SMD soldering using Hot Air Rework Station - Page 1
Forum section: electronics / beginners

author: cidcorp
author_rank: Unknown
timestamp: October 19, 2013, 04:26:38 pm
content: Not sure if this is the best place for this question but couldn't seem to find anywhere else that it fit, andseeing as I'm new to doing the surface mount components myself I figured I'd ask here.I don't do alot of surface mount, so generally I leave it up to a third party to do the assembly. After seeing a videoon doing it with a hot air rework station I decided to give a simple board a go. I got a cheap plastic stencil doneand applied the paste to each of the 5 boards I was doing then proceeded to populate the boards with the components.After populating all 5 boards I one by one started using the hot air on the components.My problem was that every few components (specifically the resistors - all were 1206 - starting big to get the feel) aftermoving the nozzle around and around (with the air flow very low) the component would appear to *pop* off the board.Now I've heard of tombstoning and such but this was literally 'jumping' across the board.I figured it could be one of the following:a] heating paste too quickly and not allowing for a soak (correct term?)b] paste was out on board for a little over an hour (as I did all boards cause I was slow going) and dried leaving little flux?c] paste I have is a couple months past exp date (kept refrigerated and not used often) I did leave out to get to room temp.d] unbalanced paste or heat to each 2 pads on component - paste appeared to coat really well using plastic stencil.My end results were good but, I would like to improve and I'm wondering if anyone can identify what I'mdoing wrong that causes the resistor and occasional capacitor popping/jumping?I tried several temps on the hot air station tried 200 to 285 in 15 degree increments to see if it helped, but found littledifference other than the low temps took too long to get the paste to reflow, got better results >250 degrees.I will attempt a toaster over (with home made controller with my solder paste reflow profile) in upcoming weeks... but ifanyone can help point out my errors it would be greatly appreciated.Chris

author: fluxcapacitor
author_rank: Frequent Contributor
timestamp: October 19, 2013, 06:22:13 pm
content: If youre blowing parts off the board youve got the air set too high.Set your temps higher, 260 - 300 ,just experiment with the temps .

author: cidcorp
author_rank: Unknown
timestamp: October 19, 2013, 07:03:12 pm
content: I'm pretty sure it isn't air flow, these component seem to pop like the solder paste has bubbles and the release of air causes thecomponent to be projected.Good to know I'm in the right temperature range.

author: fluxcapacitor
author_rank: Frequent Contributor
timestamp: October 19, 2013, 07:30:42 pm
content: Ive had flux popping on reflow but never had a problem with solder paste,i don`t use it much though,i solder most parts by hand.What brand of paste are you using ? ,its got to be the culprit .

author: Rory
author_rank: Frequent Contributor
timestamp: October 19, 2013, 07:33:54 pm
content: I wonder if following the soldering temperature vs time profile would help. Ramp up and then preheat at lower temp for prescribed time to give flux time to activate and to drive moisture off, then raise temp to get solder to reflow.

author: cidcorp
author_rank: Unknown
timestamp: October 20, 2013, 01:08:32 am
content: AMTECH 4300 Sn63/Pb37 (often wondered about the CHIPQUIK label on the top of the jar tho - thought CHIPQUIK is the low temperature stuff).Looks like it has a 2 minute preheat to 140 degrees, I think I'll try what you say and see if it's just caused by moisture.Thanks.Chris

author: Capital
author_rank: Newbie
timestamp: October 20, 2013, 01:38:29 pm
content: A few thoughts,First the jumping chips- This is actually likely caused by a steam explosion. Yes steam! you see most paste formulations are hydroscopic meaning they absorb moisture from the air. Now an hour seems like a reasonable open time but if it is very humid a good bit of water can get absorbed. I have witnessed this in a commercial manufacturing situation when the air conditioning was not working on a very humid day and long open times where the printed boards were sitting around waiting for reflow. Its more than tombstoning- its throwing parts around.Pre-Heat-it serves a few functions. First it can bake out moisture as noted above, but it also activates flux. In commercial manufacturing this pre-heat (soak) takes place anywhere from 30-60 C below the melt temperature. In low budget table top set-ups I have seen a lot of novel approaches to do this. A heat lamp placed under a piece of sheet metal and controlled either by distance or variable transformer can make a great pre-heater. Another way is the use of cheap hot plates and adding a piece of aluminum to provide thermal mass and keep boards away from the electric heating elements which get very hot and would otherwise damage boards. Some effort is needed to get the setting right on the control and don't expect it to be very precise.Both cases require a bit of trial and error but one can ultimately get very good results. In the early days of surface mount some commercial equipment was glorified heat lamps and IR heating elements.

author: fluxcapacitor
author_rank: Frequent Contributor
timestamp: October 20, 2013, 02:57:10 pm
content: Quote from: cidcorp on October 20, 2013, 01:08:32 amDid you buy it from ebay .

author: peteroakes
author_rank: Contributor
timestamp: October 20, 2013, 10:23:26 pm
content: I would agree with Capital , pre heating the board for a few minutes or more should help a lot, one suggestion would be to use the oven in the kitchen (If you don't have the IR lamp or other devices), keep it real low and place the boards on a metal tray to retain the heat when removed and to make it easier to remove all the boards without having to handle them right awayyour hot air re-soldering should then work more reliably without the popping of the componentsIt is always a risk when buying expired materials but usually this should not be an issue (If they where not stored correctly then this could cause an issue), I have never personally experience the popping issue but have used oven heating where boards have a large copper mass and therefor made it harder for the hot air gun to get the required temp on the board without excessive temp or airflow settings.Please let us know if the pre-heating fixes your problem