Introduction
Transport systems with the standard spring loaded V shaped finger hold the boards by applying a light clamping force. The system allows expansion of the board and compensate small width dimension tolerances without adjusting the setting. The standard machine has one finger per inch conveyor length on each side of the conveyor.
Clamping force
The minimal clamping force for the V shaped finger equals the board weight. Knowing that 500 gram load at the center of the V groove gives a displacement at that point of 6.5 mm, we can calculate the correct settings for the different sections. For boards with a width over 150 mm one has also the thermal expansion to take into account due to the soldering operation (λFR4 =15.10-6 and λFR3 = 25.10-6).
Example
We have to solder a bi-layer board. The board is FR4 epoxy and has a width of 200 mm. The board length is 250 mm and the weight is 750 grams. Transport speed 1.2 m/min, solder temperature 250 °C. What are the correct settings for the conveyor?
Calculations
The board is held by 250/25.4 = 10 fingers on each side.
The load per finger is than 750/2x10 = 37.5 grams
The finger displacement is 6.5/500 x 37.5 = 0.5 mm
This means that the finger conveyor setting must be set 1 mm smaller than the infeed section.
The infeed and outfeed sections must be set so that the boards will easy pass these sections. For the outfeed section it may be necessary to compensate for thermal expansion of the board due to the heat absorbed in the board from the soldering process.
Due to the type of board and the process setting we may expect a temperature increase in the board material of about 130 °C. (Note: If the transport speed is less the temperature rise will increase. Also if we have to deal with multi-layer boards the temperature can become higher).
The thermal expansion of the FR4 board will be:
ΔT . λ . W
which gives:
→ 130 . 15 . 10-6 . 200 ≈ 0,4 mm
This expansion gives an extra finger displacement:
0.4/2= 0,2 mm
This gives an extra clamping force of:
0.2/6.5 x 500 = 15 gram per finger, which is in this case 150 grams on the
board side, if the boards are not mounted in a pallet.
NOTE: If the side forces due to thermal expansion become too high so that they will give cause for board deformation, one may adjust the conveyor setting to a wider exit setting. This can be done by unlocking the chain of the width adjustment drive mechanism. Adjust the mechanism so that the transport exit width is compensating for the board expansion by rotating the chain wheel. Fix the chain and make a final check if all works properly.