The three main elements that determine the quality of SMT screen printing
The three main elements that determine the quality of SMT screen printing
In SMT, a typical PCB (Printed Circuit Board) may have several hundred components and 600 to 1,000 coupling points (i.e. pad pads) on it. The solder failure rate at these endpoints must therefore be kept to a minimum. In general, 60% of PCBs that fail testing and have to be reworked are due to poor solderpaste silkscreen quality.
Below, we will discuss the fundamentals of screenprinting and explore the techniques required to consistently ** screenprint quality in production.
There are three key elements in solder paste screen printing, here called the three S's: Solderpaste, Stencils, and Squeegees. The right combination of these three elements is the key to consistent screen quality.
Elements that determine the quality of SMT screen printing I: Solder Paste (the first S)
Solder paste is a combination of solder beads and rosin, the function of which is to remove oxides from component pins, pads and beads during the first stage of reflowing in the soldering oven, which lasts about three minutes at 150°C. (Resin is sometimes called rosin. (Resin is sometimes called rosin, and technically, resin is a natural product while rosin is man-made.) Solder is an alloy of lead, tin and silver that is reflowed in the reflow oven** at about 220°C. The silver and rosin both play a role in helping to melt the solder. Both silver and rosin play a role in helping to melt the solder and wetting to achieve reflow, i.e. the role of the flux. (Wetting: is a descriptive term for the effect of soldering, where the object being soldered appears to be "wetted" by the tin.)
Spherical solder particles are manufactured into a variety of mixed sizes, and then screened and graded. Solder paste is graded according to the size of the beads, as follows:
Type 2: 75-53mm
Type 3: 53-38mm
Type 4: 38-25mm (m=micron=0.001mm)
The Law of Three Balls
The Law of Three Balls gives production a simple formula for selecting a silkscreen stencil; the size of the solder bead in the solder paste must match the silkscreen stencil, as described below:
Empirical formula:
At least three *large diameter tin beads can be lined up vertically in the thickness direction of the screen printing stencil.
At least three *large diameter beads can be aligned horizontally across the width of the *small holes in the screen printing template.
The calculation is slightly more complicated because the beads are measured in metric microns (m), while the industry standard for the thickness of screen printing templates is the US-specific unit thou!
(1mm=1x10-3mm,1thou=1x10-3inches,25mm?1thou.)
Solder Paste Type 3x*Large Bead Size *Close Stencil Thickness
Type 2: 75-53m 3x75m=225m=9.0thou 9 thou
Type 3: 53-38m 3x53m=159m=6.4 thou 6 thou
Type 4: 38-25m 3x38m=114m=4.6 thou 4 thou
The size of the silk-screened hole is determined by the component pin spacing (pitch), and the size of the pad is generally half the pin spacing. (The size of the silkscreen hole may actually be a little smaller than the size of the pad.) For example, for a 25 thou (0.63mm) spacing, the silkscreen hole is 12.5 thou. Therefore, the solder paste must be selected to meet the *smallest* silkscreen hole on the silkscreen stencil:
Component *Small pin spacing *Small silkscreen hole Suitable solder paste type
16 thou 8 thou Type 2: 75-53m
12 thou 6 thou Type 3: 53-38m
8 thou 4 thou Type 4: 38-25m
Therefore, the thickness of the silkscreen stencil is usually the deciding factor, and for most applications the standard 6 thou thick silkscreen stencil with type 3 solder paste is selected.
Viscosity
Viscosity is an important characteristic of solder paste, from the dynamic aspect, in the screen printing stroke, the lower its viscosity, the better the mobility, easy to flow into the screen printing holes, printed on the PCB pads. From the static side of the consideration, silk screen after scraping, solder paste stays in the silk screen hole, its viscosity is high, it maintains the shape of its filling, and will not collapse downward, which is more important for waiting for the patch before the silk screen in the pads of the solder paste.
The viscosity of the paste in its container tank is measured using a delicate and often expensive laboratory viscometer. Standard viscosities are in the range of about 500kcps-1200kcps, with a more typical 800kcps being ideal for stencil screen printing. A more practical and economical method of determining whether a solder paste has the correct viscosity is as follows:
Stir the paste with a spatula inside the container jar for about 30 seconds, then pick up some paste, three or four inches above the container jar, and let the paste drip down on its own; it should slide down like thick syrup at first, then break off in sections to fall into the container jar. If the paste can not slide down, it is too thick, if it has been falling with