I think that the 4C still might be from an older reel. All of the newer bins seem to be have higher forward voltages.
Ahhh - could be. I know Simon's and yours were high amps - maybe both from older reels. The way supplies go, seems like CREE releases different bins in batch's - never seems to be a good selection available at the same time. Still the T5 4C's were older batch's - still got one. The T6 4C's came later, but still could be older than these newer U2 neutrals and U3's/U4's.
Beautiful reflow job!
Any chance you could be talked into posting a picture of a board with the solder paste on it before the reflow? I’m starting to put together boards now and I’m still feeling out how much solder paste should actually be on there. Whatever you’re doing is the way I want to be doing it, it looks great.
The OP has a pic of what you want - I think. It's probably too much paste on the pads for the MCU. I've found I was over-doing it on those pads, and then you end up getting solder pooling on the top of the MCU pins, whcih seems it will make it difficult to program with the clip at times.
Also, I've learned if you use hot air like I do, you really want to pre-heat or heat the boards from below - the solder flows better. That's why I use a coffee mug warmer to sit the boards on when I use my hot air gun. The warmer provides bout a perfect 80-100C temp or so.
Paste? Minimum, unless you have some strange paste. If you are doing many PCBs then its best to get a sheet that is cut out where the paste is to be applied. Often made from this high temperature material also used for tapes. Brown/copper nearly see throughm, sorry forgot the name but it’s an old NASA kind of stuff 
You just wipe the pate over it and the paste gets only to places where it’s supposed to.
Same as soldering LEDs to MCPCBs minimum paste is enough 
Do a board and see, if it looks too much to you, then reduce. If you get bad connections due to lack of solder, then add more. In general minimum paste is fine for SMD soldering.
Personally I heat up a small piece of copper 40x20x3mm with a soldering iron, the driver or MCPCB is placed on the copper. Works even for soldering stacked copper discs or copper to brass pills.
The heat builds up from below unlike when using hot air that is more useful for patching up than reflowing.
Stencils - yes, some guys use them - I don't and others don't as well. With the rate of change these boards go thru, I didn't want to invest in stencils. I've had a high rate of success doing the solder paste by hand in my small batch runs (3-6 boards or so). I know stencils is better - faster and more accurate with the amt of solder.
I've tried stencils on several different boards and for me I don't really like them--especially on boards like these that have fairly large pads and a small number of pads at that. The setup, cleanup, and mess of using a stencil just isn't worth it IMO.
I use stencils almost exclusively and love how even and precise the amount of solder paste is with em.
Having to do more than one by hand is a PITA, in my opinion… Just built a panel of 20 with no stencils and about went bonkers!
That's why I like this forum: you get a good debate from differing opinions. Dale and I are friends, but anyone who has followed us long enough will notice that we often have opposite opinions about a lot of different things.
A question for the driver gurus;
I'm planning on using four XHP70's for my DIY comp build.
If I were to piggyback 3 or 4 FET's on the same board (Older v1.0 type board, DPAK-2 FET), will the 13a have a strong enough drive signal to operate them all?
Unless you're only planning on a 100% mode level with a big gate resistor, no. We're already pushing the little attiny pretty hard switching one of them at the faster PWM levels. You'll find that the attiny will glitch out big time trying to drive multiples in the middle levels, and even a single FET sometimes.
I think you could get away with a single FET for that build with a little creativity: use a small PFET to power the NFET gate directly from the 8V+ input. It will load the attiny way less and will give the FET access to higher gate voltages, which will decrease the resistance of the FET. You could also easily power more than one FET off the same attiny, if needed. You would need to basically invert the PWM values, since the PFET turns on when the gate is pulled low.
It is a crazy untested idea, but in theory it should work. I don't think you need a dedicated gate driver IC since we're switching relatively slow.
Thanks Richard.
I can see this quickly getting over my head.....
Using the method you described, are you suggesting that all four XHP70's could be powered by a single DPAK-2 FET?
So the flow for the PWM signal would be;
Attiny pwm---->PFET gate, PFET drain----->NFET gate.
Can you point me in the direction of an appropriate PFET for this application?
Hmmm - I'm having difficulty understanding how I'd wire this up.
Am I on the right track here? (I realize it's not the Zener schematic)
Yep. That works. You can use a very small PFET, just use one that has sufficient voltage ratings. You may also need a pull up resistor to keep it off, but you'll have to figure that out in use.
Is there a reason I can't use the LFPAK56 or DPAK-2 in lieu of the PFET in that diagram?
Or even a 7135 ?
Thanks.
Do you understand the difference between a P channel and N channel FET?
I tried reading up on them, but I'll just accept a "no", & move on to locating a suitable P channel.
All I know is that the FET doing the main switching is rather loaded with a lot of current, so the AtTiny is having dramas moving the switch(NFET), so we are using an intermittent switch (PFET) to lend a hand.
I know that's probably the least technical description, & will make someone who knows what they are talking about cringe.... ;)
-- edit --
I now understand enough to see why the N channel would not work.... from a Positive feed.
Will this one do the trick?
Transistor Polarity: P Channel
Continuous Drain Current Id: 11A
Drain Source Voltage Vds: 200V
On Resistance Rds(on): 500mohm
Rds(on) Test Voltage Vgs: -10V
Threshold Voltage Vgs Typ: -4V
Power Dissipation Pd: 125W
Package: TO-220
Pulse Current Idm: 44A
I'd get one with a much lower voltage rating (you'll get better overall specification for your application). I'd look at an SOT-23 package. That 44nC gate charge is really high and you don't need a huge TO-220 package.