Alignment tolerances

Rules for the welded fabrication of pressure vessels cover welding processes, manufacturer s record keeping on welding procedures, welder qualification, cleaning, fit-up alignment tolerances, and repair of weld defects. Procedures for postweld heat treatment are detailed. Checking the procedures and welders and radiographic and ultrasonic examination of welded joints are covered. 

As before, if the segments are larger or the primary is faster, all the alignment tolerances will get tighter. However, for this example, which is typical of some designs under discussion, the testing does not appear to be a major problem. Like any test where a few nm rms precision is required it will not be easy and close attention will have to be paid to the environment, but nothing looks unreasonable. 

There is a variety of exposure technologies, each with a fundamental resolution limit resulting from radiation interactions with both the hardware and the resist. From a practical standpoint, factors such as alignment tolerance and resist swelling usually degrade resolution to a point where minimum feature sizes are larger than theoretically expected. Nevertheless, examining the fundamental resolution limits of the various techniques can be instructive. 

Another resource derived from BLOCKS and PRINTS is IDENTIFY, which, instead of encoding the exact information at each position in an alignment, tolerates alternative residues according to a set of prescribed groupings of related biochemical properties. IDENTIFY and its search software, eMOTIF (Huang and Brutlag, 2001), are accessible for use from the protein function Web server of the Biochemistry Department at Stanford University, http //dna.Stanford.edu/identify/. PANAL (http //mgd.ahc.umn.edu/panal/run panal.html) of Computational Biology Centers at the University of Minnesota is a combined resource for ProSite, BLOCKS, PRINTS, and Pfam (Bateman et al., 2000). 

With more stringent requirements on alignment tolerances and planarization in multilevel designs, it is important to fill in the contact and via holes with metal to maintain acceptable step coverage. The step coverage by sputtered aluminum rapidly decreases with increasing aspect ratio. Instead of the sputtered aluminum, tungsten metal has been widely used to fill in the contacts and vias by the CVD technique. 

Warner K, Chen C, D Onofrio R, Keast C, Poesse S. An investigation of wafer-to-wafer alignment tolerances for three-dimensional integrated circuit fabrication. Proceedings of The IEEE International SOI Conference 2004. p 71-72. 

Photolithography can be performed using chromium or emulsion masks. Chromium masks can hold tighter resolution and alignment tolerance and remain cleaner than emulsion masks, but quality emulsion masks are available which can be used down to 4 microns or better resolution. 

Relative to the other double-patterning approaches, the double exposure is the simplest to implement since it does not require additional follow-up process steps. Its main challenge is meeting alignment tolerance requirements. 

Maintain stringent member alignment tolerances and QA/QC protocol. 

Synchronous drives have an advantage over gears and chains in that they can transmit high loads over a wide range of speeds with low noise and no Inbrication requirements. Synchronous belts, like V-ribbed belts, require closer alignment tolerances in order to avoid premature wear or failure. 

Sn-A4.0g-0.5Cu balled, 0.4mm (0.0157 in.) pitch CSP components on Sn-3.9Ag-0.6Cu solder paste, placed up to —50% off-pad, can self-align to the pad after reflow in air (Ref 62). By considering the PWB pad size/location tolerances and solder paste alignment tolerances, the required alignment accuracy for the pick and place equipment was calculated to be 50 pm at 30 (standard deviation). 

The silicon device layer is metallized with gold (500 nm Au/20 nm Cr) and patterned with the PAD METAL mask using a photolithographic lift-off process that is capable of defining 3 pm lines and spaces with a 3 pm alignment tolerance. This metal layer is exposed to high temperatures during the subsequent process steps, so it does not provide an optical quality surface for mirrors like the second metallization that is patterned with the BLANKET METAL mask. Any metal features that are defined in the first metal deposition will be in electrical contact unless they are separated by a trench etched in the device layer since the surface of the device layer is heavily doped with phosphorus. 

ASTM list two specifications for the tensile testing of butt joints. The first, ASTM D 897-78 uses short, stubby, circular specimens of metal (or wood) with a cross-sectional area of 1 in, while the second (ASTM D 2095-72) allows for longer specimens of metal or reinforced plastics. For the latter test, specimens (round or square) should be prepared according to ASTM D 2094-69 which designates alignment tolerances and fixtures to minimize misalignment problems. 

Installation and maintenance. If a gear unit is correctly sized, properly installed, ind properly maintained, it can be expected to last indefinitely. Proper installation includes (1) proper initial alignment, both internal and external, and (2) a rigid foundation that will not settle, crack, or elastically or thermally deform under operating conditions in amounts greater than the gear-alignment tolerance. 

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