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            使用IPA(異丙醇)清洗PCBA助焊劑殘留的注意事項

            首先,醇類產品屬于易溶于水的物質,當水溶于IPA中后,會大大降低它的溶解能力,而且為了降低成本,很多本身就是摻了水的產品。所以為了達到很好的清洗效果,務必使用污水的IPA.

            其次,IPA的KB值很低,所以需要使用大量的IPA來清洗板子。為了達到很好的清洗效果,建議使用酒精瓶和毛刷,并且定時更換酒精和毛刷,確保都很干凈。

            其三,IPA清洗電路板會出現發白現象。

            第四,IPA的閃點很低,屬于及易燃品,不能夠使用噴霧罐或者氣霧形式清洗,否則十分危險。

            I assume you are speaking of cleaning PCBs with a brush and a small bottle of alcohol, or sometimes in an open top cleaning machine of some sort. In that case, you will find there are at three main problems with alcohol:

            First, alcohol is "hygroscopic." This means it absorbs water from the air, which dilutes the alcohol and weakens its ability to clean. Alcohol also is often diluted commercially by 20% or more, to keep the price down.

            This dramatically reduces its cleaning strength. Using weak, water-filled alcohol is a bad process that can lead to labor-intensive cleaning. Make sure you specify the purest, most water-free alcohol you can find.

            Secondly, alcohol is a very weak cleaner. It has a low Kb value and it saturates with flux residues at 2% by weight. So it takes a lot of solvent to successfully clean a typical circuit board. It's not the cost of the solvent that is the issue -- it's the cost of time lost cleaning inefficiently. So here are some steps you can take that will help cleaning work better:

            Use clean bottles and clean brushes, and make sure your techs clean both items daily.
            Make the techs clean with the board held at an angle, so the excess solvent runs off the board and doesn't stay and redeposit the contamination. If the solvent stays on the board, you're going to have dirty boards no matter how much the techs scrub.
            Provide the techs with clean, lint-free wipes to absorb alcohol and flux residues. Make sure they use them, and dispose of them, rather than re-using them.
            Consider upgrading to a more powerful solvent, a more sophisticated dispensing system and/or even an automated cleaner for faster, more reliable cleaning.
            Special Note: In Circuits Assembly Magazine in Dec. 2007, Mr. Terry Munson published a detailed cleanliness study using SIR benchmarks. He found that cleaning using the "dip-and-brush" process actually made the boards dirtier and more prone to premature failure. So if this is your plan, well, maybe you should re-think it.

            Most people using alcohol develop problems with white residues, especially when cleaning lead-free materials. If these residues are streaky lines across the board, your techs are not rinsing thoroughly. Have them re-clean the board. But if the residues are an even, smooth layer of white film across a large area this usually indicates you have the wrong solvent for that flux. You will need to upgrade to a stronger solvent and the problem will go away.

            Now, about worker safety. This is the third weakness of alcohol cleaners. Alcohol has a flashpoint of about 15 degrees C -- around room temperature. In the U.S., anything with a flashpoint below 55 degrtees F / 15 degrees C is considered "extremely flammable" and a significant safety hazard. Do not use these cleaners in open-topped degreasers. Do not use them in uncontrolled aerosol sprays.

            Do not use them in open trays where they might spill. Do use them with good ventilation and keep fire extinguishers available in every area where they are used. Remember, alcohol burns with an invisible flame! You can walk right into a alcohol fire and never see the flames. This makes it, in my mind, an extremely poor choice when so many other, stronger and safer options are available.

             


             

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