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August 7, 2006

Studies showing the effects of PCB finish on manufacturability

I have been having trouble finding any studies showing the effects of PCB finish on manufacturability of assemblies. I can find studies on reliability. I have heard lots of talk about ENIG, immersion silver, etc of being superior to HASL in fine pitch applications, but have yet to find a study proving this. Any comment?



P.D.

Experts Comments

HASL is generally not great for fine pitch assemblies...I would recommend flash gold which has much better resolution.

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Peter Greenland
Global Sales Director
Photonis USA
Mr. Greenland has 20 years electronics industry experience ranging from circuit board design to product support & repair services. Peter has spent the last 4 years at Pycon Inc. supporting customers that require high mix, low volume manufacturing solutions.

I am not aware of any specific published studies regarding HASL versus other finishes for fine pitch applications.

The reason why some manufacturers have problems with the HASL finish and fine pitch is that during the HASL process pads are produced with a dome or sloped shape. For very fine pitch devices a pad with a dome can cause parts not to sit squarely and therefore leading to bridging. In very a very poor process the hot air knife that is sometimes used to blow and cool the solder can sometimes push the solder off the pads, exposing the underling intermetallic, when then oxidizes in the air, and is unsolderable. This can happen when trying to produce a very flat surface for fine pitch devices.

The immersion finishes (ENIG, Silver, and Tin) as well as the OSP all produce generally flat surfaces to the pads. But, each of them has their own pluses and minuses (cost, storage, shelf life, robustness, etc.). As with any other material or process the manufacturer needs to weight the pros and cons and determine the best materials (and process) that meets their needs. A properly manufactured HASL finish may be suitable for fine pitch applications, but a more consistent finish may be obtained using one of the other common finishes available.

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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.

Unfortunately I do not know of any manufacturing studies where defects were tracked between different board finishes. They all have good characteristics and bad characteristics.

ENIG has possible black pad however it seems to be a good reproducible finish in the last few years. If you are building a device that drop testing is required this may not be your first choice since the nickel intermetallics tend to be more brittle than the copper intermetallics. Emersion silver is chemistry dependent. Not all emersion silver finishes are equal some are more susceptible to atmospheric corrosion from sulfur than others. There are also handling procedures for these, creating a tighter process window.

Having said this they are a good finish when manufactured and handled properly. HASL lead free nothing solders like solder. A pre-soldered finish will solder very well. The pads on lead free are much flatter than the old tin lead HASL. The only issue is possible warping of the board during the HASL process.

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Karl Seelig

Deck Street Consultants
In his 32 years of industry experience, Mr. Seelig has authored over 30 published articles on topics including lead-free assembly, no-clean technology, and process optimization. Karl holds numerous patents, including four for lead-free solder alloys, and was a key developer of no-clean technology.
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