| ASK THE EXPERT |
 |
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
|
|
| See the interview of BEST commenting on it being the recipient of an IPC technology award |
|
|
|
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!
Bob Wettermann
President, BEST Inc
|
 |
|
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.
|
|
