SMT Solder Paste Printing for SMTIn surface mount assembly reflow soldering, solder paste is used for theconnection between surface mount component leads or terminations and the lands.There are many variables, such as paste, screen printer, paste applicationmethod and printing process. In printing solder paste, the substrate is placedon the work holder mechanically or by vacuum, and aligned with tooling pins orvision. Either a screen or stencil is used to apply solder paste. In thiscolumn, I will focus on some key paste printing issues, such as stencil designand printing processes; in next month’s column, I will discuss printingprocesses for fine-pitch and through-hole components in a mixed surface mountassembly.
Printing Process and Equipment
In the solder paste printing process, the printer is crucial for achievingdesired print quality. Screen printers available today fall into two maincategories: laboratory and production. Each category has further subdivisionsbecause companies expect different performance levels from laboratory andproduction printers. For example, a laboratory application that is R&D forone company could be prototype or production for another. Moreover, productionrequirements can vary widely depending on volume. Because a clear?cut equipmentclassification is not possible, the best thing to do is to select a screenprinter to match the desired application.
In manual or semiautomatic printers, solder paste is placed manually on thestencil/screen with the print squeegee at one end of the stencil. In automaticprinters, paste is dispensed automatically. During the printing process, theprint squeegee presses down on the stencil to the extent that the stencilbottom touches the top board surface. Solder paste is printed on the landsthrough the openings in the stencil/screen when the squeegee traverses theentire image area length etched in the metal mask.
After the paste has been deposited, the screen peels away or snaps offimmediately behind the squeegee and returns to its original position. This gapor snap?off distance is a function of the equipment design and is about 0.020to 0.040". Snap?off distance and squeegee pressures are two importantequipment?dependent variables for good quality printing.
If there is no snap?off, the operation is called on?contact printing. This isused when an all?metal stencil or squeegee blade is used. If there is asnap?off, the process is called off?contact printing. Off?contact printing isused with flexible metal masks and screens.
Squeegee wear, pressure and hardness determine print quality and should bemonitored carefully. For acceptable print quality, squeegee edges should besharp and straight. A low squeegee pressure results in skips and ragged edges,while a high squeegee pressure or a soft squeegee will cause smeared prints andmay even damage the squeegee and stencil or screen. Excessive pressure alsotends to scoop solder paste from wide apertures, causing insufficient solderfillets.
Two squeegee types are common: rubber or polyurethane squeegees and metalsqueegees. When using rubber squeegees, 70 to 90 durometer hardness squeegeesare used. When applying excessive pressure, paste bleeding underneath thestencil may cause bridging and will require frequent underside wiping. Toprevent underside bleeding, the pad opening must provide a gasketing effectwhile printing. This is dependent on the roughness of the stencil aperturewalls.
Metal squeegees also are commonly used. Their popularity has grown with the useof finer pitch components. They are made from stainless steel or brass in aflat blade configuration, and are used at a 30° to 45° print angle. Somesqueegees are coated with lubricating material. Because lower pressure is used,they do not scoop paste from apertures, and because they are metallic, they donot wear easily like rubber squeegees and hence do not need to be sharpened.They cost significantly more than rubber squeegees, and can cause stencil wear.
Using different squeegee types has ramifications in printed circuit assemblies(PCA) with both standard and fine-pitch components. The solder paste volumerequirement is very different for each component type. Fine-pitch componentsrequire much less solder volume than standard surface mount components. Padarea and thickness control solder paste volume.
Some engineers use dual-thickness stencil to apply the appropriate paste amountat fine-pitch and standard surface mount pads. This is the conventionalapproach and requires a rubber squeegee to force paste through the stencilholes. Other engineers take a different approach — they use a more expensivemetal squeegee that does not require frequent sharpening. It is easier toprevent variation in paste volume deposition with a metal squeegee, but thisapproach requires a modified stencil aperture design to prevent excess pastedeposition on fine-pitch pads. The approach has become more popular in theindustry, but rubber squeegees with dual-thickness printing have not vanished.
Important print quality variables include accuracy and smoothness of thestencil aperture sidewalls. Maintaining a proper aspect ratio between stencilwidth and thickness is important. The recommended aspect ratio (aperture widthdivided by stencil thickness) is 1.5. This is important for preventing stencilclogging. Generally, solder paste remains in the opening if the aspect ratio isless than 1.5. In addition to aspect ratio, it also is good to have an arearatio (area of pad divided by area of aperture walls) of greater than 0.66, asrecommended by IPC-7525, Stencil Design Guidelines. (This document can serve asa good starting point for stencil design.)
The process by which the aperture is made controls both the smoothness andaccuracy of aperture walls. There are three common processes for makingstencils: chemical etching, laser cutting and the additive process.
Chemically Etched Stencils
Metal mask and flexible metal mask stencils are etched by chemical milling fromboth sides using two positive images. During this process, etching proceeds notonly in the desired vertical direction but also laterally. This is calledundercutting — the openings are larger than desired, causing extra solderdeposit. Because 50/50 etching proceeds from both sides, it results in almost astraight wall tapering to a slight hourglass shape in the center.
Because electroetched stencil walls may not be smooth, electropolishing, amicroetching process, is one method for achieving a smooth wall. Another way toachieve smoother side walls in the aperture is nickel plating. A polished orsmooth surface is good for paste release but may cause the paste to skip acrossthe stencil surface rather than roll in front of the squeegee. This problem canbe avoided by selectively polishing the aperture walls without polishing thestencil surface. Nickel plating further improves smoothness and printingperformance. However, it does reduce aperture opening and requires artworkadjustment.
Laser cutting also is a subtractive process, but it does not have theundercutting problem. The stencil is produced directly from the Gerber data, soaperture accuracy is improved. The data can be adjusted to change dimensions asnecessary. Better process control also improves aperture accuracy. Another benefitof laser-cut stencils is that the walls can be tapered. Chemically etchedstencils also can be tapered if they are etched only from one side, but theaperture size may be too large. A tapered aperture with an opening slightlylarger on the board side than on the squeegee side (0.001 to 0.002" toproduce an angle of about 2°) is desired for easier paste release.
Laser cut is capable of producing aperture widths as small as 0.004" withan accuracy of 0.0005", so it is very suitable for ultra-fine-pitchcomponent printing. Laser-cut stencils also produce ragged edges because thevaporized metal is transformed into metal slag during the cutting process. Thiscan cause paste clogging. Smoother walls can be produced by microetching.Laser-cut stencils cannot make stepped multilevel stencils without prechemicaletching of the areas that need to be thinner. The laser cuts each apertureindividually, so stencil cost depends upon the number of apertures to be cut.
The third process for making stencils is the additive process, most commonlycalled electroforming. In this process, nickel is deposited on a copper mandrelto build the aperture. A photosensitive dry film is laminated on the copperfoil (about 0.25" thick). The film is polymerized by ultraviolet (UV)light through a photomask of the stencil pattern. After developing, a negativeimage is created on the mandrel where only the stencil apertures remain coveredby the photoresist. The stencil is then grown by nickel plating around thephotoresist. After achieving the desired stencil thickness, the photoresist isremoved from the apertures. The electroformed nickel foil is separated from themandrel by flexing — a key process step. Now the foil is ready for framing asin other stencil making processes.
Electroforming step stencils can be done at added cost. Because of the closetolerances possible, electroformed stencils provide a good gasket effect, whichminimizes under-stencil paste seepage. This means that the frequency ofunderside stencil wiping is reduced drastically, which reduces potentialbridges.
Chemical etching and laser cutting are subtractive processes for makingstencils. The chemical-etch process is the oldest and most widely used. Lasercut is a relative newcomer, while electroformed stencils are the latest rage.
To achieve good printing results, a combination of the right paste material(viscosity, metal content, largest powder size and lowest flux activitypossible), the right tools (printer, stencil and squeegee blade) and the rightprocess (good registration, clean