Patent Application
20080142252
In today's high density printed circuit board assemblies, in order to connect one layer to another using a conventional multilayer printed circuit board, via holes are generally drilled through the entire board thickness. During testing, an interposer 10 is mated to an ATE tester 20 with a printed circuit board 150 using contacts 100 to access various circuits, elements and features (note shown) on the board for testing. Some circuit elements may be on the back side of the board or otherwise not readily accessible on the top side of the printed circuit board. It is desirable to utilize the via holes to access various circuit elements with the tester interposer during test.
However, via holes are not solid and therefore, cannot be used to mate mechanically with interposer contacts. Thus, for some electrical contacts, there must be a land pad, a dog bone trace, and a via hole, as shown in
As will be readily apparent from
It is desirable to have printed circuit boards capable of denser land pads and vias, while supporting larger diameter vias; thicker printed circuit boards; more layers in the printed circuit board; more signals routed to contact pads; more reliable current carrying capabilities of vias; and reduced crosstalk between signals on adjacent layers.
An understanding of the present teachings can be gained from the following detailed description, taken in conjunction with the accompanying drawings of which:
In the following detailed description, for purposes of explanation and not limitation, example embodiments disclosing specific details are set forth in order to provide a thorough understanding of the present teachings. It is apparent to one having ordinary skill in the art with benefit of the present disclosure that other embodiments according to the present teachings that depart from the specific details disclosed herein remain within the scope of the appended claims. Moreover, descriptions of well-known apparatus and methods may be omitted so as to not obscure the description of the example embodiments. Such methods and apparatus are considered within the scope of the present teachings.
With specific reference to
With specific reference to
The present teachings permit the printed circuit board 250 to be approximately up to two times thicker than prior printed circuit boards and thus have more signal layers for routing by placing the via holes 260 substantially directly under and substantially in the center of the land pads 210 and filling the via holes 260 sufficiently so that it is flat enough to mate to an interposer. This invention saves valuable printed circuit board 250 real estate by placing the land pad 210 directly on top of the via hole 260 and substantially eliminates much of the space previously taken up by the dog bone trace and the land pad.
As the via hole 260 is now drilled in the substrate substantially directly under and in essentially the center of the land pad 210, the via 260 may be of a substantially larger diameter than prior via holes. This will, in turn, permit the printed circuit board 250 to be thicker, since the maximum thickness of the printed circuit board 250 is a function of the via hole 260 diameter. Standard plating processes permit the printed circuit board to be fifteen to thirty-six times thicker than the smallest via hole diameter on the board. Permitting the printed circuit board 250 to be thicker, further enables more layers within the board thickness, which will allow for more routing layers and thus a higher density of signals can be routed to the contact area underneath the interposer contacts. There may also be additional ground layers between signal routing layers of the printed circuit board 250 to improve crosstalk of signals from adjacent layers that may previously not have had a ground layer between them due to the limitation in board thickness. For example, typical via holes are 6 mils to 20 mils in diameter and via holes according to the present invention may be 15 mils to 35 mils in diameter.
The via structure of the present teachings enables more layers within the printed circuit board 250 that mates to the tester interposer by increasing the size of the via holes 260 for the signals. Also, larger via holes 260 can carry more current more reliably than smaller via holes. The signal fidelity may also be improved by placing additional ground layers in the printed circuit board 250, which is able to be thicker with larger via holes 260. This may improve crosstalk between signals on adjacent layers. The additional ground layers may also be used to create controlled impedance transmission lines in the printed circuit board 250.
While the invention had been particularly shown and described with reference to specific embodiments, it will be readily appreciated by those skilled in the art that changes in the form and details of the disclosed embodiments may be made without departing from the spirit or scope of the invention. For example, some of the descriptions of embodiments herein imply a certain orientation of various assemblies of which the system is constructed or a certain order of fabricating or mounting the assemblies. For example, the surface planing may be completed before the epoxy material is cured. It will be understood, however, that the principles of the present invention may be employed in systems having a variety of spatial orientations or orders of fabrication or mounting and that therefore, the invention should not be limited to the specific orientations or orders of fabrication or mounting disclosed herein.
