RT FAQ

Laser Resistor Trimming FAQs

 

1. What’s involved in doing thin- or thick-film trimming?

Mainly spot size and laser power.  <25 micron spot size for thin- and >50 microns for thick-film.   >1w laser power for thick- and <1w for thin-film.

 

2.  Can you do thin- and thick-film trimming? 


Yes.

 

3.  Can you do passive or active trimming?

Yes.  Active trimming is usually to a circuit voltage output.  Passive trimming is to a resistance value.  We use a HP DVM,  0.0015% for dc voltage, 0.05% for ac voltage.  6.5 digits.  Computer interfaced to the laser.

 

4.  How do laser trimmers operate?


They are ablative and therefore increase resistance value.  Decreasing the resistance involves laser thermal annealing which can happen if the laser goes cw (loss of  Q-switched pulses).  Annealed resistors generally are unstable.

 

5.  What is your expected yield after trimming?

Close to 100%.  A typical problem is resistive material that is screened on too small, making the initial resistance higher than the target resistance.  So the resistor can not be trimmed.

 

6.  What is your accuracy of laser placement?


This is an absolute spec for comparison to a standard.  It is usually given as +/- 0.1 mil per inch of travel.  However, thick-film resistors are not screened with “perfect” accuracy.  So laser accuracy is not the issue, resistor accuracy is.

 

7.  What type laser do you use?


Q-switched Nd:YAG (lamp pumped)  or Vanadate (diode laser pumped).

 

8.  How do initial & final values affect trimming?


The further apart the initial & final values, the longer to trim.  This will vary considerably due to process variables.

 

9.  Do you have fixed probe cards with up/down probe motion?


No. We use flying probes.

 

10.  Tell me about your flying probes.


Our flying probes allow for flexibility of setup.  Our laser trimmers can be supplied with an overhead moving beam for trim motion and can have a lower X/Y stage for moving the substrate.