SMD Reflow , solder paste dispensing , stencils

How do you generally apply solder paste , I had ordered around 550 pcb's from oshpark for a small project i was doing , each board is a 1"x1" and has 9 smd components, SOT-23 , 0805 and LFPAK33 packages, i can populate them alright but they are a b*tch to apply solder paste one by one, on a good day i was able to make 25 of them in about 4 hours.. and my hand hurts from the make-shift 20cc syringe needle..

someone could make me a stainless steel or mylar stencil please. i have the .brd files for Eagle.

Any help / suggestions are appreciated.

Why do you need solder paste for those packages? They can easily be soldered manually using temperature controlled iron with suitable tip and solder wire. Is Pb-free process (RoHS) a necessity/requirement?

There is no Stencil making setup in Pakistan. These stencils are usually made with a high precision LASER cutting of steel sheets of desired thickness.I often do SMT PCB assembly but I get my stencils made from abroad. This may cost about $300-350 which will be very expensive for you.

You have two options

1) Etched stencil using thin aluminum sheet (like from a can of Pepsi)

2) Lasercut Mylar or some other sheet of suitable thickness.

If you are interested in option #2, I might help since I have a Lasercut machine which is able to cut plastic/mylar sheets. I have made and used some stencils like you are looking for but those were for LEDs which have big pads and pad to pad spacing is adequate. I doubt if it can work for LFPAK33 package since pads are very close to each other.

Even if you get the stencil, how do you plan the reflow soldering. Have you got some kind of well controlled oven ? The solder paste needs to be subjected to a manufacturer recommended temperature vs time profile in order to give best results. The hot air or fixed oven soldering joints often fail in the long run.

[quote=“Asad, post:2, topic:21975”]

Why do you need solder paste for those packages? They can easily be soldered manually using temperature controlled iron with suitable tip and solder wire. Is Pb-free process (RoHS) a necessity/requirement?

[/quote]

I guess i can if nothing else works, its not like i didnt try , took me half a day to reflow and program 25 of those suckers, still a lot more to go so im trying to improve the process with atleast a stencil and i prefer not to lose the rest of my vision :(

[quote=", post:3, topic:21975"]

There is no Stencil making setup in Pakistan. These stencils are usually made with a high precision LASER cutting of steel sheets of desired thickness.I often do SMT PCB assembly but I get my stencils made from abroad. This may cost about $300-350 which will be very expensive for you.

You have two options

1) Etched stencil using thin aluminum sheet (like from a can of Pepsi)

2) Lasercut Mylar or some other sheet of suitable thickness.

If you are interested in option #2, I might help since I have a Lasercut machine which is able to cut plastic/mylar sheets. I have made and used some stencils like you are looking for but those were for LEDs which have big pads and pad to pad spacing is adequate. I doubt if it can work for LFPAK33 package since pads are very close to each other.

Even if you get the stencil, how do you plan the reflow soldering. Have you got some kind of well controlled oven ? The solder paste needs to be subjected to a manufacturer recommended temperature vs time profile in order to give best results. The hot air or fixed oven soldering joints often fail in the long run.

[/quote]

Ijaz bro, thanks as always your suggestions have been extremely helpful.

1. Seems doable, but a lot of corrosive chemicals involved i wouldnt mind doing it unless there is no other option.

2. Mylar should work fine, are you going to make me one ? what do i have to do sir

I have hooked up my multimeter temp sensor to a toaster oven and it has served me well in the past

I can try making one for you, no problem.

I will need a Gerber file of PASTE layer or the SOLDER MASK layer to proceed further.

When I have to do small sized PCB assembly, I always make a panel of many PCBs. This makes life very easy in SMT assembly.

How well experienced are you with manual soldering and what kind of equipment and soldering wire are you using? 550 1"x1" PCBs with 9 components each aren't that many. If you are having trouble, you might want to try no-clean flux. Alternatively, A/MA flux could also be used but would require the PCBs to be washed with appropriate solvent after soldering. It would ease the manual soldering process if you go that route.

[quote=“Asad, post:6, topic:21975”]

How well experienced are you with manual soldering and what kind of equipment and soldering wire are you using? 550 1"x1" PCBs with 9 components each aren't that many. If you are having trouble, you might want to try no-clean flux. Alternatively, A/MA flux could also be used but would require the PCBs to be washed with appropriate solvent after soldering. It would ease the manual soldering process if you go that route.

[/quote]

I thought so too, but took me entire day to finish a batch of 25 , I have a 63/37 mechanics , all purpose solder paste and RMA-223 amtech flux (generic) chinese bga rework stuff, IPA (isopropyl alcohol) as a solvent.

[quote=", post:5, topic:21975"]

I can try making one for you, no problem.

I will need a Gerber file of PASTE layer or the SOLDER MASK layer to proceed further.

When I have to do small sized PCB assembly, I always make a panel of many PCBs. This makes life very easy in SMT assembly.

[/quote]

In all honesty i would hate to be a bother , your suggestions and expertise is very much appreciated.

I got a few quotes in for a SS stencil from aliexpress not nearly as expensive as i had thought.

$16.00 , and $7.00 shipping.

30*30mm SS Stencil , without frame. 1:1:2 aspect.

What stencil thickness do you recommend and percentage , and they asked if i needed a frame.. not sure if I do..

P.S: Just realized i mentioned incorrectly , the Mosfet is LFPAK56 and not 33. There is a 8-pin MCU in a SOP-8 package and rest of the components are 0805.

1) 30x30 size is too small to handle while trying to print 25x25mm PCBs. You need to have a more bigger (like 100x100mm) size to handle the printing.

2) You don't need a framed stencil, just a plane SS sheet. Framed stencils are only used in Automatic Solder Paste printing machines.

3) Regarding thickness, for the components you have stated, 150 um should be OK. For fine pitch parts (like 0.5mm QFPs) 125 um is usually OK.

4) 1:1:2 Aspect ? I am not sure about it. I never used any Chinese source for stencil.

PCB's are 17.02mm x 17.02mm so i went with a smaller one, there were options for 50mm and 100mm but i figured its a thin sheet of metal smaller will be more stable/rigid..

SA,

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We give 10% discount for new customers, students and startups. Feel free to send your enquiries and check out our PCB assembly calculator and SMT Stencils calculator online, visit http://www.taraztechnologies.com/

Thanks and kind regards.