Surface Mounted vs Metal Film resistors

Thread Starter

RAMBO999

Joined Feb 26, 2018
244
Hi

Does anyone know the decision making process behind using a SMD vs metal film resistor on a PCB?

I have a damaged board and I want to work around the damaged area which I think is doable.

One of the damaged components is a 105 SMD resistor.

I want to replace this with a 1M metal film resistor.

Do you guys see any problems with this? Heat and dissipation springs to mind for example?

What do you think?

Thanks
 

Thread Starter

RAMBO999

Joined Feb 26, 2018
244
It is probably a safe substitution.
If you explain the application in more detail maybe you can get a more confident answer.
It's a TV power board with a blown cap that has taken out a few neighbouring components including the SMD. I put a meter across it and its basically open circuit. There is board circuit damage as well. What I want to do is bridge the damaged board and connect components directly to each other and the wire either side of the metal film is ideal for that purpose.
 

kubeek

Joined Sep 20, 2005
5,718
Does anyone know the decision making process behind using a SMD vs metal film resistor on a PCB?
Ehm, what? SMD and metal film are two completely separate things, you can have metal film resistors that are SMD, or through-hole wire wound resistors, or many other types.
 

Thread Starter

RAMBO999

Joined Feb 26, 2018
244
Ehm, what? SMD and metal film are two completely separate things, you can have metal film resistors that are SMD, or through-hole wire wound resistors, or many other types.
Maybe you should read the entire question. Not to mention the follow up.
 

kubeek

Joined Sep 20, 2005
5,718
Maybe you should read the entire question. Not to mention the follow up.
Did that, and still have no clue why you mix two unrelated things. Do you mean to ask if you can replace an SMD resistor with a THT resistor? Metal film or not has nothing to do with that distinction.
 

Thread Starter

RAMBO999

Joined Feb 26, 2018
244
Did that, and still have no clue why you mix two unrelated things. Do you mean to ask if you can replace an SMD resistor with a THT resistor? Metal film or not has nothing to do with that distinction.
Ok. Here's the question "Does anyone know the decision making process behind using a SMD vs metal film resistor on a PCB?"

I will expand :

Electrically they are the same. They both provide 1M resistance when inserted into a circuit. So we can conclude that the decision to use one rather than the other at design time is not because of their electrical properties. Right? So when someone, a circuit designer, needs a 1M resistance in their circuit what criteria do they use OTHER THAN the electical property to decide whether to use a SMD or a MF resistor. Is it heat and heat dissipation? Does a MF get hotter than a SMD? Which we can agree is a serious consideration. Or is it purely the available space on the board that drives the decision making process? A less critical consideration one might think. Perhaps there are other considerations that I have not mentioned here.

Instead of writing all of that I chose to condense it into the question "Does anyone know the decision making process behind using a SMD vs metal film resistor on a PCB?" for the benefit of the members thinking that its meaning would be obvious. It seems it wasn't.

Bearing in mind that what I want to do is replace an SMD with an MF. You can see the reason for the consideration.
 
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Thread Starter

RAMBO999

Joined Feb 26, 2018
244
Most SMDs are metal film. So I still don´t understand what you are asking.
SMD is a kind of package, metal film is how the resistive material is made. Are you asking "SMD vs. THT", or "metal film vs carbon film"? SMD vs metal film makes no sense.
I didn't come here for an argument. Ok?
 

DarthVolta

Joined Jan 27, 2015
394
What ever slight difference there would be (at AC frequencies), shouldn't matter at all on a TV's PSU, go for. It's not worth ordering 1 just for that
 

Thread Starter

RAMBO999

Joined Feb 26, 2018
244
What ever slight difference there would be (at AC frequencies), shouldn't matter at all on a TV's PSU, go for. It's not worth ordering 1 just for that
EXACTLY!! I looked at buying the 105 SMD last night but, apart from not being able to find a 105, you have to buy a boat load. I have dozens of 1M MFs. Besides the location on the board is damaged anyway. Hence the workaround. Thanks for your help.
 

narkeleptk

Joined Mar 11, 2019
442
I didn't come here for an argument. Ok?
I don't think they are auguring with you at all. Only trying to correct a simple mistake about what your asking.
As @kubeek pointed out "metal film" is just one of the the ways a resistor can be made and it can be a smd or tht package. So its not a clear way of asking the question.

The choice between using a surface mounted device (smd) vrs through hole technology (or THT) in most applications is probably for size. You should be safe to swap between them if you have the room to do it. I have on many occasions with no problem.
 

djsfantasi

Joined Apr 11, 2010
6,338
I don't think they are auguring with you at all. Only trying to correct a simple mistake about what your asking.
As @kubeek pointed out "metal film" is just one of the the ways a resistor can be made and it can be a smd or tht package. So its not a clear way of asking the question.

The choice between using a surface mounted device (smd) vrs through hole technology (or THT) in most applications is probably for size. You should be safe to swap between them if you have the room to do it. I have on many occasions with no problem.
Your question, “Does anyone know the decision making process behind using a SMD vs metal film resistor on a PCB?" is similar to the question, “Should I buy a Toyota or a car?”

No one is arguing with you. Some of us are trying to figure out if you mean a Toyota/SMD or a car/metal film?
 

SteveSh

Joined Nov 5, 2019
105
The choice between using a surface mounted device (smd) vrs through hole technology (or THT) in most applications is probably for size. You should be safe to swap between them if you have the room to do it. I have on many occasions with no problem.
Almost all of our products are 100% surface mount technology. The only time we use a leaded part is if that part is not available in a surface mount package. Even then we will attach it to the board with some sort of adhesive, and wire it to pads on the surface of the board.

Size, as narkeleptk point out is one reason for using SMT. But also in many of of our applications the PWB (printed wiring board) is bonded to a metal heat sink on the back side for thermal reasons. Having leads from parts poking through the board does not work with that arrangement.
 

WBahn

Joined Mar 31, 2012
25,556
Ok. Here's the question "Does anyone know the decision making process behind using a SMD vs metal film resistor on a PCB?"
You need to revise your question, as this one is a false dichotomy. I can use an SMD resistor that is metal films and I can use a non-SMD resistor that is not metal film. These are essentially independent aspects of a resistor, so there's no way to know what it is you are really asking about. Ask yourself the following questions to help you gain clarity:

"If I have to choose between two metal film resistors, one of them through-hole and the other surface mount, what factors would help me decide which to use?"

"If I have to choose between two surface mount resistors, one of them metal film and the other some other type, what factors would help me decide which to use?

Which of those two questions is closer to what you are actually trying to learn about? Or can you craft a still better one.

I will expand :

Electrically they are the same. They both provide 1M resistance when inserted into a circuit. So we can conclude that the decision to use one rather than the other at design time is not because of their electrical properties. Right?
Wrong. Nominal resistance is just ONE of MANY electrical parameters that a resistor has. Any component is going to have parasitic properties such as inductance and capacitance. In addition, each of these three parameters (R, L. and C) are going to have tolerance and stability characteristics. For instance, the resistance will change with temperature and different types of resistors will change in different ways as the temperature changes. It may also change due to other factors such as humidity. They will also change with mechanical stress, perhaps even to the point that they will act like microphones in some circuits.

So the circuit designer may have to determine which characteristics are important for the circuit they are designing and choose accordingly. In many situations it doesn't matter and we just need some resistance that is roughly some value and we don't care too much if it changes by quite a bit as the circuit operates. That same circuit may well have resistors in it that we care very much about how it behaves under different conditions. Usually we are trying to minimize the impact of those parasitic changes, but sometimes it is those changes in parasitic values that we are expressly trying to exploit -- that is how many sensor technologies work. Consider a strain gauge circuit -- if we used resistors that were real "good" about not changing value no matter how much we deformed them, we would defeat the entire point of the circuit!

So when someone, a circuit designer, needs a 1M resistance in their circuit what criteria do they use OTHER THAN the electical property to decide whether to use a SMD or a MF resistor. Is it heat and heat dissipation? Does a MF get hotter than a SMD? Which we can agree is a serious consideration. Or is it purely the available space on the board that drives the decision making process? A less critical consideration one might think. Perhaps there are other considerations that I have not mentioned here.
The number of considerations that might need to be taken into account is almost unlimited and depends very highly on the specific application. Some very common considerations are power dissipation, physical size, and cost. But others might be radiation hardness (how much does the resistance change when exposed to long duration of radiation, such as for space flight hardware), corrosion resistance, thermal expansion coefficient, operating temperature range, and the list goes on and on. For most applications there only a small number of factors that are really important and so you look for the type of component that performs well against those factors and check if you can live with how they perform against any others that are of possible concern.

Instead of writing all of that I chose to condense it into the question "Does anyone know the decision making process behind using a SMD vs metal film resistor on a PCB?" for the benefit of the members thinking that its meaning would be obvious. It seems it wasn't.

Bearing in mind that what I want to do is replace an SMD with an MF. You can see the reason for the consideration.
Not really. Why are you wanting to replace the resistor in the first place?
 

SLK001

Joined Nov 29, 2011
1,543
Why are you wanting to replace the resistor in the first place?
Already explained.

Let me answer this: Yes, you can make the substitution.

Sometimes there are reasons to use a particular resistor kind (carbon, carbon film, metal film, wire-wound), but in this case there isn't one. The SMD was selected because of the board and assembly requirements. If you can fit a THT part where the 105 SMD was, you will be okay - your power supply will not know the difference.
 

Thread Starter

RAMBO999

Joined Feb 26, 2018
244
Almost all of our products are 100% surface mount technology. The only time we use a leaded part is if that part is not available in a surface mount package. Even then we will attach it to the board with some sort of adhesive, and wire it to pads on the surface of the board.

Size, as narkeleptk point out is one reason for using SMT. But also in many of of our applications the PWB (printed wiring board) is bonded to a metal heat sink on the back side for thermal reasons. Having leads from parts poking through the board does not work with that arrangement.
Thanks for your input. That's great. I was only concerned really about heat differences to be fair. That seemed like the only plausible reason for opting for one over the other at design time. It's not an unreasonable concern when repacing one with the other. It's worth seeking a second opinion.
 
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