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Metal foil is pulling away from the screen.

bphilips

#5263

Metal foil is pulling away from the screen. | 19 February, 2001

We use an ultrasonic stencil cleaner (not Smart Sonic) and have problems with stencils falling apart. The metal foil is pulling away from the screen. Is this a common problem with ultrasonic stencil cleaners?

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#5267

Metal foil is pulling away from the screen. | 19 February, 2001

The ultrasonic frequency used in today�s stencil cleaners is usually a 40 kHz �sweep� frequency (also used on semiconductors and other sensitive cleaning applications). This frequency is not low enough to cause stencil damage (the lower the frequency, the more aggressive the cleaning action).

A comparison would be to put your hand into an ultrasonic stencil cleaner bath and you can hardly feel the cavitation (ultrasonic cleaning action). Put your have into a high-pressure spray cleaner and you could draw blood!

There is an article on our Website written by a stencil manufacturer, Richard Clouthier (formerly of AMTX stencils and Photo Stencil). Mr. Clouthier sites two primary causes of stencil failure - heat and moisture.

Stencils are sensitive to elevated temperatures because the adhesive used to bond the screen to the frame and metal etched foil is a heat-cured at approximately 160 degrees F (71 degrees C). If a stencil is washed in hot water or dried with hot air, the adhesive bond can weaken. In addition, the expansion and contraction of the different metals (aluminum frame, stainless steel or nickel etched foil) and screen material are different than that of the adhesive, which will cause additional stress on the adhesive bond. Most stencil manufacturers recommend that their product not be exposed to temperatures in excess of 110 degrees F (43 degrees C).

(Side note: Heat can also cause distortion of the etched image. Stainless steel has a very poor memory. Once it expands, it doesn�t like to contract back to the exact same position. Repeated washing or drying at elevated temperatures may cause distortion of fine-pitch apertures leading to misregistration and misprint problems.)

Excessive exposure to moisture can cause stencil failure in that the adhesive is hygroscopic (absorbs moisture). Long cycle times in a spray washer or if the stencil is allowed to �soak� for long periods in the ultrasonic bath prior to cleaning, the adhesive will absorb the moisture and swell. When allowed to dry, the adhesive bond weakens and fails. Cleaning cycles should be kept to a minimum, cycles less than 10 minutes in a spray-type cleaner and less than 5 minutes in an ultrasonic or spray-under-immersion cleaner are good rules of thumb.

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#5270

Metal foil is pulling away from the screen. | 19 February, 2001

Bill, It is my understanding that in using the Smart Sonic to clean Adhesives from stencils and mis-printed boards it is recommended to have the system up to a certain heated temperature. What is that recommended temperature? Does this temperature have any long term effects on the life of the stencil ? I have also been inquiring stencil manufacturers on Heat and Cleaning restrictions for their stencils and they are all coming back not to go over 130 to 140 F !

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#5271

Metal foil is pulling away from the screen. | 19 February, 2001

Hi Mark,

If you are a user of stencils, you already know that there are literally hundreds of stencil manufacturers out there. If you found one that will allow such hot temperatures, I would be interested in learning who it is. However, I have only been able to find one written article on the subject and it states that "temperatures in excess of 43 degrees C. will most likely damage the stencil." ref. Richard Clouthier, "Improving Screen Print Yields," EP&P, July 1996.

If I had a choice of using a chemistry or drying method that required 95 degrees F. or one that required 140 degrees F. and all else was equal, I would certainly select the process that operated at the lower temperature. It would be safer on the stencil (and personnel), more energy efficient and not generate excess water vapor. Water vapor can be a key contamination source for an SMT assembly area. Humidity levels are usually controlled and you don't want excess water vapor around. Temperatures of 130 - 140 degress F will generate a significant amount of water vapor. Similar to opening your dish washer at home after the cycle is complete.

You are correct, cleaning solder paste in a Smart Sonic system, the wash solution is not heated. This assures the integrety of the stencil (and eliminates the potential hazard of scalding water), conserves energy and eliminates water vapor. However, when cleaning adhesives, the wash temperature needs to be warm (approximately 95 - 100 degrees F. or 35 - 38 degrees C.) depending on the type and brand of adhesive to be cleaned. 95 - 100 degrees F. is still safe on the stencil, safe for personnel and minimizes water vapor.

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#5300

Metal foil is pulling away from the screen. | 21 February, 2001

Bill, Are you recommending the heat range of 95-100F (35-38C)to clean stencils with the Smart Sonic using the 440-RT SMT Detergent? The reason I am asking is were are looking at potentially the Model 2003 to clean our paste stencils and our adhesive stencils ( using Heraeus PD955PY Adhesive). The literature on your web page for this model recommends cleaning adhesives from stencils with a elevated temperature of 110-125 F (43-52C). What is right for our application ? 95-100 F or 110-125F ???

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#5305

Metal foil is pulling away from the screen. | 21 February, 2001

Hello Mark,

As with any cleaning process, there are a number of variables that will affect the cleaning of adhesives with a Smart Sonic Stencil Cleaner: brand of adhesive, concentration of cleaning chemistry, temperature of wash solution, etc. Both temperature ranges will work. However, the 95 � 100 degrees F. may take 1 or 2 minutes longer to remove some adhesive brands. For example, your Heraeus PD955PY will clean well at 100 degrees F. in approximately 3 minutes using a 10% concentration of 440-R SMT Detergent. If you cleaned at 120 degrees F., it may only take 2 minutes to clean, but you are approaching the threshold of potentially weakening the adhesive bond on the stencil. I advise using the cooler temperature and waiting the extra minute or two.

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