SMD Solder Pads - Is it possible to share pads?

Thread Starter

Ford Prefect

Joined Jun 14, 2010
173
I am curious. I cannot seem to find the answer on the Internet but...
Because there is a lack of space on the PCB I am designing I want to put a row of SMD (1206) LED's on the PCB.
The supply will be 12vDC and will be available to supply to the LED's.
I will therefore need to put SMD (1206) resistors in series for each of the LED's.
My question is:
Is it possible to share the SMD pads so that one SMD pad is shared with a resistor and LED?
See photo..
The red squares are SMD solder pads.
The top row of pads are for one side of the resistors.
The bottom row of pads are for one side of the LED's
The centre row of pads are 'shared' for the other side of the resistor and other side of the LED's
I understand that it may be tricky to solder between the resistors and LED's but...Can this be done?

ResLED.JPG
 

kubeek

Joined Sep 20, 2005
5,657
The ultimate question is what or who is going to solder the components. With hand soldering it is easy, you solder the outer pads first and when you have both components you solder the middle pad. With reflow it most likely will not work correctly, with wave soldering it might be ok.
 

SLK001

Joined Nov 29, 2011
1,543
What you are really doing is overlapping pads - which should give you a warning (or error) depending on you DRC settings and your PCB package. It is obvious that it can be done, but the question is "should it be done"? If this is a production part, then the answer is a resounding "NO"! If you are soldering by hand, then the answer is "MAYBE" - it depends on who is doing the soldering.

But it would be better to redesign the circuit so that you only need one resistor and current share all the LEDs. Since all LEDs are getting the same current, you probably won't have to use resistors to balance the outputs. It you LEDs are WHITE or BLUE, then you will have all four in one string. If RED, GREEN or YELLOW then 9 or 10 can be in the same string (depends on your Vf of the diodes).
 

ebeowulf17

Joined Aug 12, 2014
3,040
I am curious. I cannot seem to find the answer on the Internet but...
Because there is a lack of space on the PCB I am designing I want to put a row of SMD (1206) LED's on the PCB.
The supply will be 12vDC and will be available to supply to the LED's.
I will therefore need to put SMD (1206) resistors in series for each of the LED's.
My question is:
Is it possible to share the SMD pads so that one SMD pad is shared with a resistor and LED?
See photo..
The red squares are SMD solder pads.
The top row of pads are for one side of the resistors.
The bottom row of pads are for one side of the LED's
The centre row of pads are 'shared' for the other side of the resistor and other side of the LED's
I understand that it may be tricky to solder between the resistors and LED's but...Can this be done?

View attachment 179338
Just eliminate all the dog leg shapes and raise the resistors up so their pads are where the horizontal trace is now. Then you'll have plenty of room.

I had the same question recently and kubeek's answer perfectly summarizes the advice I got.
 

MisterBill2

Joined Jan 23, 2018
4,847
For typical 3 volt LEDs and a 12 volt supply you can put 3 in series and one resistor to limit the current. That cuts the soldering effort a bit and saves money on resistors, in addition to being more efficient and wasting less power.
My answer about soldering repeats what the others have said, although if all the parts are glued down then it may even work with some automation systems. Almost anything can be hand soldered by somebody.
 

Tonyr1084

Joined Sep 24, 2015
4,028
I've seen it done before. Not good practice however. Two issues come to mind; 1) tombstoning - where a part will stand up on edge leaving the other edge of the component flying in mid air. 2) repair - what happens when you need to remove one part? You're exposing the other component to double the heat. Remember, when you heat one pad you're heating two components (on the center pad). That extra heat excursion could cause latent damage that may not manifest itself right away. So when it does manifest, you repair it, exposing the other component to two extra heat excursions. First excursion is when you desolder the defective component, the second is when you solder in a new component. So the unoffending component has seen at least three heat excursions; one when it was built and two more when its neighbor was repaired.

That's my take on it anyway.
 

MisterBill2

Joined Jan 23, 2018
4,847
My observation is that SMD circuit boards are usually replaced, not repaired. Of course there are exceptions for some high value products, but mostly not. Just look at the relative number of TV and appliance repair shops compared to pre-SMD construction. Of course there are a number of causes, but the fact is that mostly PCB assemblies are scrapped and not repaired.
 

Tonyr1084

Joined Sep 24, 2015
4,028
@MisterBill2 Did a job for Vivint. Six weeks cutting a trace, adding a capacitor over that cut trace and swapped four SMT resistors. Quite small but not the smallest. Don't remember the size; 4020 maybe? Don't remember how many boards were needed but there were a lot. Worked with two irons at once.
 

MisterBill2

Joined Jan 23, 2018
4,847
@MisterBill2 Did a job for Vivint. Six weeks cutting a trace, adding a capacitor over that cut trace and swapped four SMT resistors. Quite small but not the smallest. Don't remember the size; 4020 maybe? Don't remember how many boards were needed but there were a lot. Worked with two irons at once.
Probably it was not a production run of ten thousand boards. Or maybe it was. Using two irons is certainly the very fastest and probably the neatest way to remove parts on many circuit boards. My guess is that by the time you were done with the job you were really good at it. AND those 0204 parts certainly are small. And I know that the magnifier was your best friend.
 
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