At What Point is SMT Worthwhile?

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thermionic

Well-known member
Joined
Jun 3, 2004
Messages
1,671
Hi,

Recently, I've seen highly specialist items that I know will be sold in low volume, assembled entirely in an SMT format. How are they doing this if they are only likely to sell a handful of assembled PCBs a year? I would've thought it'd be far more prudent to make such items via hand-soldering? Arethey hand-soldering the parts? Some of the items I've seen have hundreds of parts, and SMT by hand would take far more time than TH parts.

Because TH components are getting phased out in some ranges, are some of these specialist items being forced into an SMT format because that's the only way to get certain ICs etc now? 

At what point did you go to SMT with your widget? Did you apply a rule of thumb?

How do companies make SMT prototypes that have hundreds of parts? Do they just poney-up for a one-off set up cost? Maybe they have their own P+P machines? Have you seen these at any small OEMs?

NB - I realise this is more of a mechanical / financial question than anything, but considering this is the Drawing Board, I'm going to presume that all aspects of design can be discussed here.
 
thermionic said:
Are they hand-soldering the parts? Some of the items I've seen have hundreds of parts, and SMT by hand would take far more time than TH parts.
For me hand soldering 0805/SOIC-size SMD parts is easily twice as fast as TH parts. If I get to use a solder stencil plus tweezers, placement speed doubles again. And I'm no spring chicken.

thermionic said:
At what point did you go to SMT with your widget? Did you apply a rule of thumb?
I don't believe I've designed a TH-only board this century. Main reasons for going SMD are component availability, loop area, less leadframe/package parasitics, easier routing/decoupling, faster/cheaper assembly, easier process control... Of course, through-hole parts still win for most connectors where mechanical ruggedness matters.

thermionic said:
How do companies make SMT prototypes that have hundreds of parts? Do they just poney-up for a one-off set up cost? Maybe they have their own P+P machines? Have you seen these at any small OEMs?
We have a semi-automatic P&P-machine which hardly sees any use. Proto work happens almost exclusively by hand, for boards up to 600 parts. Our full auto P&P, on the other hand, is still running unattended after everyone has left the lab.

JDB.
[in my current design the connectors, the only through-hole parts, are giving me the most headaches as they have to be hand soldered. We did some experiments with pin-in-paste, but we found that would require a respin of our panels as our V-cut tool can't depanelize the boards once the connectors are mounted]
 
thermionic said:
Recently, I've seen highly specialist items that I know will be sold in low volume, assembled entirely in an SMT format. How are they doing this if they are only likely to sell a handful of assembled PCBs a year? I would've thought it'd be far more prudent to make such items via hand-soldering? Arethey hand-soldering the parts? Some of the items I've seen have hundreds of parts, and SMT by hand would take far more time than TH parts.

As JD notes, hand-soldering SMT parts is generally faster than the same for through-hole parts. All you need is a magnifying lens and a pair of tweezers. Put a drop of solder on an 0805 resistor pad, then using tweezers place the resistor on its footprint, heat of that drop of solder to attach the part, then solder the other pad. Done. No board flipping, no snipping excess leads, no holding the parts in place as they're upside down.

Because TH components are getting phased out in some ranges, are some of these specialist items being forced into an SMT format because that's the only way to get certain ICs etc now?

I'm sure that has a lot to do with it. If you're doing machine assembly, you want all SMT because the through-hole stuff is an additional assembly step that isn't free. Also, once you're forced into using an SMT package (say you choose a modern microcontroller), then doing one part SMT and the rest THT really doesn't make sense.

At what point did you go to SMT with your widget? Did you apply a rule of thumb?

SMT for easily 20 years here.

How do companies make SMT prototypes that have hundreds of parts? Do they just poney-up for a one-off set up cost? Maybe they have their own P+P machines? Have you seen these at any small OEMs?

At my day job, we are a small OEM, and we have some boards that do have hundreds of parts. Prototypes are all hand-assembled, even the BGA-packaged processors and FPGAs.  When PCBs are ordered, we also order a stencil. The board and stencil go into a jig, solder paste is spooged onto the board, and then parts are hand placed and the board goes through the reflow oven. Smaller boards will often be hand soldered. We don't have a P&P machine. The pros and cons of having one has been discussed, and so far the time to assemble boards by hand is faster than the time to set up the P&P machine for prototype quantities.

My home projects? With the exception of connectors, everything is surface mount.

-a
 
Andy Peters said:
At my day job, we are a small OEM, and we have some boards that do have hundreds of parts. Prototypes are all hand-assembled, even the BGA-packaged processors and FPGAs.  When PCBs are ordered, we also order a stencil. The board and stencil go into a jig, solder paste is spooged onto the board, and then parts are hand placed and the board goes through the reflow oven. Smaller boards will often be hand soldered. We don't have a P&P machine. The pros and cons of having one has been discussed, and so far the time to assemble boards by hand is faster than the time to set up the P&P machine for prototype quantities.

My home projects? With the exception of connectors, everything is surface mount.

-a

Andy stole what I would have said.  You can get a laser-cut stencil for about $10-$15 nowadays (even as one-offs).  It will allow you to put solder-paste on every pad on your board at once, and then stuffing the board is simply tweezering the component onto the pads, where the solder paste will hold it.  Bake in a reflow over and ZAP! Your board is completely stuffed.  It's about an order of magnitude faster for doing 4-5 boards at a time:  my record is I did 10 large-ish boards measuring about 4"x5" in the time it takes me to do a single PTH board, since you can stuff in each component on to all boards at the same time (e.g. place all the 0.1uF caps, then all the 10k's, etc).
 
Yes, there are small OEMs with P&P machines...... I am one of them :)

The path to personal P&P has been this.....

My first heavy-duty experience of SMT in the pro-audio production environment was in the mid-1990s when I was a contract designer at a West Yorkshire based console manufacturer that was transitioning over to SMT. The advantages were huge for a mass-producer. To gain further personal experience of SMT, I started regularly doing my own designs with SMT and hand-placing & hand-soldering.

Moving forward 15 years to 2007, there was an increasing amount of work that I was doing that required SMT (very little of it audio), and my remit was to design and build. I decided to investigate getting my own SMT facility set up...... and said goodbye to several years-worth of very hard-won savings, plus a bit more. The facility comprises Contact Systems CS3 P&P machine, >100 component feeders, solder reflow oven, paste printer, visual inspection equipment, and specialised air compressor (for the P&P).

Now for the detailed reply......

One-off and small quantity board assembly is possibly, but there are balancing factors. The biggest amount of time is in setting-up the P&P machine. There are a core number of types of components that are common across many designs (10k & 100k resistors, TL072, NE5532, 22uF, 100uF, etc), and these parts are permanently on the machine and in the same places. Non-regular parts need to be loaded into component feeders and placed on the machine.
Getting the placement data (part type, X-Y position, rotation) from a CAD design into the P&P is a simple matter of data manipulation and formatting.
Solder pasting is ideally done by stencil in a process similar to screen printing. One-off small boards - prototypes - with less than 10 minutes worth of manual pasting will sometimes get hand pasted. The aforementioned air compressor and a syringe of paste is the method used. If there are several designs going through the production cycle, then a multi-image paste mask will be made. A paste-mask is always the best method to use as there is a guaranteed consistency of paste quantity applied to the board surface.

Even as a small business there is a balance in choice between hand assembly and machine assembly.

For one or two PCBs with, say, 10 parts, all different, and none present on the P&P machine, it is actually quicker to assemble by hand, including hand pasting and this is due to the set-up time. Two 10-part boards could be hand placed in maybe 20 minutes starting from a parts list and having to go & get each part as compared to a couple of hours of machine set-up time, assuming none of the parts were on the P&P machine. Fine pitch devices such as processor ICs really do need a paste mask as the finely spaced pads do not lend themselves to a syringe needle depositing paste..... it's a bit like using a fire hose to fill a cup and not spilling a drop!
There is a bit of a caveat to this: If most of the aforementioned 10 parts are on the P&P machine, then it may be quicker to do the assembly on the machine. Likewise, if the board quantity rises to around 5, it's a machine-job... every time.

Solder reflow is always through the oven for consistency of finish.

Move on to a different scale of PCB, such as the one in the (hopefully attached photo), where the design has over 1000 SMT parts with about 50 different part types..... hand assembly is possible, but not wise. It would take over 10 hours and the solder paste would probably have gone off in that time. The P&P machine assembles such a board in around 17 minutes, and the machine has NOT been tuned-up for high-speed assembly.

From a product point of view, the board shown would not have been possible in the same size (critical for the end product) using conventional through-hole parts. The PCB is 4-layer and much use is made of the inner layers for trace screening, so I can guarantee that a through-hole version would have much poorer performance.

Component size is a huge factor. My personal preference is to use 1206 passive parts where possible. It keeps the parts inventory to a sensible level, the placement reject rate is less than 5%, the parts are easy to handle in rework/repair/modification. The slightly smaller 0805 parts have a poorer reject rate and are that little bit harder to work with by hand. Then there's the even smaller 0603. Just don't go there!



 

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Here is a photo of part of the workshop.

Rear left is the R&D bench, rear right is the SMD inspection table with magnifier.
The main part of the pic shows the P&P machine with a PCB on the right of the machine. The screen shows a pair of 1206 parts than the vision system has displayed. Both parts are shown off-centre, but in the brief time between dipslaying this image and the placement head arriving over the PCB, the machine has analysed the image and auto-corrected the rotation and centering so that the parts are placed accurately. See my previous post's photo that shows the consistency of placement.
 

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john12ax7 said:
I would just add if you're gonna go the SMT route, use 1206 parts. Easier to handle, and better performance for audio.
Easier to handle for sure, but better performance, not for sure.
If we talk about resistors for example, at one time it might be the case, but nowadays, standard size for thin film resistor is 0805 (and we are moving to 0603), so good 1206 parts from sellers (farnell, digikey, mouser) are quite rare, specialized parts and/or expensive.
For these reasons, and when voltage or power rating is not an issue, I go for 0805 in thin film in the audio path, and more and more 0603 for thick film (digital, psu).
 
Chris_V,
If you are worried about performance, then the MELF types are the ones to use in audio. In most E24 values at 1% they are cheaper than thin-film types, and I'm thinking of Farnell and RS as suppliers. A typical price for a couple of hundred is under 3 pence each.
The DIN0204 size is compatible with 1206.
 
I am using MELF 0204 and 0207 when power/voltage rating is an issue, however they are not as cheap as Susumu RG (0.5%, 25ppm) and far more expensive than Susumu RR (0.5%, 25ppm too), which have been measuring very well in audio application (almost noise free compared to metal film)
 
Thanks for the info, Chris. Interesting.
... although I am trying to keep the parts inventory down to having only the parts that are needed, but when its time for a new reel...
 
Gareth Connor said:
Yes, there are small OEMs with P&P machines...... I am one of them :)

The path to personal P&P has been this.....

My first heavy-duty experience of SMT in the pro-audio production environment was in the mid-1990s when I was a contract designer at a West Yorkshire based console manufacturer that was transitioning over to SMT. The advantages were huge for a mass-producer. To gain further personal experience of SMT, I started regularly doing my own designs with SMT and hand-placing & hand-soldering.
SMD offers other benefits in console land as the tighter layouts can reduce loop area affecting noise pickup.
Moving forward 15 years to 2007, there was an increasing amount of work that I was doing that required SMT (very little of it audio), and my remit was to design and build. I decided to investigate getting my own SMT facility set up...... and said goodbye to several years-worth of very hard-won savings, plus a bit more. The facility comprises Contact Systems CS3 P&P machine, >100 component feeders, solder reflow oven, paste printer, visual inspection equipment, and specialised air compressor (for the P&P).

Now for the detailed reply......

One-off and small quantity board assembly is possibly, but there are balancing factors. The biggest amount of time is in setting-up the P&P machine. There are a core number of types of components that are common across many designs (10k & 100k resistors, TL072, NE5532, 22uF, 100uF, etc), and these parts are permanently on the machine and in the same places. Non-regular parts need to be loaded into component feeders and placed on the machine.
Getting the placement data (part type, X-Y position, rotation) from a CAD design into the P&P is a simple matter of data manipulation and formatting.
Solder pasting is ideally done by stencil in a process similar to screen printing. One-off small boards - prototypes - with less than 10 minutes worth of manual pasting will sometimes get hand pasted. The aforementioned air compressor and a syringe of paste is the method used. If there are several designs going through the production cycle, then a multi-image paste mask will be made. A paste-mask is always the best method to use as there is a guaranteed consistency of paste quantity applied to the board surface.

Even as a small business there is a balance in choice between hand assembly and machine assembly.

For one or two PCBs with, say, 10 parts, all different, and none present on the P&P machine, it is actually quicker to assemble by hand, including hand pasting and this is due to the set-up time. Two 10-part boards could be hand placed in maybe 20 minutes starting from a parts list and having to go & get each part as compared to a couple of hours of machine set-up time, assuming none of the parts were on the P&P machine. Fine pitch devices such as processor ICs really do need a paste mask as the finely spaced pads do not lend themselves to a syringe needle depositing paste..... it's a bit like using a fire hose to fill a cup and not spilling a drop!
There is a bit of a caveat to this: If most of the aforementioned 10 parts are on the P&P machine, then it may be quicker to do the assembly on the machine. Likewise, if the board quantity rises to around 5, it's a machine-job... every time.

Solder reflow is always through the oven for consistency of finish.

Move on to a different scale of PCB, such as the one in the (hopefully attached photo), where the design has over 1000 SMT parts with about 50 different part types..... hand assembly is possible, but not wise. It would take over 10 hours and the solder paste would probably have gone off in that time. The P&P machine assembles such a board in around 17 minutes, and the machine has NOT been tuned-up for high-speed assembly.

From a product point of view, the board shown would not have been possible in the same size (critical for the end product) using conventional through-hole parts. The PCB is 4-layer and much use is made of the inner layers for trace screening, so I can guarantee that a through-hole version would have much poorer performance.

Component size is a huge factor. My personal preference is to use 1206 passive parts where possible. It keeps the parts inventory to a sensible level, the placement reject rate is less than 5%, the parts are easy to handle in rework/repair/modification. The slightly smaller 0805 parts have a poorer reject rate and are that little bit harder to work with by hand. Then there's the even smaller 0603. Just don't go there!
It seems that 1206 are large for SMD parts these days. I saw one board that used 0204 resistors (well I could just barely see them). For non-audio circuits I like to use quad resistor packs. One machine placement for 4 resistors. The x8 resistor packages were often problematic (lead bridging etc) so not worth the degree of difficulty, but x4 was cost effective. I actually use one 2512 1W resistor in my outlet tester, not because I need the power dissipation, but it's a fraction of the cost for a 1206 resistor rated for the same 500V. No doubt a quirk of distribution (high voltage 100M parts are not widely used). The smaller package SMD resistors don't tolerate high voltage.

======
Now all you need to do is figure out how to cover the back side of that board with SMD for 2x the circuitry. You could probably hand pop parts on the back.  ;)

Back last century (at Peavey) we mixed our (cheap) in house single sided PCB technology with SMD parts on the solder side. This was pretty cost effective for modest sized value mixers, where TH components like pots, jacks, switches, etc were inserted in the top side with SMD circuitry on the bottom side. In my experience these SMD versions out performed the earlier TH versions (better and cheaper). 

Back in the day we could fill a landfill with the wire leads cut off TH parts. SMD is so much greener. I recall visiting a capacitor factory in Mexico that was basically attaching TH leads to chip capacitors, this was before SMD was widely used. Now that whole operation is an unnecessary added cost.

JR
 
I design and prototype mixed signal topology boards with components down to the 0402 size. The small package seems daunting at first but once you get the feel it goes pretty quick. I use a microscope and an iron with a .2mm tip
 
Those of you working with mostly SMD parts at home, how do you end up prototyping?  Do you get boards made for each revision, scratch away at copper clad with an xacto, or something else?
 
That's a good question, DF

Thank you all for your input - much appreciated.

The project I'm working on at the moment has a main PCB with 390 parts and is 210mm x 280mm. It has several plug-in PCBs which are up to 280mm x 60mm and up to 100 parts each.

Volumes are likely to be limited, due to the nature of the product (it's highly specialised). The question is, at what point will it be economical to use an SMT fabricator over the current hand-soldering people (assuming they are in the UK and not on Chinese wages!)?
 
Chris_V said:
john12ax7 said:
I would just add if you're gonna go the SMT route, use 1206 parts. Easier to handle, and better performance for audio.
Easier to handle for sure, but better performance, not for sure.
If we talk about resistors for example, at one time it might be the case, but nowadays, standard size for thin film resistor is 0805 (and we are moving to 0603), so good 1206 parts from sellers (farnell, digikey, mouser) are quite rare, specialized parts and/or expensive.
For these reasons, and when voltage or power rating is not an issue, I go for 0805 in thin film in the audio path, and more and more 0603 for thick film (digital, psu).

1206 are certainly more expensive and less common. But , in general, noise and distortion increases as the package size decreases (for audio frequencies). So 0805 will have more noise and distortion than 1206, although granted it may not matter for a given application.

And as you mentioned, go with thin film over thick film for noise sensitive applications.
 
In my experience, you only truly worry about SMD in higher voltage applications.
They don't have much capability to dissipate heat, so a lot of current through a low value resistor (running from, say ±15V rails) is going to heat it up and cause a value change.
Dynamic value change = distortion.

We use 0805 and 0603 for most of our applications, however, we chill out in the 3.3 and 5V zone for most stuff.

/R
 
thermionic said:
That's a good question, DF

Thank you all for your input - much appreciated.

The project I'm working on at the moment has a main PCB with 390 parts and is 210mm x 280mm. It has several plug-in PCBs which are up to 280mm x 60mm and up to 100 parts each.

Volumes are likely to be limited, due to the nature of the product (it's highly specialised). The question is, at what point will it be economical to use an SMT fabricator over the current hand-soldering people (assuming they are in the UK and not on Chinese wages!)?

It really depends on where you go, but with your project I would guess as few as 10 boards would be cheaper with SMT given UK labor rates. A lot of assemblers just populate through hole by hand and then wave solder.

You can find online calculators like this one to get a vague idea, although it's China based
http://www.7pcb.com/PCB-Assembly-Quote.php

Edit: As a frame of reference I recently asked multiple US based assemblers about 25pcs for a board about half your dimensions and components. And they all agreed I would be better off SMT.
 

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