You ever tried heating the board up with hair dryer or heatgun to help the solder iron any? Ill have to try it on the next one to see if it helps any
Only time I have ever preheated a PCB prior to soldering it was when I use to work for Rockwell Automation and they needed me to cover the production of circuit boards in the wave solder machine when the guy that normally ran it was out sick or on vacation. Its a machine that grabs the PCB on titanium fingers and these act like a conveyor system in which you have to adjust the feet per second of travel, how much flux is bubbling/foaming out of flux pot, and preheaters to heat the PCB's up prior to the boards bottom skimming the molten lead in the solder pot, and after this you have some burning hot boards to handle with mits and put into these dish washing type racks in which the boards were then placed into a board washer that removed flux and solder balls etc, and then they went into a dryer that removed the water, and then off to QC where they were inspected or back to production to remove a special mask tape or latex and other components hand soldered on that couldnt be part of the initial assembly process because they conflicted with the titanium fingers or were heat sensitive etc.
The heating of the boards after the flux was applied to the bottom of the boards was mainly to avoid shocking the boards when they came into contact with the molten lead pool that was flowing like water out of a Z baffle screen and a pump that pumped the molten lead up to the surface in which it was a fine adjustment for missing the board completely or too much flow of the pumping molten lead in which it spills over the top and bottom of the board and you possibly destroyed a board worth $150 in parts and labor up to that point.
Also the heat assisted with the flux that we were using. We initially had alcohol based flux in which you had to mix 100% isopropyl alcohol with 100% flux solution and get the specific gravity right for a specific combination of flux to alcohol.
This was VERY flammable and we had a few easy bake oven experiences with flames if too much flux was added to boards and the isopropyl alcohol flux was dripping on the red hot heating rods below the boards in which it would flash over and set boards and the foaming flux pot on fire. And the flux pot with compressed air to get it to foam made the fire worse if the rubber air hose burst and you had it feeding the fire fresh air.
Before I left there I was involved in a conversion to Lead Free Soldering and Alcohol Free Flux and it was quite the fun trying to get the machine not designed for this type of operation to work with the Lead Free Temp and the increased preheating temp to get the water based flux to function. With the right temperature and board speed on conveyor recipe the boards came out good. However the newer more environmentally and humanly safe manufacturing came at a cost of production problems with solder balls everywhere.
The solder ball problem was because the water based flux was in the holes and when what made contact with molten lead-free solder it spattered the same as dropping a drop of water into hot oil. The spattering shot tiny molten lead-free solder balls out of the holes and they would get everywhere and adhere to the worst of locations such as behind non thru hole surface mount chips and J-lead IC sockets etc. In QC at one point you could take a freshly manufactured board and tap the end of the board on the ESD rubber mat and off the board would roll these pen point size solder balls and worst of this was that not all fell off.
We would try to inspect to remove them all but occasionally boards would make it to testing and fail to post and then your chasing address or data lines that are held high or held low or joined with other address or data lines by solder balls.
I liked the older manufacturing better as for it made manufacturing easier for higher probability of trouble free boards rolling off the production line.
Sorry to go into extreme detail with this question... But simple answer is that heat has been used to avoid shocking the board due to a room temp board hitting molten lead or lead free solder caused problems, warpage etc. While a board heated to around 200F bottom side getting to the solder pot of the wave solder machine was safer for the boards.
However I could see that preheating the board as you have done making it easier for a soldering iron to have a higher reference temperature to start from to heat solder pads, traces with masking scratched off of, and thru holes vs the soldering iron having to bring it from room temp all the way up to the melting temp of lead or lead-free solder. So if it works for you without stressing the components or damaging the board then I would continue to do it.
When it comes to surface mount soldering of single components. The surface mount soldering station heats up a specific area of the board with the solder paste applied to the pads and the IC with suction cup lowered to and aligned leads to pads with a magnification camera. Where air is passed through a heating element in the unit and channeled out of a metal cone to concentrate the hot air on the area to apply the soldered surface mount chip etc.
So heat gun or hair dryer if you heating 1 spot vs the entire board evenly you should be ok as long as you dont overheat and start to burn the board. If you start to smell the mask material ( burning plastic/fiberglass type smell ) then your heating the board too hot.
If you ever have any questions on soldering feel free to ask. I have worked in manufacturing for many years for Rockwell Automation, Allen-Bradley Company which was bought by Rockwell, as well as Allied Electronics ( Not the catalog company but a small electronics business, as well as got mil-spec training and cert etc on some special project manufacturing that I had to follow strict manufacturing guidelines above the normal guidelines. I left the manufacturing to IT in 2003 and then went back into electronics career with gov in 2009. So I work in both ET and IT/MIS.
