A typical surface-mount printed circuit board may be populated with a couple of hundred components with 600 to 1,000 connections (solder pads). For a successful manufacturing process, the failure rate of these solderable terminations must be kept to a minimum. The use of state-of-the-art screenprinters, and a review of screen printing basics will help to produce an accurate, repeatable print.

There are three critical areas in solder paste printing: solder paste, stencils and squeegees. The correct combination of these areas is vital.

Solder Paste
Solder paste consists of solder balls and flux in a binder. The spherical solder particles are manufactured in mixed sizes and then sieved to grade them into ranges. Pastes are graded according to the size of solder balls according to the Three Ball Rule. The three-ball rule provides a simple formula for stencil selection in a production environment. The grading size of solder balls in the paste should match the stencil used. At least three of the maximum solder ball diameters should be vertically located in the stencil thickness. At least three of the maximum solder ball diameters should be horizontally located in the finest aperture width on the stencil (Figure 4-1). The aperture size will be dictated by the size of the pad.

Stencils
The majority of solder paste printers now use stencils. They usually consist of a thin metal sheet with open apertures where the solder is to be printed. No pump action is required of the squeegee, because the solder paste can flow easily into the apertures. The three major types of stencils are Chemically Manufactured Stencils, Laser-Cut Stencils and Electro-Formed Stencils.

Chemically Manufactured Stencils
Chemically manufactured stencils are the main types of stencils in use today. A thin sheet of brass or stainless steel is taken and coated on both sides with photosensitive acid-resist. A photopositive (black solder pads) master is sandwiched on either side of it and the assembly is exposed to powerful ultraviolet light. The exposed areas harden, allowing the soft pad areas to be washed away. The board is then placed in an acid etch bath, which etches away the desired apertures from both sides.

Laser-Cut Stencils
Another technique used in the manufacture of stencils is through the use of a computer-controlled CO2 or Yttrium Aluminum Garnate (YAG) laser to cut away the apertures from one side only. The time to manufacture a stencil this way is dependent on the amount of open area but is normally less than one hour.

Electro-Formed Stencils
Electro-formed stencils currently account for between two and three percent of stencils in use. The manufacturing process is additive, unlike the others. The internal walls of the aperture are very smooth and can be made trapezoidal - that is, slightly wider at the bottom than the top. This can help the solder paste to transfer through the stencil out onto the board.

Squeegees
While good results can be obtained with polyurethane squeegees under controlled conditions, metal squeegees offer advantages under production conditions. Unlike a polyurethane blade, they have a perfectly straight and durable edge and require no adjustment before use. The fact that the edge cannot break away and deform into apertures gives some advantages. A far greater range of pressures can be exerted (e.g. 4-15kg) and will produce a good print irrespective of the fineness of the apertures. Also, a 6 thou stencil will give a 6 thou thickness of print, which eliminates variations because of different operators and conditions.

The speed at which the squeegee travels over the stencil during printing is an important process parameter because the solder paste needs time to roll and flow into the apertures. If insufficient time is allowed, the deposition may be uneven across the pad in the direction of squeegee travel. At a speed as low as 20mm per second, the squeegee can pass over small apertures in as little as a few tenths of a millisecond. For each of the finest component pitches on the board (i.e. two times the finest aperture size), allow 1mm per second of maximum print speed.

Recently, the EMPF received a new R29 batch printer from Reprint Services, Dorset, England (Figure 4-2). The R29 was developed as a medium volume batch printer with a capability to handle a full size 29" x 29" stencil frame. With the supplied multi frame adapter, this capability can handle stencil frames down to a 6" x 6" size. A vacuum pump for tooling is standard. An easy-to-use Windows™ based interface combined with state-of-the-art micro-controller based servo electronics provides precise repeatable control of all printer functions. The software is simple to use and requires only the basic understanding of the print process to operate, with all of the set up and print parameters software controlled.

This printer is being used at the EMPF on printed circuit boards for power devices. It is also being used by the EMPF Learning Center in its SMT and Boot Camp training classes. In this class, the students are taught the fundamentals of solder paste printing. The R29, through its mechanical design and software platform, enables the students to easily learn to print any PCB. The R29 stencil printer provides all the functionality and flexibility required in today's production environment. With the benefit of a Windows based user interface, operators can easily access the extensive features of the printer and be assured that new product file generation or product changes over time are performed quickly and smoothly. This machine is currently offered in the USA through RPAmericas (www.rpamericas.com). If you would like a demonstration of the Reprint Services R29 Printer, please contact Jeff Stong at (610) 362-1200 x224.

Summary
Controlling the failure rate of surface-mount printed circuit boards can be accomplished by implementing proper screenprinting techniques, and utilizing state of the art printers. The critical areas of solder paste, stencils and squeegees must be combined correctly to ensure a quality print.