Stencil Printing Defects

Stencil is damaged which will result in Excess paste on the BGA pads resulting in Bridging after Reflow

Stencil Aperture Blocked after Laser Cutting which will result in no paste on to the pads.

Bridging or Shorts on two pads which can result in soder short after reflow

Solder Slump which is very rare mainly occurs due to Temperature.

Scooping which is mainly caused due to High Squeegee Pressure resulting in less solder.

Bleeding is caused because of the Board Vacuum on the printer which results in solder balls

Residues is because of improper cleaning of the board resulting in solder balls.

Incomplete print is due to improper cleaning of the stencil.

Slamp is because of the contact with the stencil, the stencil has either flexed during separation with the board or the board has lifted

Paste Misalignment is result of poor alignment which will result in solder shorts

Stencil Printing

Stencil Types

Stencil Types

Important print quality variables include accuracy and smoothness of the stencil aperture sidewalls. Maintaining a proper aspect ratio between stencil width and thickness is important. The recommended aspect ratio is 1.5. This is important for preventing stencil clogging. Generally, solder paste remains in the opening if the aspect ratio is less than 1.5. In addition to aspect ratio, it also is good to have an area ratio (area of pad divided by area of aperture walls) of greater than 0.66, as recommended by IPC-7525, Stencil Design Guidelines. (This document can serve as a good starting point for stencil design.)

The process by which the aperture is made controls both the smoothness and accuracy of aperture walls. There are three common processes for making stencils: chemical etching, laser cutting and the additive process.

Chemically Etched Stencils

Metal mask and flexible metal mask stencils are etched by chemical milling from both sides using two positive images. During this process, etching proceeds not only in the desired vertical direction but also laterally. This is called undercutting — the openings are larger than desired, causing extra solder deposit. Because 50/50 etching proceeds from both sides, it results in almost a straight wall tapering to a slight hourglass shape in the center.

Because electroetched stencil walls may not be smooth, electropolishing, a microetching process, is one method for achieving a smooth wall. Another way to achieve smoother side walls in the aperture is nickel plating. A polished or smooth surface is good for paste release but may cause the paste to skip across the stencil surface rather than roll in front of the squeegee. This problem can be avoided by selectively polishing the aperture walls without polishing the stencil surface. Nickel plating further improves smoothness and printing performance. However, it does reduce aperture opening and requires artwork adjustment.

Laser-cut Stencils

Laser cutting also is a subtractive process, but it does not have the undercutting problem. The stencil is produced directly from the Gerber data, so aperture accuracy is improved. The data can be adjusted to change dimensions as necessary. Better process control also improves aperture accuracy. Another benefit of laser-cut stencils is that the walls can be tapered. Chemically etched stencils also can be tapered if they are etched only from one side, but the aperture size may be too large. A tapered aperture with an opening slightly larger on the board side than on the squeegee side (0.001 to 0.002" to produce an angle of about 2°) is desired for easier paste release.

Laser cut is capable of producing aperture widths as small as 0.004" with an accuracy of 0.0005", so it is very suitable for ultra-fine-pitch component printing. Laser-cut stencils also produce ragged edges because the vaporized metal is transformed into metal slag during the cutting process. This can cause paste clogging. Smoother walls can be produced by microetching. Laser-cut stencils cannot make stepped multilevel stencils without pre-chemical etching of the areas that need to be thinner. The laser cuts each aperture individually, so stencil cost depends upon the number of apertures to be cut.

Electroformed Stencils

The third process for making stencils is the additive process, most commonly called electroforming. In this process, nickel is deposited on a copper mandrel to build the aperture. A photosensitive dry film is laminated on the copper foil (about 0.25" thick). The film is polymerized by ultraviolet (UV) light through a photomask of the stencil pattern. After developing, a negative image is created on the mandrel where only the stencil apertures remain covered by the photoresist. The stencil is then grown by nickel plating around the photoresist. After achieving the desired stencil thickness, the photoresist is removed from the apertures. The electroformed nickel foil is separated from the mandrel by flexing — a key process step. Now the foil is ready for framing as in other stencil making processes.

Electroforming step stencils can be done at added cost. Because of the close tolerances possible, electroformed stencils provide a good gasket effect, which minimizes under-stencil paste seepage. This means that the frequency of underside stencil wiping is reduced drastically, which reduces potential bridges.

Solder Paste Printing

In surface mount assembly reflow soldering, solder paste is used for the connection between surface mount component leads or terminations and the lands. There are many variables, such as paste, screen printer, paste application method and printing process. In printing solder paste, the substrate is placed on the work holder mechanically or by vacuum, and aligned with tooling pins or vision. Either a screen or stencil is used to apply solder paste. In this column, I will focus on some key paste printing issues, such as stencil design and printing processes; in next month's column, I will discuss printing processes for fine-pitch and through-hole components in a mixed surface mount assembly.

Printing Process and Equipment

In the solder paste printing process, the printer is crucial for achieving desired print quality. Screen printers available today fall into two main categories: laboratory and production. Each category has further subdivisions because companies expect different performance levels from laboratory and production printers. For example, a laboratory application that is R&D for one company could be prototype or production for another. Moreover, production requirements can vary widely depending on volume. Because a clear-cut equipment classification is not possible, the best thing to do is to select a screen printer to match the desired application.

In manual or semiautomatic printers, solder paste is placed manually on the stencil/screen with the print squeegee at one end of the stencil. In automatic printers, paste is dispensed automatically. During the printing process, the print squeegee presses down on the stencil to the extent that the stencil bottom touches the top board surface. Solder paste is printed on the lands through the openings in the stencil/screen when the squeegee traverses the entire image area length etched in the metal mask.

After the paste has been deposited, the screen peels away or snaps off immediately behind the squeegee and returns to its original position. This gap or snap-off distance is a function of the equipment design and is about 0.020 to 0.040". Snap-off distance and squeegee pressures are two important equipment-dependent variables for good quality printing.

If there is no snap-off, the operation is called on-contact printing. This is used when an all-metal stencil or squeegee blade is used. If there is a snap-off, the process is called off-contact printing. Off-contact printing is used with flexible metal masks and screens.

Squeegee Types

Squeegee wear, pressure and hardness determine print quality and should be monitored carefully. For acceptable print quality, squeegee edges should be sharp 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 and may even damage the squeegee and stencil or screen. Excessive pressure also tends to scoop solder paste from wide apertures, causing insufficient solder fillets.

Two squeegee types are common: rubber or polyurethane squeegees and metal squeegees. When using rubber squeegees, 70 to 90 durometer hardness squeegees are used. When applying excessive pressure, paste bleeding underneath the stencil may cause bridging and will require frequent underside wiping. To prevent underside bleeding, the pad opening must provide a gasketing effect while printing. This is dependent on the roughness of the stencil aperture walls.

Metal squeegees also are commonly used. Their popularity has grown with the use of finer pitch components. They are made from stainless steel or brass in a flat blade configuration, and are used at a 30° to 45° print angle. Some squeegees are coated with lubricating material. Because lower pressure is used, they do not scoop paste from apertures, and because they are metallic, they do not 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 volume requirement is very different for each component type. Fine-pitch components require much less solder volume than standard surface mount components. Pad area and thickness control solder paste volume.

Some engineers use dual-thickness stencil to apply the appropriate paste amount at fine-pitch and standard surface mount pads. Other engineers take a different approach — they use a more expensive metal squeegee that does not require frequent sharpening. It is easier to prevent variation in paste volume deposition with a metal squeegee, but this approach requires a modified stencil aperture design to prevent excess paste deposition on fine-pitch pads. The approach has become more popular in the industry, but rubber squeegees with dual-thickness printing have not vanished.

To achieve good printing results, a combination of the right paste material (viscosity, metal content, largest powder size and lowest flux activity possible), the right tools (printer, stencil and squeegee blade) and the right process (good registration, clean sweep) are necessary.

Flux

Flux is a chemical agent which helps for soldering, brazing and welding of metals by removing the oxide layer from the metal parts. Different fluxes, mostly based on sodium chloride, potassium chloride, and a fluoride such as sodium fluoride, are used in foundries for removing impurities from molten nonferrous metals such as aluminum, or for adding desirable trace elements such as titanium.

In high-temperature metal joining processes (welding, brazing and soldering), the primary purpose of flux is to prevent oxidation of the base and filler materials. Tin-lead solder (e.g.) attaches very well to copper, but poorly to the various oxides of copper, which form quickly at soldering temperatures. Flux is a substance which is nearly inert at room temperature, but which becomes strongly reducing at elevated temperatures, preventing the formation of metal oxides. Additionally, flux allows solder to flow easily on the working piece rather than forming beads as it would otherwise.

In soldering of metals, flux serves a threefold purpose: it removes oxidation from the surfaces to be soldered, it seals out air thus preventing further oxidation, and by facilitating amalgamation improves wetting characteristics of the liquid solder. Flux is corrosive, so the parts have to be cleaned with a damp sponge or other absorbent material after soldering to prevent damage.

There are following types of Flux:

R ( Rosin)

RA (Rosing Activated)

RMA (Rosin Mildly Activated)

WS ( Water Soluble)

Any of these categories (except WS) may be no-clean, or not, depending on the chemistry selected and the standard that the manufacturer requires.

The function of flux is primarily to remove oxide, with the general formula being:

Metal oxide + Acid → Salt + Water

Solder Paste Composition

Solder Paste is normally used for connecting the terminations of the components with the land patterns on the PCB The paste is usually applied either by Dispensing or by printing the solder using the stencil. Most of the problems are related to paste printing or dispensing only. So the characteristics of the solder paste, Viscosity needs to be checked periodically to get perfect joints.

Composition

Solder paste is basically comprised of Powdered Metal suspended in thick medium called Flux. Flux is added to act as a temporary adhesive for holding the components until the soldering process. The paste is a gray, plasticine-like material. The composition of the solder paste varies with purpose the paste is used for. For example, with plastic packages on a FR-4 board the solder composition used is eutectic Sn-Pb (63%Sn 37%Pb) or SAC alloys (Sn/Ag/Cu), or if one needs high tensile and shear strength tin-antimony (SnSb) alloys. Generally, solder pastes are frequently made up of an alloy of tin and lead, with possibly a tertiary metal alloyed, though environmental protection legislation is forcing a move to lead-free solder (see also:solder, RoHS).

Solder paste is thixotropic, meaning that its viscosity changes with applied shear force (e.g. stirring). The thixotropic index is a measure of the viscosity of the solder paste at rest, compared to 'worked' paste. Hence it may be very important to stir the solder paste before it is used.

To produce a quality solder joint, it's very important for the spheres of metal to be very regular in size and have a low level of oxidation.

Classification based on size

The solder particle size and shape determines the paste print-ability. A solder ball is spherical in shape; this helps in reducing the surface oxidation and ensures good joints formation with the adjoining particles. Irregular particle sizes are not used as they tend to clog stencil causing printing defects. The pastes are classified based on the particle size by JEDEC J-STD 005 . This is a standard body governing the electronic industry. The table below gives the classification type of a paste compared with the mesh size and particle size.

Type designation[JEDEC]	Mesh size in micrometres	Particle size in micrometres	Average size in micrometres
Type 2	-200/+325	75-45	60
Type 3	-325/+500	45-25	35
Type 4	-400/+500	38-20	31
Type	-500	25-15	18

Classification based on flux

According to J-STD-004, solder pastes are classified into three types based on the flux types: rosin based pastes, water soluble pastes and no clean pastes. Rosin based pastes are made of rosin, a natural extract from the pine trees. These fluxes need to be cleaned after the soldering process by using CFC. Due to the ban on this material the usage of rosin fluxes is not predominant. Water soluble fluxes are made up of organic materials of glycol bases. There are wide varieties of cleaning agents for these fluxes. A no-clean flux is made up of resins and various levels of solid residues. No-clean pastes not only save cleaning cost but capital expenditure and floor space. However these pastes need very clean assembly environment and inert re-flow environment.

Properties of solder paste

In using solder paste for assemblies we need to test and understand the various rheological properties of a solder paste. A few of them are explained in this section:

Viscosity: Viscosity of a material is internal property of the material which resists the tendency of flow. In this case, solder paste is desired to have varying viscosities at different stress levels. Such kind of material is called thixotropic. When solder paste is moved by the squeeze on the stencil, due to the application of stress on the paste the viscosity breaks down making the paste thin and helping it to flow easily through the apertures on the stencil and when the stress on the paste is removed it regains it shape preventing it from flowing on the PCB. Viscosity for a particular paste is available from the manufacturers catalog and in-house testing is needed sometime to judge the usefulness of paste after some usage.

Slump: Slump is the characteristic of a material to spread after application. Theoretically it is assumed that the paste side walls are perfectly straight after the deposition of paste and remain like that until the part placement. If the paste has high slump value we see a deviation form the expected behavior, as now walls of the paste are not perfectly straight. Slump in a paste should be minimized as it risks formation of bridges between two adjacent pads.

Working life: Working life is the amount of time paste can stay on a stencil without affecting it printing properties. Manufacturer gives this value.

Use

Solder paste is typically used in a screen-printing process, in which paste is deposited over a stainless steel or polyester mask to create the desired pattern on a printed circuit board. The paste is dispensed pneumatically, by pin transfer (where a grid of pins are dipped in solder paste and then applied to the board) or by jet printing where the paste is distributed on the SMT pads like an inkjet printer.

As well as forming the solder joint itself, the paste carrier/flux must have sufficient tackiness to hold components while passing through the various processes, or perhaps moved around the factory.

Printing is followed by pre-heating and reflow (melting).

As with all fluxes used in electronics, residues left behind may be harmful to the circuit, and standards (eg J-std, JIS, IPC) exist to measure the safety of the residues left behind.

In most countries, 'no-clean' solder pastes are the most common, whereas in the US, water soluble paste (which have compulsory cleaning requirements) are common.

Storage

Solder paste should be stored in an airtight container at low temperatures (above freezing) but should be warmed to room temperature for use. Air exposure to the solder particles in the raw powder form causes them to oxidize so exposure should be kept to a minimum.

The paste manufacturer will suggest a suitable reflow temperature profile to suit their individual paste, however one can expend too much energy on this. The main requirements are a gentle rise in temperature (preheat) to prevent explosive expansion (solder balling) and to activate the flux. Thereafter the solder melts and the time in this area is known as Time Above Liquidus. Reasonably rapid cool down is a requirement after this.

A good tin/lead solder joint will be shiny and relatively concave. This will be less so with lead-free solders.

SMT Process Requirements