SMD reflow hotplate

this morning i did a last test-run with the tweaked Felder ISO-Cream profile:

yeah... at the top i thought it is in the cooling step already and opened the window - with ~3°C cold air from outside it dropped fast.. then i found it is in the middle of the reflow - sorry... and closed the window again - until it really switched to cooling..

the old left-over pcb i use for these is done now.. i comes from my LEDBoard_4x4_16bit project - and if i remember correctly i backed it while assembling the  boards in the oven multiple times back then.. now grilled it again ~4-7 times. it smells very bad - is super dark discolored.. i think that is ok with about ~12 solder cycles..

and then started to assemble a simple board to really test the profile :-)

then reflowed:

i added a paper-lid to have stable air inside..

reflow was successful :-) my profile is just a little bit to long for my right angle touch switches:

they melted away :-( - lesson learned - have a look in the datasheet and you know that they are very heat sensitive!

in general i have the feeling that my heating elements get a little bit to hot - the pcb also slightly discolored at on place...
so will keep an eye on this and improve it..

Open Points

• add housing
  • i would like to have class at the top for a good view what is happening inside..
• metal frame for heating-elements
• quite 5V fan with PWM control for cooling
• add second temperature sensor
  • contact-less melexis MLX90614ESF ??
• spring thing to hold board down
• way to fix sensor position on board
• more heating elements for bigger working area
  • switchable configuration for long or more square pcbs?!
• bigger power supply ?! (~750W)

for tuning i followed more or less the tutorial PID Without a PhD from Tim Wescott and the tutorial and video from PID Explained Team.

first i just checked with low temperatures of 20..40°C as i went on and tested up to 260°C i noticed that the current did decrease. and the temperature did not increase any more. i could see this in my graph as the heating got slower and slower with the rising temperature… (also the pid already saturated at the output..)

so i measured the resistance during the cool down of the heating elements to get some insights: (4x in series → 48V/4=~12V/Module)

Resistance (Ohm)	Power @48V (W)
40	57
39	58
38	59
36	60,5
38	60,3
34	64,8
26	88,6
24	96
22	104
20,9	110
19,5	118

result: the ~57W is not enough to get to more than 255°C…

i rearranged the Modules into 3-in-series connection. this means ~16V/Module – and tested again:

Resistance (Ohm)	Power @48V (W)
27,5	70
27,0	72
26,6	77
25,6	80
25,3	82
24,1	86
18,8	100
17,5	130
16,7
15,6
14,3

with this i found that i can go above 255°C.

i then tested the profile for the Felder ISO-Cream “Clear” and found that in the reflow stage the heat-up is a little to slow:

in the *my setup* picture is a temporary cardboard thing with a 80mm 12V fan (connected to 5V) to cool down faster between tests. for the final setup i think i will buy 1 or two 5V and PWM capable fans…. and also exchange the *chamotte* ston with some metal frame. this way i also can cool the bottom side..

so i again switch the configuration – now i have a 2-in-series config: 24V/Module CURRENTLY THIS TABLE IS ONLY CALCULATED VALUES!!

Resistance (Ohm)	Power @48V (W)
20	115
19,5	118
19	121
18	128
17,5	132
17	135
13	177
12	192
11	209
10,45	220
9,75	236

i also tested this with the Felder profile:

this time the heat-up is fast enough! 🙂 the nice and working pid tuning i had for the 4-in-series arrangement is now out of tune… so i will have to re-tune it to get less overshoot / swing.

while having a break i thought about the maximal power in this configuration – and found that this way i only be able to power 2×2 modules with my 250W power supply. for now i leave it this way. in the long run i hope with the other frame concept i get more heat to the pcb and less into the stone and this way be able to use the 3S config.

Tuning

after a day of mostly waiting til the system cooled down again – one test cycle <=60°C needs 400s → 6:40min – i just rebuild my hw mounting setup.

this way i can warm up quicker and cool down much quicker as i do not store heat in the stone. – at least that is what i hope..

hmmm – does not seem to change much..

i then tested the actual Felder Profile:

seems i have a working profile. i will add a little more time for the prepare phase. so the pcb is really fully at the 50°C. at the top i have a little bit of a mis-match – i saw on my temp sensor directly connected to the heating elements at the top ~265°C – so that is hot… the pcb seems to increase its temperature resistance at higher temperatures… at the peak i have 230°C to 245°C error. and to the heating this results in ~35°C difference…

i will report when i solder the first real board. 😉

there was a response to my request in the forum: with the links to

• this CircuitPython Toaster Oven project
• B3AU micropython PID.py

i have created a [new repository for my cp_reflow_controller – with this example on hand i will use a adafruit PyBadge as main controller – and use the EZ-Make-Oven as guide how to design the software… (all the parts i have on hand are a little bit different….)

As temperature sensor i use the Adafruit MAX31855 Thermocouple breakout board

parts have arrived and i have build the hw..

and over the holidays i also have written the firmware...

and just *along the way* written a CircuitPython library for nonblocking user input...

with this setup i can experiment with the PID...

the idea came from this Applied Science Video: Electroluminescent paint and multi-channel control circuit 21 Nov 2018 starting at 11:25

there is a link to amazon for a element – and it is not available to delivery to germany 🙁 so i went on with some help of friends and found

voltage	current	power@ 6V	power@ 12V	link
12V	5,8A	17W	70W	HALJIA 12V 70W Wired MCH Metal Ceramic Heating Plate Heating Element 70mm x 15mm
24V	4,6A	7W	27W	Haljia 24V 110W Wired MCH Metal Ceramic Heating Plate Heating Element 70mm x 15mm
12V	4A	12W	24W	Haljia 12 V48 W Wire MCH Metal Ceramic Heating Plate Heating Element 40 mm x 40 mm
24V	4	24W	48W	Haljia 24 V96 W WIRED MCH Metal Ceramic Heating Plate Heating Element 40 mm x 40 mm

to get an idea of how much power i actually need i had a look at the small commercial IR-Heaters and Hot-Plates – they all have about 800W:

180mm * 235mm = 42300 mm² = 423 cm²
a = 60mm * 60mm = 3600 mm² = 48 cm (1x4)
b = 60mm * 80mm = 4800 mm² = 48 cm (2x2)
c = 60mm * 90mm = 5400 mm² = 54 cm² (1x6)
d = 60mm * 120mm = 7200 mm² = 72 cm² (1x8)
e = 120mm * 60mm = 7200 mm² = 72 cm² (2x4)
f = 120mm * 90mm = 10800 mm² = 108 cm² (2x6)
g = 120mm * 120mm = 14400 mm² = 144 cm² (2x8)

423 cm² == 800W
1 cm² == x

x = 800W *   1cm² / 423cm² = 1,89W
a = 800W *  36cm² / 423cm² = ~68W (1x4) 
b = 800W *  48cm² / 423cm² = ~91W (2x2) 
c = 800W *  54cm² / 423cm² = ~102W (1x6)
d = 800W *  72cm² / 423cm² = ~136W (1x8)
e = 800W *  72cm² / 423cm² = ~136W (2x4)
f = 800W * 108cm² / 423cm² = ~204W (2x6)
g = 800W * 144cm² / 423cm² = ~272W (2x8)

30x40mm: ~23W/module
60x15mm: ~17W/module

then i calculated the resistance of the found element to check on what wattage i can do at what voltages:

U = R*I
P = U*I
→ P = U*(U/R)

(i added these *guesses* in the table above)

So I decided to go with the 70x15mm 24V model. and will update here if i found how this works out..

and for the first test setup i will go with the concept 12V→ 27W / module so definitive more then enough..

as power supply i will use a MeanWell GST280A48-C6P (reichelt) with an fitting connector (reichelt) to get a 5V for the controller i will go with a recom R-78HB50-05 (VIN: 9-72V) and for switching the power to the heating elements i will use IRLB4030PBF – MOSFET N-LogL 100V 180A 370W 0,0043R TO220AB and to drive this a BC 550C as mentioned in this nice article: Schalten und Steuern mit Transistoren III – Mit MOSFETs höhere Ströme schalten

so when all the parts arrive i can go on.. with building.

for the Controller i plan to write it in CircuitPython and run it on an adafruit ItsyBitsyM4. and maybe later add an LCD – or use a PyBadge – for now i just want to use the arduino serial plotter or similar with an second CDC-device enabled to log the progress and the flash-drive function of CircuitPython for a text-file with the temperature-profile. i have written a request in the adafruit CircuitPython forum if there are any PID controller things out there…