Not sure I would agree with this premise.
I had 100 or so boards done my usually ok local pcb maker/assembler. It took me over two weeks to go round all 28 pins of the QFN on each board under the mircoscope and hand check/fix many of the connections to get the boards to work. This is after giving him £1,500 for his trouble to use his supposed good kit, ie Auto P&P and Vapour Phase Reflow setup etc.
I vowed after this experience that if I was going to fix mistakes it might as well be my own rather pay for some one else to produce them for me.
I have invested a modest amount of money and have been able to get reasonable protoyping and small batch production capabillities.
I have “toaster” oven reflow with PIC based profile controller, works great and gives good results on Lead Free (cost ~= £100).
I produce D/S prototype PCB’s with 0.6m vias and 10thou track/space, no problem with bubble etch tank and home made UV box (now 15yrs old). (Cost £300)
I have old but servicable Auto P&P machine (cost £300). It does QFN and 0603 fine but “looses” about 30% of 0402’s if pressed.
I have new ERSA I-CON soldering iron, (cost £280), this has been an invalubale investment and worth every penny. For leadfree SMT soldering works great.
Bausch&Lomb stereo zoom boom microscope, (cost £120), can’t see how I would manage without one.
I have £120 IR rework station.
I have cheep manual stencil printer.
I have “home made” microplace station which can place chips to within a 2 or3thou using stero microscope vision and x,y,z & theta micrometer adjsutment table.
I have found that using mylar laser cut stencils I can easily produce boards with stuff down to SSOP chips and 0402’s.
I have to admit doing QFN’s has been a challange on the pasting front. Hence my enquires here as to why its going wrong for me.
At then end of the day it should be matter of getting the paste printing process under control, which I am not sure mine was. But other people manage this so why should I not be able to learn how ?
If its only possible to paste print QFN type boards in dust free air conditiondion workshops with auto screen printers costing $20,000 then perhaps someone will stick there neck out and say so.
My enquiry here is to ask if its reasonable possibly how have other people faired and where did I go wrong ? If the answer is its only possible if “your a proffessional” then I have to say my direct experience is even they can get it wrong :!: .