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September 2014 • SMT Magazine 71 4. Clogged stencil apertures or poorly formed apertures Dirty apertures will limit the effective paste fill and release of the aperture. Paste that binds to the aperture edges will reduce the effective aperture opening, add surface roughness, and change the effective area ratio. Regular cleaning is a must for proper transfer and release of the solder paste. 5. Stencil thickness If the stencil thickness doesn't meet the ex- act target for thickness, the volume of the paste that is transferred will be inaccurate. For smaller pattern features, a small variation in the stencil thickness can make a big difference in the effec- tiveness of the stencil. prObLem: Inconsistent paste transfer across the board Potential causes of inconsistent paste trans- fer across the surface of the board include: 1. Inadequate support of the board Good under-board support is a must in or- der to keep the board flat and achieve high- quality stencil printing. Particularly when there are components on both sides of the assembly, there can be trouble due to flexure of the board as paste is applied. For proper support, the PCB must be locked into position absolutely parallel to the stencil. To do this requires positive sup- port, particularly when printing the second side of a double-sided board, to eliminate any down- ward movement and flexure when the squeegee blade moves across the board. The edge of the support tool should be flat to the PCB edge, or the stencil will coin. Over time, that will result in a shortened stencil life. 2. Non-homogeneous application of sol- der paste This is most often attributed to reused or dried-out solder. Application can have localized swaths where the paste is not consistent over the full application area. Below are some other commonly encoun- tered problems, possible causes, and potential cures. Solder balls Possible Causes • Excess solder paste applied to the assembly • Excess solder paste under the chip component • Outgassing of solder paste in a confined area • Misprint leaving excess solder paste • Poor gasketing of the stencil to the board Potential Cures • Special apertures for chip components • Adjusting the reflow profile tombstoning Possible Causes • Unequal wetting of pads for chip components • Too little solder paste on one pad of chip or possible solder thieving • Excess solder paste on the chip pad • Reflow profile too steep which will typically cause one pad to reflow before the other • Shielding of a chip pad by a larger component causing uneven heating • Spacing between chip pads is too big or there is an unbalanced pad geometry Potential Cures • Change to a C-shaped chip component stencil aperture • Characterize the board and make adjustments to the reflow profile • Review and correct the pad layout design and look for solder thieving off of the component pads • Decrease solder paste volume on the chip pads bridging or Shorts between Leads Possible Causes • Bad stencil aperture pad alignment • Smeared solder bricks • Good brick, but too much volume • Contaminant, like fiber, lies across the leads • Excess solder paste left on squeegee side surface of the stencil • Excess insertion pressure at pick-and-place The ShorT SCooP ImprOvING SteNCIL prINtING reSULtS continues