After you’ve dispensed solder paste it’s time to reflow all that solder. This is where the reflow oven comes in. It seems like reflow hot plates are a more popular choice in most small shops because they’re smaller, cheaper, and it’s easier to monitor the reflow process. A reflow oven is a lot more convenient though, and it follows the recommended heating profile for the solder paste.
A reflow oven is an automated way of soldering boards. Stencil or dispense solder paste onto a PCB, place the parts, then pop it in the oven. 10 minutes later, if you’re lucky, nicely soldered boards come out. A lot can go wrong though. Solder might not reflow, heating may be uneven, boards may burn, and results may generally be inconsistent.
We shipped back a Qinsi QS-5100 from China, this is the same brand used by Seeed Studio and SmartMaker. List price is about $220 for the 2 tube model in Huaqiangbei market, with a larger version 4 tube version available for about $440.
More info and a review follow below.
Sjaak’s RGB LED driver board will be our test subject today, it was part of his Mardi Gras costume. Three FETs with large heatsink pads might not completely reflow, and a DC power connector could deform from excessive heat. Both of these devices have large thermal mass which should provide a good test of our oven and heating profile.
Several people recommend placing the board to be soldered on top of two smaller PCBs in the oven drawer. The smaller boards keep the PCB off the metal grate so it heats more evenly. At least that’s what we’re told, we haven’t tested it without standoffs to compare.
What’s cool about a reflow oven is that they follow a pre-programmed heat profile, ideal soldering conditions and you just push a button. At least in theory it’s “set it and forget it”.
A “temperature-over-time” graph shows how the oven will heat and cool the PCBs. This particular oven has 5 preset profiles and 1 user programmable profile. The presets don’t match the manual so we documented them on the tool list wiki. Presets, as well as language, are set by holding RUN then turning the oven on by pressing the ON/OFF button. Preset 0 is user defined, and the only one we’re tried.
The heat profile is divided in 5 parts: preheat, soak (heat in Qinsi terminology), solder, natural cool-down (keep in Qinsi-eese), and active cool-down with a fan.
Preheat to 150C, hold for 60 seconds. Board and components are heated to 150C to reduce thermal shock as as we move to higher temperatures. Our preferred profile for the cheap “Mechanic” lead-tin solder paste from eBay preheats to 150C and keeps it at that temperature for 60 seconds.
Heat (soak) to 180C, hold for 58 seconds. Next the boards are heated close to soldering temperature to activate the flux in the paste. Our profile heats up to 180C and holds it for 58 seconds. Most paste profiles will call this the soak period, Qinsi calls this the “heat” phase which seems less jingoistic.
Solder (reflow) at 210C, hold for 30 seconds. Soldering is done during the reflow phase (called “solder” by Qinsi). The temperature is brought above the solder’s melting point and kept there for a few seconds.
Cool naturally (keep) to 180C. After soldering, boards are allowed to cool to a preset temp before turning on the cooling fan. Slow cooling gives better joints. We let the oven naturally cool to 180C before starting active cooling.
Active cooling to 150C. When the boards reach 180C the fan kicks in to quickly finish cooling the oven. Cooling usually continues until the oven automatically shuts down, despite the mention of 150C. We usually wait for the oven to reach 70C before removing boards.
The board came out perfectly soldered. There’s bound to be some limits to the machine, and we’ve yet to explore the quality of heating on the drawer periphery, hotspots could still be a lurking problem, but overall this is a fantastic little reflow oven. Well worth the price we paid, and seemingly much better than similar ovens.
The T-962, commonly found on eBay for around $300, is slightly larger than the Qinsi QS-5100. Both the Qinsi and the T-962 use only two infrared heating tubes. Two seem barely enough for the smaller work area of the QS-5100, so we chose it over the T-962. The larger Qinsi with 4 tubes and greater vertical separation between tubes and boards probably heats more evenly. Given unlimited money and space we’d choose a four tube oven instead.
Some mention (and others, and others) that various T-962s exhaust bad fumes, timers are off by double or more (5 minutes becomes 12 minutes), menus are buggy, buttons are not debounced, or that the heat doesn’t ramp up fast enough. These were all nagging concerns we had buying this oven, but we have not run into a single one of these issues.
Next time we’ll look at the reflow hotplate, though we just got confirmation that the TM220A pick and place will be here within a day.