Application Note #20002 Rev: 2008-02

Background

For electronics assemblers running high mix-low volume production, maximizing throughput can be critical. Changing over the screen printer and the pick and place rarely takes more than ten minutes. For changeover at the reflow oven, if the board has been previously run, the operators look up the recipe and set the oven to this recipe. The problem is that when setpoint temperatures are changed, it takes time for the oven to stabilize, especially if zones need to cool down, as it takes longer for the oven to cool off than it does to heat up. It can take 5-20 minutes for the oven to stabilize, which can greatly increase the amount of time required for line changeover.

One method for reducing the amount of time required for reflow process changeover is to find a common set of oven setpoints for all board types processed in that oven, requiring only changes to conveyor speed to setup product profiles. This allows the oven to be ready to process product almost immediately, within a minute, which can significantly reduce line changeover time. The SlimKIC 2000/KIC Explorer with the KIC Navigator option can be used to efficiently develop common recipes.

Procedure

  1. Weigh and measure (length and width) all boards to be reflowed with the common recipe.
  2. Identify the boards with the highest grams/cm2 (density) and the lowest grams/cm2.
  3. Attach at least three thermocouples to each board, selecting high and low mass components plus any sensitive components. If one board is significantly larger than the other, four or five TC’s can be attached to the larger board.
  4. If there are sensitive components and you have ver. 2.0.2 or greater of the KIC 2000 software, you may set process limits for individual thermocouples in the process specification screen.
  5. Run a profile with the thermocouples from both boards plugged into the SlimKIC 2000/Explorer. It is best to place the air TC on the lowest density board and place it on the belt first (in front), leaving 8-10 inches of space between it and the highest density board. The space between the boards needs to be great enough that each will have an individual profile uninfluenced by the other.
  6. At the completion of the profile you will have profile data for both boards under the same recipe, on the same graph. Make sure that the settings in the Optimization tab are to minimize the PWI and allow both setpoint and conveyor speed changes. The Navigator will now find a recipe solution that will get both boards into spec.
  7. Continue profile iterations with the SlimKIC 2000/Explorer using the Navigator suggested solutions until the PWI of the Navigator’s predicted solution is less than 10% different from the current PWI. When you get to this point you have the setpoints for your Common Recipe.
  8. Load the Common Recipe setpoints into the oven and profile each board individually. On the Optimization tab of the graph screen, deselect or uncheck “Allow Setpoint Changes”. The Navigator will now use the Common Recipe setpoints and optimize the profile for the individual boards by only changing the conveyor speed.
    If you have a minimum or maximum line speed you can adjust the Conveyor Speed constraints thus defining the range of conveyor speeds the Navigator will try. Caution: This decreases the possible solutions the Navigator can search, along with disallowing the setpoint changes. If you have trouble getting a particular board in spec you may have to open up the range, and possibly have a separate setpoint recipe if the board is particularly difficult.
  9. When the process needs to be setup for new products, users should be able to find an in-spec profile by changing the conveyor speed only.

Caveat

It should be noted that while a common recipe will process multiple products within spec, it will not provide the optimal recipe for each individual product. The greater the differential in product density, the less optimal the recipe will be. It is not recommended that users attempt to develop common recipes for boards of radically different size and density, for example, a mother board and a cell phone board. Further, common recipes should not be used for high value or high reliability products. There is a tradeoff implicit in the use of common recipes: greater throughput for a potential reduction in solder joint quality and yield.