Interposers and Space Transformers
Space Transformers / Interposers
Space transformers are used extensively in the vertical probe card industry, in order to transform the fine pitch pattern on the die, under test, to a coarser pitch compatible with a larger probe card (printed circuit board). Traditionally these cards are built with ceramic processing. However, due to improvements in the printed circuit board industry, great savings in time, and money can be realized when building these space transformers with printed circuit board materials, and techniques. The following is a design guideline:
Pitch: Down to .157 mm
Traditional printed circuit boards consist of multiple etched core layers laminated together, and drilled through. When routing density is so extreme, as in a .25 mm pitch and below, conventional through drilling become impossible due to the space required on internal layers in order to fan out the I/O from inner rows of the device. This necessitates the incorporation of blind vias.
If a blind via could be drilled to the layer for which that pin is to rout to the periphery of the space transformer, no matter the depth. It would be an easy proposition to rout even the densest part.
However, we must consider the limiting factor of the fluid dynamics when plating these high aspect ratio holes. Since at this time we cannot plate greater than a 1:1 ratio, (the ratio of the blind via diameter compared to the depth it penetrates). As you can see, in the picture to the left, it was necessary to stack blind vias on top of buried vias to achieve the (overall) hole depth compared to it’s diameter.
This method can also be incorporated with blind vias stacked on top of blind vias. The only limiting factor being the number of lamination cycles the material can withstand before the dielectric begins to fail and delaminate.
Please refer to the following chart for design specs:
Fab Specs Good Yields Medium Yield Low Yields
Trace and Space 2/2 @ .5 oz 1.5/1.5 @ .5 oz 1/1 @ .5 oz
Min Blind via diameter .002” .0015” .001”
MinimumThrough via .003” .002”
Minimum blind via aspect ratio 1.5:1 1:1 .8:1
Minimum blind via stop pad diameter, over hole diameter .002” NA NA
Number of sequential laminations 3 4 5
Dielectric thickness .001” min
Materials Available:
There are a host of materials available for use in space transformer designs, including high-speed materials. Please contact a R&D sales engineer to help you make a selection or visit our website @ rdcircuits.com to down load material data sheets.
Mechanical Specs:
The mechanical specifications vary from design to design depending on the material chosen and the stack-up of the design. However typical values are listed below:
Planarity per inch: .001”-.0014” /20-30 um
Pad to pad true position: +-.0002” / 5um
Tooling hole to pad true position: +-.001”