Blind via printed circuit board fabrication supporting press fit connectors

An information handling system circuit board interfaces storage device surface connectors and storage device controllers disposed on opposing sides by coupling a first circuit board portion having a controller press in connector to a second circuit board portion having plural surface connectors. The first and second circuit board portions couple to each other with an adhesive activated by curing. Resistant ink is printed over openings of the first circuit board portion where adhesive is applied in order to prevent the adhesive from flowing into the openings at or before the curing of the adhesive.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates in general to the field of information handling system circuit boards, and more particularly to blind via printed circuit board fabrication supporting press fit connectors.

2. Description of the Related Art

Server information handling systems often include a number of storage devices to store information, such as hard disk drives. For example, a server rack supports multiple server information handling systems disposed in multiple slots and interfaced with each other by a switch. Each of the server information handling systems in the rack has a printed circuit board (PCB) that interfaces components of the server information handling system, such as one more processors each disposed in a socket, random access memory, a chipset with BIOS and similar firmware, one or more network interface cards (NIC), a baseboard management controller (BMC) and one or more hard disk drives. By directly connecting hard disk drives to a server information handling system, access is provided to information stored at local hard disk drives in a rapid manner as compared with retrieval of information through the switch. For example, Serial attached SCSI (SAS) hard disk drive connectors at a PCB provide information transfer rates of 6 Gb/s with planned transfer rates of 12 Gb/s.

Server information handling systems that include storage devices typically have a mid-plane circuit board assembly for connectivity in an enclosure, such as a rack slot. Generally, when a server information handling system's primary function is service of storage requests, the hard disk drives or other persistent storage devices are placed at the front portion of the chassis that holds the server information handling system in the rack to provide ease of access in the event that a storage device needs replacement. One typical goal is for the front portion of the server information handling system mid-plane circuit board to have 100% utilization for supporting storage devices with a maximum storage device connector density and the rear portion of the mid-plane circuit board having an interface to the storage controllers. For instance, SAS surface mounted connectors extend outwards from the upper surface of the mid-plane circuit board to interface with storage devices and press fit connectors extend outwards from the lower surface of the mid-plane circuit board to interface with storage controllers. Press fit connectors provide high speed backplane connections with very high pin counts to interface with multiple storage devices through circuit board connections with multiple surface mounted connectors.

One difficulty with upper and lower surface connector configurations is that press fit connectors are inserted into relatively large through hole vias in the mid-plane circuit board. Due to the large pin count of the press fit connectors, large sections of the mid-plane circuit board have through holes formed to accept connectors of the connector pin fields. SAS surface mount connectors located on the upper surface often coincide with the location of press fit connectors on the bottom surface. In order to co-locate storage device and press fit connectors, two mid-plane circuit boards are assembled in a sandwich configuration so that one mid-plane circuit board accepts the press fit connectors and the other mid-plane circuit board accepts the storage device surface mounted connectors. The two separate mid-plane circuit boards are then connected to each other with another set of press fit backplane connectors that avoid co-location with the first set of press fit connectors and the storage device surface mounted connectors. However, including a second set of press fit connectors to interface the two mid-plane circuit boards increases the cost of the system and introduces signal integrity issues that further increase manufacture costs. For example, back-drilling of press fit connectors vias eliminates stubs from assembly but increase manufacture steps and cost.

SUMMARY OF THE INVENTION

Therefore a need has arisen for a system and method which supports press fit connectors and surface mounted connectors assembled to opposing sides of a circuit board.

In accordance with the present invention, a system and method are provided which substantially reduce the disadvantages and problems associated with previous methods and systems for colocation of press fit connectors and surface mounted connectors at opposing sides of a circuit board. A counter bore opening in the bottom of a blind via that passes plating fluid through during manufacture of a first circuit board portion is covered by a blocking material to prevent fluids from entering the blind via through the opening during lamination of a second circuit board portion to the first circuit board portion over the opening.

More specifically, an information handling system motherboard with a storage device controller interfaces to a storage device circuit board through a press in connector coupled to the storage device circuit board. The press in connector communicates signals from the storage device controller through the circuit board to a surface connector that couples with a storage device. In order to provide freedom of placement of the surface connector and press in connector on opposing sides of the storage device circuit board, such as colocation on opposite sides, the storage device circuit board is assembled from first and second circuit board portions. The first portion has blind vias formed with a small diameter opening in the bottom of each blind via that allows plating fluid to pass through. After plating of the blind vias with conductive material, the openings in the bottom of the blind vias are blocked with a blocking material, such as by printing ink across the openings. The blocking material prevents adhesive used to couple the first and second portions together from entering into the blind vias during lamination of the first and second portions to each other. The ink printed on the first circuit board portion to block the openings has a viscosity that covers the openings but does not add significantly to the thickness of the circuit board portion. After lamination of the first and second portions to each other, interconnects are formed to interface wirelines disposed in the first and second portions so that the press in connector interfaces a storage device controller with plural storage devices coupled to plural storage device surface connectors.

The present invention provides a number of important technical advantages. One example of an important technical advantage is that press fit connectors and surface mounted connectors are collocated on opposing sides of a circuit board to provide efficient use of space within an information handling system enclosure for supporting storage devices. Integrating press fit connectors and surface mounted connectors to a common circuit board for interfacing storage devices with storage controllers reduces signal integrity issues to support high speed information communication, such as 12 Gb/s SAS interfaces. Laminating two circuit boards together provides reduced cost of assembly of an information handling system since fewer parts are involved in the assembly, better signal integrity since signals are transferred through circuit board without intermediary connectors and reduced circuit board costs with simplified manufacture.

DETAILED DESCRIPTION

Referring now toFIG. 1, a server information handling system10is depicted having a press in connector12that interfaces plural storage devices14to a storage controller16. Information handling system10processes information with a CPU18that executes instructions and random access memory (RAM)20that stores the instructions. In the example embodiment, information handling system10provides a storage server function by interfacing a motherboard22supporting storage device controller16with a storage device circuit board24supporting plural storage devices14, such as hard disk drives, through press in connector12. For example, a network interface card (NIC)26receives requests to access storage devices14from a client information handling system through a network. Firmware running on a chipset28manages communication of the requests through motherboard22for action by CPU18. Access to storage devices14is handled by storage device controller16with communications sent through press in connector12, storage device circuit board24, a storage device surface connector30and a storage device14. Storage device controller16controls access to plural storage devices14. To provide communication between storage device controller16and plural storage devices14, press in connector12has a high density of pins pressed into circuit board24.

Storage device circuit board24has a relatively great thickness due the layer count needed to route signals between multiple storage devices14and storage device controller16, and also to provide rigidity for mechanical support of storage devices14and press in connector12. Storage devices14interface to the upper surface of circuit board24by coupling to surface connectors30, such as SAS surface connectors. Press in connector12extends from the lower surface opposite surface connectors30. Press in connector12for providing high speed communication with storage device controller16is a press fit type of connector that couples into hole vias for pin insertion. Surface connectors30also sometimes use hole mounted vias for insertion into circuit board24. Since one goal for server information handling system10is a minimal footprint, surface area constraints can lead to colocation of surface connectors30and press in connector12on opposing surfaces of circuit board24. In order to provide a thicker circuit board24that supports colocation of press in connector12and one or more surface connectors30on opposing surfaces, press in connector30inserts into blind vias of a first portion of circuit board24that is coupled to a second portion of circuit board24during manufacture of circuit board24. Since blind vias for accepting press in connector12at a bottom surface of circuit board24do not protrude through the upper surface of circuit board24, the pin field of press in connector12does not interfere with surface connectors30or the pin fields that support surface connectors30.

Referring now toFIG. 2, a side blown-up view depicts a circuit board24assembly for interfacing storage device surface connectors30and press in connectors12. Circuit board24is assembled from a storage device circuit board portion32and a press in connector circuit board portion38. An adhesive34is applied between portions32and38to couple the portions together in a lamination process, such as with a pre-preg resin that is activated to form a hardened plastic. In order to prevent adhesive34from entering vias40formed in press in connector circuit board portion38, a blocking material36having resistive characteristics, such as a UV activated ink or an epoxy ink, is applied over vias40before adhesive34is applied. Press in connector12inserts into vias40to a depth less than the thickness of press in connector circuit board portion38so that wirelines formed in portion38do not interfere with wirelines formed in storage device circuit board portion32to support storage device surface connectors30. The wirelines of portions32and38are then interfaced with each other by drilling vias after portions32and38are coupled with each other. Because the connections for press in connector12and storage device surface connector30are in separate portions of circuit board24, press in connector12can collocate on an opposing side of a surface connector30without introducing undue complexity to circuit board24and with management signal interference.

Referring now toFIG. 3, a side cutaway view depicts a circuit board38manufactured to provide press in connector blind vias. Circuit board portion38is initially manufactured with lamination formed blind vias42using multiple laminations to form blind via42with an aspect rations suitable for use with press in connector, such as a 10:1 aspect ratio in which the depth of blind via42is ten times greater than its width. Controlled depth drilling may also be used in some embodiments to form initial blind vias42, however, controlled depth drilling is limited in that the vias are plated in copper with an electro plating process having a general guideline of a 1:1 aspect ratios that will not typically work with press in connectors. The use of a blind via means that the thickness of circuit board portion38is greater than the depth to which the pins of press in connector12enter so that the bottom surface of circuit board portion38can remain solid without any openings.

In order to pass plating fluid through blind via42, an opening is formed with a counter bore drilling process. A larger diameter first drill width44is drilled in each blind via42with a controlled depth drill to approximately the depth to which connector pins will insert. A second smaller drill width46is then concentrically drilled down the first hole through the thickness of circuit board portion38. The finished press in connector via opening48thus includes an opening50at the lower surface of portion38that will allow plating fluids to pass through for plating via48with conductive material. The aspect ratio of 10:1 is met for the lower surface opening50to provide passage of the plating fluid, however, the diameter of lower surface opening50can vary as desired to adjust to the viscosity of a blocking material disposed over opening50as set forth below. Although the description set forth forFIG. 3starts with a lamination formed blind via42, in alternative embodiments, counter bore drilling may be performed on an unformed circuit board material as permitted by the connector pin insertion depth. In alternative embodiments, the counter bore drilling process may be avoided where the blocking material is viscous enough or the via small enough that the blocking material spans the hole associated with the main via. In such a situation, one via hole is formed with a constant depth through the thickness of circuit board portion38and then covered by the blocking material. The use and size of a smaller counter bored opening depends upon the viscosity of the blocking material.

Referring now toFIG. 4, a side cutaway view depicts a circuit board38with press in connector blind vias48prepared to couple with a second circuit board section32. Finished press in connector via opening48has a conductive material52deposited with plating fluids that flow through openings50. After completion of the plating process, ink36is printed over openings50to block the openings from allowing adhesive to enter during lamination of circuit board portion32to circuit board portion38. The viscosity of ink36is selected to have a value that will cover openings50without ink36adding excessive thickness to the circuit board lamination. The size of opening50determines the viscosity of the ink36so that a low enough viscosity is used to avoid adding excessive thickness while a high enough viscosity will provide sufficient blocking of opening50. In various embodiments, various diameters for opening50may be used to have a large enough diameter to ensure adequate plating fluid passage, such as with a 10:1 aspect ratio, while also having as small a diameter as possible to obtain coverage of openings50with blocking material, such as printed ink, that does not add excessive thickness to the circuit board laminations.

Referring now toFIG. 5, a side cutaway view depicts circuit board38with press in connector blind vias48assembled to a surface connector circuit board portion32to form a circuit board24for use in information handling system10. Once press in connector circuit board portion38is coupled with storage device portion32, circuit board portion interface openings54are drilled between portions38and32to form interconnects that interface press in connector12and surface connectors30. Referring now toFIG. 6, an assembled circuit board24is depicted having a co-located press in connector12and surface connector30on opposing surfaces of circuit board24. Colocation of press in connectors12and surface connectors30provides more efficient use of information handling system space and more flexibility in design of circuit board layout for supporting storage devices14.

Referring now toFIG. 7, a flow diagram depicts a process for manufacture of a circuit board having a co-located press in connector and surface connector on opposing surfaces. The process begins at step56by forming a press in connector circuit board portion using multiple laminations to have blind vias for accepting a press fit connector. At step58, two holes are drilled for each blind via with a counter bore drill process that leaves a blind via opening large enough to accept a press in connector pin and a through opening having a smaller diameter that has sufficient diameter to allow plating fluid to pass. At step60, the blind vias are plated by flowing plating fluid through the small openings. Once the blind vias are plated with conductive material, the process continues to step62to print ink over the small openings so that the small opening are blocked from allowing adhesive to flow into the blind via. The viscosity of the ink used to print over the small openings provides complete blocking without having the ink flow into the blind vias. The ink has resistive properties to prevent conduction of signals through the ink between blind vias. For example, the ink is a UV activated or epoxy ink. At step64, the surface mount circuit board portion is laminated to the press in connector circuit board portion over the ink and small diameter openings by applying an adhesive over the ink, clamping the portions together and activating the adhesive, such as with heat. At step66, vias are drilled through the laminate portions to form interconnects for communicating signals between the surface connectors and the press in connectors. At step68, the interconnects are plated and the assembled circuit board is prepared to accept a press in connector and surface connector couple on opposing sides and collocated over each other.