Figure 1. When thermal paste dots drift in size, the cause is usually in the material handling or the dispensing setup - and it can be measured.
Thermal pastes are highly filled, so small changes in the material or setup change how much comes out. The usual causes are: filler that has settled in the container, so the paste is not uniform; air trapped during transfer or loading; temperature changes that shift viscosity; time-pressure dispensing, where the shot size changes as the syringe empties; and nozzles that are too narrow or worn by abrasive filler. Check them in that order, and measure shot weight to confirm the fix.
Thermal interface performance depends on getting the right amount of material in the right place, every time. Too little leaves dry areas and hot spots; too much squeezes out, adds thermal resistance and contaminates nearby parts. Yet thermal pastes are among the harder materials to dispense: they are thick, heavily loaded with abrasive filler, and sensitive to temperature and air.
This guide covers the most common reasons dispensing drifts on a production line, how to tell which one you have, and what to change.
Symptoms
Dot or bead size changes over a shift, or between the start and end of a syringe or container.
Random small or missing dots, or beads that break up.
Spitting or popping at the nozzle tip.
Different results between operators, lines or days with the same settings.
Shot size slowly decreasing over weeks with the same program.
Five Common Causes
Figure 2. Five common causes, from container to tip. A shot-weight check shows which one you have.
1. Filler settling in the container. Over time in storage, dense filler can settle toward the bottom of a container. Paste taken from the top is then richer in carrier oil and flows more easily; paste from the bottom is stiffer. Dispensing from an unstirred container gives a gradual drift as you work down through it. Where the supplier recommends it, stir the full depth of the container until uniform before transferring.
2. Trapped air. Air enters when paste is scooped, stirred vigorously, or loaded into syringes or cartridges. Air compresses while paste does not, so air pockets cause spitting, missing dots and shot-to-shot variation. Load syringes carefully from the bottom up, avoid whipping air in when stirring, and consider degassing for critical applications.
3. Temperature. Viscosity changes with temperature. Paste taken straight from cool storage dispenses less than the same paste at room temperature, and a dispensing area that warms during the day changes shot size. Let containers reach a stable room temperature before use, and keep the dispensing area temperature stable.
4. Time-pressure dispensing. The most common and simplest method - a pulse of air pressure on a syringe - is also the most sensitive. As the syringe empties, the air volume above the paste grows, so the same pulse delivers a different amount; small viscosity changes also have a large effect. For tighter control, auger (screw) valves and positive-displacement or piston pumps meter the volume directly and are much less affected by fill level and viscosity.
5. Nozzle and wear. A tip that is too narrow for the filler can clog or cause the paste to separate under pressure. One dispensing equipment maker recommends a tip inner diameter of at least three times the filler particle size for filled greases, and tapered tips for higher-viscosity materials. Abrasive filler also wears valves, augers and seals over time, so a slow, steady decrease in shot size can point to worn parts.
How to Find the Cause: Measure Shot Weight
The fastest diagnostic is a shot-weight check: dispense a set number of shots onto a tared plate on a precision balance and record the weight. Repeat it under different conditions to isolate the cause.
| Pattern in shot weight | Most likely cause |
|---|---|
| Steady drift from full to empty syringe | Time-pressure dispensing and fill level |
| Drift from top to bottom of a container | Filler settling - material not uniform |
| Random low or missing shots | Trapped air or partial clogging |
| Higher in the afternoon than the morning | Temperature |
| Slow decrease over weeks with the same program | Wear of valve, auger, seals or tip |
Once the process is stable, keep the shot-weight check as a routine control at the start of each shift and after every material change.
Process Checklist
Follow the supplier's preparation instructions for the container - including stirring if specified.
Bring material to a stable room temperature before use.
Transfer and load syringes or cartridges without introducing air.
Match the dispensing method to the tolerance you need - volumetric methods for tight control.
Use a tip with enough inner diameter for the filler, tapered for high viscosity.
Check shot weight at the start of each shift and after material changes.
Inspect and replace wear parts on a schedule.
Reseal open containers immediately and use them within the supplier's stated period.
Next Step: Check That the Material Form Fits Your Process
Some dispensing problems are a sign that the material form does not suit the process - for example, a very thin grease on an automated line that needs a more stable bead, or a pad where a dispensed material would cut cycle time. See Thermal Putty vs Thermal Grease vs Thermal Gel vs Thermal Pad: How to Choose.
Struggling With Dispensing Consistency?
Tell us your dispensing method, tip size, container size and what the shot-weight pattern looks like. Our engineers can help you narrow down the cause.
References
Techcon - Improving the Dispensing Process for Grease
Universal Instruments - Time Pressure Dispensing
Published by: Fong Yong Chemical Co., Ltd. - specialty chemical and thermal management materials manufacturer, Taiwan. Established 1983.
