Prior to Assembly Process

Pre-baking prior to assembly for flex and rigid-flex applications is extremely critical due to the hydroscopic nature of flexible PCB materials. We recommend the following pre-bake guidelines:

  Pre-Baking Guidelines
 Surfaces to be soldered need to be clean
 Bake circuits at 250°F (121°C) for 1 hour
 Remove enough units from the oven that can be soldered within 2 hours
 Allow Cooling (if units not soldered in 2 hours, re-bake or place in desiccant)
 Apply Flux (a rosin type flux like Kester 185 works well)
 Use Acid Brushes to apply flux; area to be soldered should covered by the flux
 A soldering iron (MetCal unit set at 700°F) with 700°F #2 solder tip ideal heat
 Soldering tip held on the pad/component lead while applying the solder
 A solder joint forms in 3-5 seconds
 Minimize the dwell time
 Longer dwell time causes solder to "run under" the covercoat or delaminate
 Remove flux using an appropriate flux remover (vapor degreaser works )
 The solder joint should have nice uniform fillet with no voids or dewetting
 Pre-bake for 45 minutes at 250°F
 Pre-bake for 3 hours at 300°F
 Pre-bake for 4 hours at 325°F

pcb-assembly-pre-bake        To Bake or Not to Bake
Yash published this article in SMT007 examining the impact of waiving Pre-Baking process prior to Assembly. Baking is critical to successful lead-free assemblies as it drives the moisture from the board. Yash addresses the "perfect storm" of reliability failures that include: moisture absorption, interlaminate adhesion strength, and water vapor pressure at lead-free assembly temperatures.
Click here to read