When printing quality goes awry, check the climate.

Your forehead feels a little moist. Palms are sweaty. Heat begins to rise out of your collar, and breathing becomes a chore. Mind you, you haven’t run a marathon; all you’re doing is moving a few boards and spatulas. Chances are if you’re feeling the heat, so is your printer. My general rule is this: If I feel OK, then the process is probably OK. Too hot, we both sweat; too cold, we both quake.

We often understandably can get so focused on the advanced options and capabilities of our printers, the impact of the print environment on the process gets overlooked. Assemblers need to be aware that the environment in which they print has a significant influence on the results, as temperature and humidity can significantly change the viscosity of the material – in most cases, solder paste – being printed. Generally, the optimum temperature range inside the printer should be 70° to 77°F (21° to 25°C). If the temperature is too high, material will start to lose viscosity, which can lead to bridging and smearing. If the air conditioning system happens to be very good and the temperature is too low, material viscosity increases, and the result may be insufficients from blocked apertures.

If printing quality is fine for a few hours, then all of a sudden, problems like bridging or smearing arise, and if there haven’t been changes to the paste, stencil or other inputs, the environment could be the culprit. Troubleshooting the issue is as simple as turning on the SPC system to monitor the temperatures inside the printer. It may reveal a cycle in which printing is fine in the morning hours when the system is maintaining a steady 71°/72°F, but when the afternoon rolls around and the temperature peaks at 79°/80°F, that’s when defects begin to appear. Using the SPC tools, it is fairly simple to determine if there is a correlation between defect rates and the environment, and in most cases, it is easily resolved.

For environmentally induced print process issues, there are several fixes to consider:

• Install a temperature control monitor. Most screen-printing equipment manufacturers offer an automatic temperature control system that will either chill or heat, depending on what’s required. These systems keep the inside of the printer at a constant temperature to within +/-1°. One note of caution: Select a system that filters and monitors airflow. Solder pastes contain high volatile solvents, and excessive airflow will dry out the print materials, which can lead to other issues. It is critical that the heat is moved out of the printer and cooled through a diffusing system that doesn’t dry out the print material.
• Relocate the printer/change the ambient temperature in the factory. Seems an obvious solution, but you’d be surprised at how often this isn’t considered. Before joining DEK, I worked at a manufacturer where we did just this; we moved the SMT line away from the external windows to aid with temperature management. I’ve also seen situations where, even when the factory is properly cooled, the line might be next to the “Goods Out” door. Not surprisingly, in the heat of summer/freeze of winter, when the door is constantly opened and closed, outside temperatures impact the print and paste performance. Consider the factory layout and line locations very carefully. And, of course, adjusting the temperature control within the factory is an option.
• Change print materials. Solder paste materials development has come a long way in the past 10 years. With today’s expertise and manipulation of the paste DNA, modern materials are formulated to be somewhat resistant to heat and humidity. I’ve seen a material change work wonders on a process. In fact, many years ago, I was working with a customer that was having bridging problems. The factory was very hot, and the company didn’t have the funds to invest in a new HVAC system. After our evaluation, we suggested they consider changing materials, which they did, and which dramatically improved the results.

These are easy remedies for a frequently misdiagnosed problem. Just as the environment impacts your own well-being, it can do the same for the print process. Remember the simple rule: If you’re not comfortable, odds are neither is your printer. Be cool.

Clive Ashmore is global applied process engineering manager at DEK International (dek.com); This e-mail address is being protected from spambots. You need JavaScript enabled to view it . His column appears bimonthly.