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April 3, 2006

Lead-free solder pastes drop-in replacements?

We're a small sub-contract shop and my boss just told me we must be ready to convert one production line to "Lead-Free" in 60 days. This line does not include a wave solder machine, so I'm hoping this may be an easy process. My question, are there new lead-free solder pastes which are drop-in replacements for the conventional pastes I've been using for the past few years?



A.D.

Experts Comments

The concept of a drop-in replacement has been one of the drivers for the industries solder paste development programs. The answer is two fold, but relatively simple.

1. For printing, and pick n place, if you have the guidance of a good supplier, then you should see no real difference. The flux chemistries developed for the Pb-free solder alloys should enable you to operate without printing and pick n place process change that you have used for traditional SbPb solder alloy assembly.

2. For reflow and inspection - the noticeable difference is the change in melting point of the solder alloy, and reconditioning the expectations in the post assembly visual inspection.

The eutectic Sn63Pb37 solder alloy has a melting point of 183C, where as the SAC (SnAgCu) eutectic solder alloys melt around 217-221C (depending on the Ag and Cu content chosen). Therefore, your board temperatures during reflow will have to compensate for this - typically 25-30C above the liquidus temperature. This is the major process change. So please make sure the oven you are using has been certified as being capable of operating at higher temperatures for an extended period of time.

The SAC alloys wet onto copper less than SnPb (due to higher surface tension), and so exposed copper on pad maybe noticeable. A method often used to eliminate this effect is to remove the aperture pad reduction often used in SnPb board assembly. However, if your pad surface is ENIG or ImAg the exposed pad phenomena is minimized. You may also notice that the solder surface looks visually different - more dull. It is purely cosmetic, and has no effect on reliability. This dull cosmetic effect can be improved by controlling the cooling process in your oven.

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Dr. Brian Toleno
Application Engineering
Henkel Electronics
Dr. Brian Toleno is the Application Engineering Team leader for Henkel Technologies. He is responsible for the technical service and application engineering for Henkel's electronics assembly materials, including solder paste, underfills, PCB protection materials, and underfills.

You shouldn't see much difference in the printing and placing. Your oven, however, may be a different matter. Many of the lead free pastes have a much higher melting temperature. You may have to run a much hotter profile and many old ovens aren't equipped for this. Check with your paste vendor for advice, and get samples of different formulations to try.

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Bob Black
President and CEO
Juki Corporation
Mr. Black was the President and Co-Founder of Zevatech in 1977 and introduced first Pick and Place System at Nepcon West 1980. Bob is now the President, CEO and Co-Founder of Juki Automation Systems. He is also a Co-Founder of the SMEMA Council of IPC. He serves as a member of SMTA and SEMI..

Yes, there are replacements available from your Solder Paste supplier. It is suggested that you contact your Supplier(s) and discuss what they have available at the moment. For further information and future use, it is recommended that you visit www.ipc.org and consider purchasing the applicable documents which will provide a wealth of process and function information for you to make use of. Another valuable site is http://www.pb-free.info/laymans_terms.htm which provides very valuable "plain English" information regarding this subject. Of subsequent concern is the types of files that you are receiving from your customers. There are some very specific layout considerations for the application of the Lead Free process. One software provider has actually incorporated system level changes to specifically address these needs. http://www.cadstarworld.com/press_200510cadstar8.asp for more information.

Best regards and I hope this helps.

Sam Burton
President
Tropical PCB Design Services Inc.
With 28+ years of direct PCB layout and design experience and 8 years as President of Tropical Design; Mr. Burton has the breadth and depth of knowledge to assist in solving most design and manufacturing related issues.
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