If your board is single sided you should convey it on a mesh belt conveyor. Unfortunately most assemblies these days are double sided which makes it difficult to convey on the mesh belt. Plus with the increasing population on SM boards the weight is ever increasing. This is why we at ETS developed our Product Support Conveyor for use with the EDGE GRABBER (tm) Conveyor. All ETS Reflow ovens can be equiped with one of two styles of product support conveyors. The first utilizes a #25 chain which is .35" wide enclosed into a rail. The chain moves with the circuit board at the same speed as the conveyor. The rail is adjustable both in height and the location across the width of the process cavity. The second style of product support conveyor is a stainless steel rail which has a series of thin, free rolling disks .035" thick. Again the rail is computer programmable for the x and y axis. This system offers the most flexibility in processing as it requires the thinnest "lane" on the bottom side of the board. We have also conducted a study about the effects of a center support during reflow. I would be happy to share that study with you if you provide me your fax number, or the article can also be found in the September 1990 issue of SMT Magazine. For more information contact Brian Stumm at 509-483-0900 or by e-mail at ets@eznet.com
| Hi Anthony! | Earl's right, things can get rather involved when you start discussing ALL the things that affect whether or not a board is going to warp. | The way I look at the issue is try to separate the warp issue into two catagories; 1. The board being flat prior to SMT, and 2. The board being flat after assembly. | I think between the two areas, it's easier to deal with the first issue than it is with the second. Obviously, the main enemy of board flatness is heat, be it in the PCB fabrication process, or the assembly process. | Just a few things to think about to address minimizing warping in the fabrication process: | 1. There are many different laminate materials that are more stable than FR4, but it comes at a price. Almost every other material is going to be more expensive than FR4. | 2. Balancing the copper content in the board, distributing it as equally as possible within each layer, and throughout the overall board. If the design allows, using cross-hatch types of patterns, or I've even seen circles distributed in a grid-pattern left in areas where there are no traces or planes to balance the copper. By doing this you not only improve the temperature stability of the board, but it makes things easier for your fab vendor to etch. | 3. I know I'll get a few zings from our HASL fans for this one, but almost every other board finish is much less warp inducing than HASL, none of the other board finishes subject the fab to the kind of heat that HASL does. | (I gotta get ready for work, I'll come back up later to give a few suggestions about minimizing warp during assembly) | -Steve Gregory-
reply »