an outer layer of a multilayer PCB. Note that blind via206 is not a through-hole via utilizing a via cap or via plug. A via that connects all layers of a PCB, including the outer layers, is called a “through via” or a “plated through hole via” (PTH).
• A[0029]vent204 is positioned inpad202. In one embodiment, thevent204 can have approximatelyparallel walls207 to form a groove shape. In one embodiment, thevent204 is sized to permit nearly all of a gas208 to escape to the atmosphere. In another embodiment, the approximatelyparallel vent walls207 are sized to a width (w) to meter escape of the gas208 such that some gas208 remains in the blind via206. In one embodiment, the depth (d) ofvent204 reaches the outer substrate layer of PCB201; while in another embodiment, the depth ofvent204 does not reach the outer substrate layer of PCB201 (discussed further in conjunction with FIGS. 6A and 6B.) In the embodiment shown in FIGS. 2A and 2B, thevent204 runs from the outer edge ofpad202 to blind via206.
• A gap placed at the outer edge of the[0030]pad202 allows clearance for gas208 to escape thevent204. In one embodiment shown in FIGS. 2A and 2B, surrounding thepad202 is thesolder mask214 where a gap (g) is between thesolder mask214 and the outer edge ofpad202. Solder will not adhere to the outer layer of substrate of PCB201 or to soldermask214. As a result, the venting path (vent204 plus gap g betweenpad202 and solder mask214) will not become filled or clogged with solder during a solder reflow process. In an alternative embodiment, a gap is placed at the outlet of thevent204 between thesolder mask214 and the outer edge ofpad202. Such a gap is does not surround theentire pad202. For example, the gap may be approximately the same width as the width (w) of thevent204.
• In a typical solder reflow process, solder paste is applied to pad[0031]202. The entire assembly is heated to a temperature that melts the solder paste (and solder balls of a BGA) so that thepad202 and contact222 form a physical and electrical connection via a solder joint226.
• During solder reflow, gas[0032]208 can form in the solder joint226 and/or blind via206. The expanding gas208 will seek a path of less resistance and travel into thevent204. By incorporating avent204 within thepad202, a pathway is established that allows the expanding gas208 to escape the solder joint226 while the solder (and solder ball) is in the molten state. All or a portion of the expanding gas208 that otherwise would contribute to expanding the solder joint226 may now escape to atmosphere throughvent204.
• FIGS. 3A and 3B illustrate another embodiment of a vented blind via in pad. Referring to FIGS. 3A and 3B, a[0033]pad302 is positioned on amultilayer PCB301. A blind via306 is configured inpad302. The blind via306 includes walls of conductive material and is electrically connected to an innerconductive layer316. Surrounding thepad302 is asolder mask314. Acontact322 of anelectrical component324 is positioned overpad302.
• Within the[0034]pad302, avent304 does not run continuously from the outer edge of thepad302 to the blind via306. In this embodiment, thevent304 can be roughly shaped like a wedge having an apex307. With this wedge shapedvent304, a solder joint326 can begin to expand due togas308 building up within the solder joint326 with no venting. As thegas308 builds up in thesolder joint326 and thesolder joint326 continues to expand, a void310 crosses over and contacts the wedge shapedvent304 at thevent apex307. Once contact by the void310 with thevent304 occurs,gas308 can begin to release or vent to atmosphere (or a lower pressure space). With sufficient pressure released, the void310 can shrink until thevoid310 is no longer positioned over thevent304 or thevoid310 is positioned over a smaller cross-section of thevent304. With venting stopped or slowed, pressure could again build up, again increasing the size of the solder joint326 to a point where thevent304 would once more begin to release pressure. This “burping” action could repeat as long as outgassing with thesolder joint326 was being generated with sufficient pressure.
• In another embodiment, the release of gas occurs without the burping action. The solder joint[0035]326 could expand to a point on thevent304 wheregas308 builds up in solder joint326 verses the rate ofgas308 escaping fromvent304 is such that that the rate of expansion of the solder joint326 would slow or cease. Once the heating process was complete and the assembly began to cool, the outgassing from solder joint326 would ultimately cease.
• The embodiment in FIGS. 3A and 3B show a gap (g) surrounding the[0036]pad302 between thesolder mask314 and the outer edge ofpad302 to create a clearance for gas to escape thevent304. In another embodiment, the gap (g) does not surround theentire pad302. For example, the gap may be approximately the same width as the outlet ofvent304 at the outer edge ofpad302.
• FIGS. 4A-4H illustrate one embodiment of a method to fabricate a vent within a blind via in pad on a multilayer PCB. FIG. 4A is an illustration of an outer[0037]conductive layer402 that has been laminated tosubstrate404.Substrate404 is laminated tosubstrate406. Innerconductive layer408 includes a circuitry trace pattern. The material ofconductive layers402 and408 may include, but is not limited to, copper, aluminum, gold, or the like. It will be appreciated that the PCB of FIGS. 4A-4H is not limited to the embodiments as shown and may contain additional layers. A blind via may electrically couple one or more of these additional layers.
• FIG. 4B is an illustration of the results of creating a[0038]window410 in the outerconductive layer402. In one embodiment, thewindow410 may be created through various photolithography processes that are well known in the art. In another embodiment, thewindow410 can be created through a laser skive process of outerconductive layer402 that is well known in the art. During laser skiving of outerconductive layer402, a vent416 (shown in FIGS. 4E-4H) can also be created at this time through laser skiving (described further below).
• FIG. 4C is an illustration of the results of laser drilling a blind via[0039]412. The laser ablates thesubstrate404 to expose the innerconductive layer408. FIG. 4D is an illustration of the results of a plating process. The walls of blind via412 are plated with conductive material to allow layer-to-layer electrical conductivity between the layers of the PCB.
• FIG. 4E is an illustration of the results of trace formation in the outer[0040]conductive layer402. In one embodiment, apad414, avent416 and atrace418 is formed through photolithography. If thevent416 is formed here through photolithography, then the depth of the vent is to thesubstrate404.
• If the[0041]vent416 was formed by laser skiving as described in conjunction with FIG. 4B, then the vent is not formed here by photolithography. The vent formed by laser skiving is positioned in the outerconductive layer402 such that the photolithography process creates thepad414 with the knowledge of the placement of thevent416 by laser skiving. In one embodiment, a vent formed by laser skiving will not pass entirely through the depth of thepad414.
• FIG. 4F is an illustration of the results of adding a[0042]solder mask420 to the remaining outerconductive layer402 andsubstrate404. Typically, thesolder mask420 covers all exposed features except for the pad414 (includingvent416 of pad414). FIG. 4G is an illustration of the results of applyingsolder422 to the exposedpad414.
• FIG. 4H is an illustration of the results of reflow soldering to couple a[0043]contact424 of anelectrical component426 to pad414.Electrical component426 was positioned so thatcontact424 was aligned withpad414. Thecontact424 was affixed to pad414 through reflow soldering, which is well known in the art. During the reflow soldering, agas430 that developed in the solder joint428 or blind via412 could escape to atmosphere throughvent416.
• FIGS. 5A-5I are top view illustrations of alternative embodiments of vent design. FIG. 5A is a top view diagram illustrating one embodiment of the present invention. A[0044]pad510 is formed on asubstrate516. A blind via512 is configured inpad510.Vent518 extends from the outer edge ofpad510 towards the blind via512, but does not reaching the opening of blind via512. Thevent518 has generally a groove shape.
• FIGS. 5B and 5C are top view diagrams illustrating embodiments of the present invention. In FIGS. 5B and 5C, a[0045]pad510 is formed onsubstrate516. A blind via512 is configured inpad510.Vents518 and519 are formed inpad510.Vents518 and519 extend from the outer edge ofpad510 to blind via512. It is understood that the embodiment shown in FIG. 5B is not limited to the angle betweenvents518 and519. FIG. 5C showsvents518 and519 such that vents518 and519 are collinear along a diameter passing throughpad510.
• FIGS. 5D and 5E are top view diagrams illustrating embodiments of the present invention. In FIGS. 5D and 5E, a[0046]pad510 is formed onsubstrate516. A blind via512 is configured inpad510. In the embodiment shown in FIG. 5D, avent518 is positioned inpad510 extending from the outer edge ofpad510 to the blind via512. Thevent518 has generally a wedge shape. In the embodiment of FIG. 5E, thevent518 has a half-wedge shape with the apex of the wedge removed. In the embodiment of FIG. 5E, thevent518 does not reach blind via512 from the outer edge ofpad510.
• FIGS. 5F and 5G are top view diagrams illustrating embodiments of the present invention. In FIGS. 5F and 5G, a[0047]pad510 is formed onsubstrate516. A blind via512 is configured inpad510. Avent518 inpad510 extends from the outer edge ofpad510 to blind via512. In the embodiment of FIG. 5F, the path ofvent518 has generally a round shape. In another embodiment (not shown), the path ofvent518 in FIG. 5F has one or more angles to form a zigzag shape. In the embodiment of FIG. 5G, the path ofvent518 has generally a spiral shape.
• FIGS. 5H and 5I are top view diagrams illustrating embodiments of the present invention. In FIGS. 5H and 5I, a[0048]pad510 is formed onsubstrate516. A blind via512 is configured inpad510. Avent518 is inpad510 extending from a first point on the outer edge ofpad510 to a second point on the outer edge ofpad510, and not intersecting the blind via512. In the embodiment of FIG. 5H, the path ofvent518 has generally a straight path. In the embodiment of FIG. 5I, the path of vent118 has generally a curved path. In another embodiment (not shown), the path ofvent518 in FIG. 5I has one or more angles to form a zigzag shape.
• FIGS. 6A and 6B are perspective view illustrations of alternative embodiments of a vent design. In FIGS. 6A and 6B, a[0049]pad610 is formed on asubstrate616. A blind via612 is configured inpad610. Avent618 is inpad610 extending from the outer edge ofpad610 to the blind via612.Pad610 has afirst face610A and asecond face610B, wheresecond face610B is the face ofpad610 coupled tosubstrate616. In the embodiment of FIG. 6A, thevent618 passes fromfirst face610A intopad610, but does not reachsecond face610B. The depth ofvent618 does not pass all the way throughpad610. In the embodiment of FIG. 6B, thevent618 passes fromfirst face610A tosecond face610B. In this embodiment, the depth ofvent618 passes all the way throughpad610 to exposesubstrate616.
• FIG. 7 is an illustration of one embodiment of an[0050]example computer system740 that can be implemented in conjunction with the presently claimed invention.Computer system740 includes aprocessor750, amemory755, an input/output controller760.Bus765 is coupled to each ofprocessor750,memory755 and input/output controller760.Processor750 may be a conventional microprocessor including, but not limited to, an Intel Corporation x86, Pentium, or Itanium family microprocessor, a Motorola family microprocessor, or the like.Memory755 includes Dynamic Random Access Memory (DRAM), Static Random Access Memory (SRAM), Synchronized Dynamic Random Access Memory (SDRAM), Rambus Dynamic Random Access Memory (RDRAM), or the like. An input/output device (not shown) coupled to input/output controller760 may be a keyboard, disk drive, printer, scanner and other input and output devices, including a mouse, trackball, trackpad, joystick, or other pointing device. Incomputer system740, any one of theprocessor750,memory755, and input/output controller760 are coupled to a PCB having at least one vented blind via in pad as described herein.
• In the foregoing detailed description, the method and apparatus of the present invention have been described with reference to specific exemplary embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the present invention. The present specification and figures are accordingly to be regarded as illustrative rather than restrictive.[0051]

Claims (24)

What is claimed is:

1. An apparatus, comprising:

a substrate; and

a via in pad positioned in the substrate, the via in pad including a pad having a vent to provide an escape path for gas from the via in pad, and a blind via disposed in the pad.

2. The apparatus ofclaim 1, wherein the vent to include a first opening at the outer edge of the pad.

3. The apparatus ofclaim 2, wherein the vent to include a second opening at the blind via.

4. The apparatus ofclaim 2, wherein the vent is a wedge.

5. The apparatus ofclaim 3, wherein the vent is a groove.

6. The apparatus ofclaim 1, wherein the pad has a plurality of vents.

7. The apparatus ofclaim 2, wherein the vent to include a second opening at the outer edge of the pad, the vent to not intersect the blind via.

8. The apparatus ofclaim 1, wherein the vent has a depth that exposes the substrate.

9. The apparatus ofclaim 1, wherein the blind via to electrically couple the pad to a trace on an inner substrate layer, wherein the substrate is an outer substrate layer of a multilayer printed circuit board having one or more inner substrate layers.

10. An apparatus, comprising:

a first substrate to form an outer layer of a printed circuit board (PCB);

a second substrate to form an inner layer of the PCB, the second substrate having a trace; and

a via in pad positioned in the first substrate electrically coupled to the trace, the via in pad including a pad having a vent to provide an escape path for gas from the via in pad, and a via, disposed in the pad, having a first end at the pad and a second end at the trace.

11. The apparatus ofclaim 10, wherein the vent to include a first opening at an outer edge of the pad.

12. The apparatus ofclaim 11, wherein the vent to include a second opening at the via.

13. The apparatus ofclaim 12, wherein the vent is a groove.

14. An apparatus, comprising:

a substrate; and

one or more via in pads in the substrate, the one or more via in pads each to include a pad and a blind via positioned in the pad, wherein the pad of at least one of the one or more via in pads having a vent to provide an escape path for gas from a gas pocket in the at least one of the one or more via in pads.

15. The apparatus ofclaim 14, wherein the vent to include a first opening at the outer edge of the pad.

16. The apparatus ofclaim 14, wherein the vent to include a second opening at the via.

17. The apparatus ofclaim 16, wherein the vent is a groove.

18. An apparatus, comprising:

a substrate;

a via in pad positioned in the substrate, the via in pad to include a pad and a blind via disposed in the pad; and

means for allowing a gas to escape from a solder joint coupled to the pad.

19. The apparatus ofclaim 18, wherein the means for allowing a gas to escape includes the pad having a vent, the vent to include a first opening at the outer edge of the pad.

20. The apparatus ofclaim 19, wherein the vent to include a second opening at the via.

21. The apparatus ofclaim 20, wherein the vent is a groove.

22. A system, comprising:

a printed circuit board (PCB), comprising:

a substrate; and

a via in pad positioned in the substrate, the via in pad including:

a pad having a vent to provide an escape path for gas from the via in pad, the vent to include a first opening at the outer edge of the pad; and

a blind via disposed in the pad; and

a memory device coupled to the PCB at the via in pad.

23. The system ofclaim 22, wherein the vent to include a second opening at the blind via.

24. The system ofclaim 23, wherein the vent is a groove.

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