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A Rough Guide to Tiny Dots
December 31, 1969 |Estimated reading time: 4 minutes
Despite the numerous technologies available to dispense dots, all is moot if these continuously shrinking dots cannot be placed accurately.
By Richard Jefferson and Steven R. Marongelli
Figure 1. Piston Position Displacement Pump with standoff tip.
It is easier for a camel to pass through the eye of a needle than for Type 3 solder paste to become a 0.014" diameter dot (with apologies to the original quotation).Why does one want to pass anything through a needle? There are many reasons for choosing needle dispensing populated boards, three-dimensional parts, flexibility, etc. Needle dispensing is the best available method for these situations and we are forced to deal with the difficulties.The forces that keep us from dispensing "tiny dots" are legion, but controllable. First is the above referenced particle size. It is simply a physical impossibility to force a 0.009" particle through a 0.008" tube. Actually, experience has taught that it is impossible to force a suspension of 0.0015" particles through that 0.008" tube. The rough guideline is particle size times seven must equal less than the needle tube's internal diameter (ID) intended for use. Anything more and the particles eventually will gang up and block the needle.
Fig. 2
So just use a larger needle! Yes, but the needle tube ID determines the ultimate diameter of the dot. That relationship usually is 1.5 times the needle's ID (although some of the newest piston positive displacement dispensing technology breaks this barrier). For example, a needle with an ID of 0.010" typically will produce a minimum dot diameter of 0.015".The particle size challenge seldom is present when dealing with surface mount adhesives (SMA), however. But, new difficulties await.Figure 2. The entire content of the syringe is compressed before material moves through the needle.
Fig. 3
The gap (defined as the distance between the substrate surface and the dispensing needle tip at the dispense point) certainly will impact the ability to dispense an accurately sized and correctly shaped dot. The rough guideline is that the gap must be about 60 percent of the intended dot diameter, although the material's rheology may require slight adjustments to this rule. The easiest way to maintain the gap is to use a standoff tip. The standoff contacts the substrate and holds the dispensing tip at the fixed gap (Figure 1). The difficulty with a standoff is that you always have to be concerned that you are not standing OFF on something you do not want to be standing ON! Additionally, you may require as many as four different dispensing heads to ensure standoff is present at the right orientation to approach a certain location, as well as a standoff that delivers the right gap for each dot size. State-of-the-art systems use some type of height sensing (i.e., contact sensors with contact force as low as 1 g or noncontact laser sensors) and sophisticated height mapping software. These features allow the machine to place the dispensing tip at the precise height required to produce the dot without a standoff. This enables one needle to produce a range of dot diameters by controlling dispensed volume in concert with controlling gap.Figure 3. Material is compressed over the entire path to the dispense tip.
Fig. 4
Another challenge is the compressibility of various materials. The pumping method is key to this problem. Typical time-pressure dispensing (also referred to as "air-over" because air pressure is over or behind the material) compresses the entire contents of the syringe before material begins moving through the needle (Figure 2). Linear pumps (motorized time pressure) also compress the entire chamber full of material (Figure 3). When the pressure is stopped, however, the compressed material must expand back to its original state. With many materials, the expansion results in drooling or an inability to accurately control the volume of material dispensed. The rotary pump (also known as an auger pump) also compresses all the material in the auger chamber (Figure 4). The piston pump compresses only the material to be dispensed. Compressibility is an issue with silver-filled epoxies, lead-free solders and some underfill epoxies, but seldom is a problem with SMAs or leaded solder (Figure 5).Figure 4. All the material around the feedscrew is compressed before material moves through the needle.
Fig. 5
Entrapped air can complicate the compressibility issue and introduce additional complications. Air bubbles within the material will compress dramatically before enough force is developed to move material through the needle. When the dispensing force is stopped, the compressed air bubbles strive to return to their original size to atmospheric pressure. Many dispensing pumps retain at least one opening directly to the atmosphere, the path through the needle. That is where the material moves as the air bubble expands, creating drool or run-on. Some dispensing pumps have added a "positive shut-off" mechanism that closes the open path to the atmosphere and other pumps embody a positive shut off in the basic design.Figure 5. Only the material being dispensed is compressed.The weight (specific gravity) of the material also can provide a complicating force. If the material is heavy enough, it can drip out of a parked or paused dispensing head. Rotary (auger) pump technology added a positive shut-off mechanism when this became an issue with encapsulation materials. Time-pressure relies on vacuum "suck back" to hold heavy materials from dripping and oozing. The positive shut-off method is the most reliable.
All of this means nothing if the dot cannot be placed in the right spot. Small dots obviously require greater placement accuracy, but that is another "rough guide" entirely.
RICHARD JEFFERSON, process engineer, and STEVEN R. MARONGELLI, technical marketing manager, may be contacted at Creative Automation Co., 11641 Pendleton St., Sun Valley, CA 91352; (818) 767-6220; Fax: (818) 767-1243; E-mail: rjefferson@creativedispensing.com and smarongelli@creativedispensing.com.