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>>> Temperature Profiling <<<
 
 
 
  What type of thermocouple wire should I use for profiling surface mount boards ?
  
  The wire most often used for this application is "K" type which is a twisted pair and used on most profilers. The "K" wire is used for temperature ranges between 150°-300° C and is approximately -0.010". For occasional use an insulated wire may be used; for regular continuous use a higher temperature insulation wire.  
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  We use one profile for all our surface mount products, we have a convection oven is this OK ?  
  Well it is fair to say that a convection oven can provide single profile for all products large and small. But it is poor practice as inevitably some products will be in the liquid state for an unnecessary period of time increasing intermetallic thickness, affecting solderability for second side reflow and possibly component damage. Be a SMART engineer, not a lazy one!  
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  How should thermocouple junctions be fixed to the board surface ?  
  There are three methods: adhesive, Kapton tape and high temperature solder. The best is quite simply the last but do not bury the probe in a pile of solder as this will affect temperature. You may need to use an activated flux to make the wire wet.  
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  When profiling a reflow process should you use a fully populated board ?  
  Yes always. You may find that soldering is successful with a profile developed with a sample board but have a look at the voiding in the joints. It is difficult sometimes to get a populated board if you are a sub-contractor, customers are loath to let you have one. Insist, this will help you provide a better quality and potentially more reliable product to your customer.  
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  would like to purchase a Mole to profile my reflow oven what should I look for in the equipment ?  
  First of all Mole, Kick and Datapaq are all trade names for the mid to top end profile units on the market. Generally the price increases with the number of thermocouple inputs to the data logger. The high end units also have excellent software for manipulating data. The cheaper units between 6-800 pounds sterling only have very basic software. It depends on what temperature measurements you want to do in production. I personally use an ECD Mole for running all my profiles on ovens, wave soldering systems and even the oven at home!!.  
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Bob Willis, Process Engineering Consultant Electronic Presentation Services
2 Fourth Avenue Chelmsford Essex CM1 4HA England Tel: (44) 01245 351502 Fax: (44) 01245 496123
Email: bob@bobwillis.co.uk