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May 2008

 

Dear Subscriber;

In this edition of the BEST  newsletter  we are discussing:

  • QFN/LGA Rework
  • Power Supply Troubleshooting

As part of  the troubleshooting discussion on power supplies we give you some practical tips on where to look first when trying to repair power supplies. We always invite you to check in with BEST technicians if you are stumped on a particular repair.

Do not miss several upcoming learning opportunities. The first is  on Friday June 13th in the Milwaukee area  as (3) excellent presenters will be discussing the state of lead free reliability testing In addition, mark your calendars for the rework symposium that will be held on Thursday August 14th at BEST Inc in Rolling Meadows where there will be (3) very knowledgeable speakers  as well as hand-on demonstrations of various rework techniques.  Please mark your calendars for these informative events!

Thanks!

CktBdAnimated

Bob Wettermann
President, BEST Inc

Solder Tips
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BEST Video Soldering Tips

BEST Instructors have put together video demonstrations of common hand soldering tips and skills. These are not designed to be in place of live one on one training where repetitive skills and subsequent one on one feedback is given. Rather, they are quick refreshers designed to be viewed if a specific skill has not been practiced in a while.

 
QFN and LGA Rework


BEST provides sound rework solutions for LGA and leadless devices such as QFNs and other MLFs. We have been reworking LGAs, QFNs and other MLF packages for many years, and continue to stay current with the latest package styles and techniques to rework them.

We have numerous "high end" LGA rework systems and have the right experiences and the right tools for the job. Rework of LGA, QFN or MLFs is performed by experienced technicians   who inspect their work with the latest in QC tools such as endoscopes and X-ray imaging.

Repair of damaged solder mask between LGA pads is a task our experienced repair technicians face on a daily basis. Damaged or missing LGA pads are replaced by following an IPC recommended procedures.

The key to successful LGA, QFN and MLF rework is the proper solder paste printing of the land areas, whether that be on the device and or the PCB. BEST has the experience to properly design the stencils, even when large ground planes exist on the device. In addition BEST has the ability to "bump" the lands of the devices using its patented StencilQuik™ stencil printing technique. Stencils are fabricated inhouse for this technique.

If you have a specific QFN, LGA or other leadless device requiring rework please call Laura Ripoli at (847) 797-9250.

 
Shorted Power Supplies-Troubleshooting Tips


First, let's look at a "shorted" supply. This is most commonly caused by shorted output or driver transistor(s), but may also be the result of any of the following ...

- Shorts between transistor case and heatsink, due to a punctured mica washer

- Open circuit bias servo circuit. The bias servo is the transistor and pot that generates the bias voltage needed to keep the transistors conducting at just the right level to avoid crossover ("notch") distortion. An incorrectly installed transistor, faulty (open circuit, wrong value, or improperly adjusted) pot, dry solder joint or broken track can all cause the output transistors to turn on fully when power is applied. In some designs, the bias "servo" is simply two or more diodes, and may also have a series resistor.

- Solder bridges between tracks or component pads.
The first thing to determine is if the short is "hard" or "soft". A "hard short" will show up as a very low resistance between the supply rails (less than 1 ohm), when measured with a multimeter with no power applied. Hard shorts always indicate either blown transistors, solder bridges or punctured mica washers. If you are lucky, it will be either of the second two, but don't get your hopes up.

A "soft short" is identified by the fact that a resistance measurement between the supply rails to each other, the output and ground does NOT show a very low resistance (less than (say) 650 ohms or so). Resistance readings of around 600-700 ohms are possible in one direction (this is actually a voltage, and is developed across diode junctions either on real diodes, or within the junctions of transistors). Resistance may be the same or much higher in the other direction - swap the meter leads for all such tests so you measure with both polarities. You almost certainly have a component (power transistor or driver) installed incorrectly if you get a soft short, but a faulty bias servo will create the same effect.

If you can vary the voltage, determine the voltage where the soft short comes into play. It is very rare for soft shorts to be present at extremely low voltages (less than +/-1 or 2 Volts), but if it is, then something is installed incorrectly. Further component tests will isolate 99% of all soft short problems.

 
TABLE OF CONTENTS
 
QFN/LGA Rework
Power Supply Repair Tips
 
QUICK LINKS
 
 
BEST Capabilities Video
EZReball(TM)
Proto Assembly Estimator
 
 
ASK THE EXPERT
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BEST's "Ask the Expert" column is designed to give one-one dialogue between a questioner and our master instructor Kris Roberson. Specification, processing and inspection questions are asked and answered here.

 
PRODUCTS & SERVICES
 

In order to contact customer care on a specific repair project click here

 

BEST INDUSTRY FORUMS
 

This Industry forum is a discussion area where opinions are given on equipment and processes, help is given on process problems and  resources for problem-solving and troubleshooting are shared.

 
IL/WI SMTA CHAPTER MEETING
 

The IL and WI Chapters of the SMTA will be holding a vendor day on June 13th at the Wyndham Hotel near the Milwaukee Airport. There will be several excellent speakers discussing the  reliability of lead freee PCB assemblies  including Dr Ron Lasky and Denny Fritz. Users are encouraged to bring their lead free reliability "issues " to an excellent roundtable with accomplished engineers from Plexus, Creation Technologies and Tellabs there to answer questions and provide opinions. More info here.

 
See an interview of BEST Inc-IPC Technology Award Recipient
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See the interview of BEST commenting on it being the recipient of an IPC technology award

 

SOLDERING TIPS

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Newsletter

WHAT'S NEW?

VIDEO: BEST at APEX 2017 Convention

ARTICLE: To Bake or Not to Bake (in Rework)

 

 

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