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BESTProto Fall 2010 Newsletter
BUILDING YOUR PC BOARD ASSEMBLIES FAST AND BUILDING THEM RIGHT
THE FIRST TIME  
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We will once again be exhibiting at the MD&M Midwest show September 28 - 30 at the Convention Center in Rosemont IL.

If you need tickets, let me know, I still have some available.

We hope to see you there!
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As part of an on-going effort to increase our ability to serve YOU, our customer, we continue to work on broadening our capability
Featured Article
We all love what surface tension does to solder to help align parts such as BGAs now watch how it works with a drop of water:  http://www.flixxy.com/water-drop.htm

 
Mixing Lead-Free and Tin-Lead Solder on a PCB Assembly

Summer is drawing to an end, the kids are back in school and companies are getting back to business. Now is as good a time as any to talk about mixing lead-free (RoHS) solder with tin-lead solder (Sn63) paste on your PCB assemblies.
 
This situation arises most often when introducing a lead-free BGA (Ball Grid Array) device with a PCB assembly that's being processed using tin-lead solder. 
 
Most all BGAs being produced today are being shipped with RoHS compliant solder on them.  Tin-lead solder balls were probably phased out and not available on the current generation BGA device and it's likely the previous generation came with RoHS solder balls as well.  Unless you've been able to strike a deal with the device manufacturer by committing to buying very large quantities, you're left holding the bag and trying to fend for yourself in making these devices work in your application.
 
If you're like many of BESTProto's customer's in the Medical, Military/Aerospace, and Industrial Equipment/Controls business, you're not only exempt from RoHS compliance, your requirements for highly-reliable PCB assemblies dictates you use tin-lead solder for your PCB assemblies. 
 
There are a few ways to handle the issue of dissimilar alloys:
 
- Switch the entire assembly to lead-free solder
 
Pros and Cons: It would eliminate the issue of the dissimilar alloys however, since the reflow temperatures of lead-free solder is about 30' C higher than that of tin-lead solder, some of the other components on the board may not be able to withstand the elevated temperatures needed for reflow. 
 
- Reball the BGA with tin-lead solder and process as a normal tin-lead assembly
 
Pros and Cons: Reballing the BGA is one of your better options but there is additional cost associated with the process and device manufacturers often discourage subjecting the device to more than 3 heat excursions although studies conducted by BESTProto sister company BEST Inc.  show the devices to be more robust then manufacturers are letting on.
 
- Process the board with tin-lead solder and place the RoHS BGAs using a BGA rework station.
 
Pros and Cons: Since BGA rework stations use localized heat that's confined to the area of the BGA device being placed; the entire board assembly isn't subjected to the additional heat.  Rework stations aren't cheap and unless you can justify the cost and have a skilled person to run it often enough to keep their skills sharp, you can have mixed results.
 
- Process the board with tin-lead solder with a lead-free BGA
 
Pros and Cons: Although this would seem to be the easiest solution, it's fraught with uncertainties.  Since you'll end up with an unknown and untested alloy, long term reliability is questionable. On the other hand, if you're just doing it to build prototypes, that may not be a concern.  Another potential issue you may face is the phenomenon known as "Head-In-pillow" whereby the two alloys don't actually bond to one another and you have a solder ball on the device and a concave pad on the board which creates more of a spooning effect and you have an open circuit.
 
Flux is an area you shouldn't overlook either because In order to get the lead-free solder to go into reflow, you'd have to run the entire board at the higher lead-free temperatures.  The problem with doing that is the flux used in the tin-lead solder paste wasn't designed for the higher temperatures and will burn off prematurely leaving the solder joint unprotected and exposed to oxides during the balance of the reflow cycle and may also cause a distorted solder joint with voiding.
 
Another issue to be mindful of is that the PC board itself may not have been produced with the ability to withstand the elevated temperatures.  Most of the PCB fab houses require you tell them if you need high temp capable substrates or key off the specified surface finish to try and determine whether the board is to be processed as lead-free or not. 
 
The bottom line: if you're faced with having to place a lead-free BGA onto a board that is primarily intended to be processed with tin-lead solder paste.  Consider reballing the BGA with tin-lead solder balls or having it placed by an experienced technician using a good quality rework station.  Or, you can always send it to BESTProto and we'll place it for you using the most appropriate process for your specific application.


 
"Garth's Garage"
Being a certifiable "gear head" most of my life, I have a passion for most things mechanical.  Whether its cars, motorcycles, bicycles or anything else with wheels and/or an engine, it gets my engine racing.  Since it's one of my favorite subjects, I've also done a fair amount of reading on those subjects and have a host of otherwise useless knowledge that I hope you find interesting and informative.

First Mass-Produced Fully Electric Car In U.S. the Nissan Leaf


Nissan will soon be releasing what is essentially the first mass-produced fully electric car called the "Leaf".  Unlike the Toyota Prius or Honda Insight there is no gasoline (or any combustible fuel) engine.  Even though the upcoming GM/Chevy Volt is primarily batter powered it has a gasoline engine that is used to regenerate the power in the batteries but it doesn't actually drive the wheels themselves.

Although GM made available a fully electric car the EV1back in 1996 to certain lease customers.  Although you couldn't really call it mass-produced even though that was the intent at the outset.  They eventually made about 800 and they weren't available for purchase, just leasing.  GM pulled the plug (pun) in 1999 as profits were no where in sight.

The Nissan Leaf will be introduced in coastal states and Michigan initially but will be available country wide over time.  If you're one of the fortunate ones, Nissan has a limited program for helping wire your home with a 240 volt A/C charger.

You can read more at  http://www.motortrend.com/roadtests/alternative/1009_2011_nissan_leaf_second_opinion/index.html
Feedback and comments are always welcome.  Please feel free to contact me any time

 
Sincerely,
 

Garth Cates
BESTProto Inc.
847-797-9250
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