Ioan:
If memory serves, Chicago Laser Systems is a manufacturer of laser trimming
equipment. When a resistor is printed, it is usually printed with a
slightly low value. When the resistor is trimmed (regardless of method),
the resistor is probed and its resistance is monitored before and after the
trimming process. In a passive trim process, the resistor's value is
adjusted by slicicng through part of the resistor with the laser to achieve
a predetermined value. In an active trim, the output of (for example) an op
amp is adjusting by trimming the bias resistors while the device is
operating to fine tune to the gain of the amplifier. The resistor initial
value must be closely controlled to maintain the width of the trimmed
resistor (read that power rating).
In the world of thick film, the resistors are in a glass matrix which is
"fired" around 800 C (I'm working from memory so don't flame me if the
temperature is off a little) which is obviously too hot for conventional
organic substrates. There are polymer thick film resistor materials, but
the downside to them historically has been a lack of temperature stability.
Another challenge to printing precision resistors on circuit boards is the
conductor height (great compared to thick film). The resistor values depend
on the length, width, thickness, and relative resistivity (ohms per square)
of the ink - printing in open areas vs. areas populated with circuitry can
affect the ink thickness.
I am aware of organic board designs utilizing polymer thick film resistor
inks but not if the resistors are laser trimmed at the assembler. One last
note: thick film types often look for .1% or .01% tolerances for their
resistors. These tolerances are ofted required in analog circuitry with
high gain amplifiers. For less demanding applications less precise
tolerances may be acceptable enabling the use of "as printed" resistors.
There are some books on thick film manufacturing, but I don't have the
titles available any more. You might try contacting ISHM (International
Society for Hybrid Manufacturing?) for more information. This got a little
long........hope it was helpful.
Don Vischulis
-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Tempea, Ioan
Sent: Thursday, October 25, 2001 10:55 AM
To: [log in to unmask]
Subject: [TN] Laser trimming of resistors
Hi Technos,
we are advancing in exploring new technologies. Some time ago I tapped your
wisdom for info on flex circuits, now I have a better one.
What is "laser trimming of resistors"? According to the small research I
did, it's related to thick films. Is that true? What should I read on this,
starting with some on-line resources?
Keeping in mind that we are assemblers, not board manufacturers, how is this
trimming related to SMT assembly, what equipment would do it (at what cost),
implementation issues, etc.
Best regards,
Ioan
---------------------------------------------------------------------------------
Technet Mail List provided as a free service by IPC using LISTSERV 1.8d
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet
To temporarily halt delivery of Technet send the following message: SET Technet NOMAIL
Search previous postings at: www.ipc.org > On-Line Resources & Databases > E-mail Archives
Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional
information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315
---------------------------------------------------------------------------------
|
