SMT IC packages

[metebalci]

I wonder how a vendor decide on packages for an IC, particularly for low pin counts, SOT, SOIC vs SOP and their variants, but also for 4 side packages. I understand one reason is pin count, 64 pin SOIC may not make sense. What are the technical and non-technical factors behind these decisions ? For example, LMH6609 is offered in SOT-23 and SOIC-8 (but why no SOP ?, and why both SOT and SOIC), whereas TL082x has I think all 2 side package options and MCP33151 (10 pin ADC) has only SOP and DFN packages (why no SOIC?).

[ajb]

When the part was initially introduced is probably a big factor.  Older ICs like the TL08x that were introduced a while ago might be introduced in whatever package is most popular at the time then, if they're popular enough, moved to more modern packages while retaining the old ones for compatibility with existing customer designs -- and in this case, since opamp pinouts are generally well standardized, they maintain interchangeability with other parts that way too. 

SSOP is half the pitch of SOIC, so would be notably more difficult to assemble in older times -- less so now, which is why it's more common for newer parts.  Improvements in assembly technology plus drive for more compact electronics reduces demand for the larger parts, hence why the MCP33151 might only be available in SSOP rather than SOIC.  The industry is still progressing, so now more parts are in leadless and even chip scale packaging, but those are still more challenging/expensive to assembly, so there's still plenty of market for the SSOPs.  Or if you're wave soldering, you still probably want SOIC rather than SSOP, but you probably wouldn't be wave soldering anything complex enough to need an ADC. 

More complex parts might be limited by their die size, so in some cases a smaller package may only become available late in a part's lifecycle after a die shrink. 

[metebalci]

SSOP is half the pitch of SOIC, so would be notably more difficult to assemble in older times -- less so now, which is why it's more common for newer parts.  Improvements in assembly technology plus drive for more compact electronics reduces demand for the larger parts, hence why the MCP33151 might only be available in SSOP rather than SOIC.

The industry is still progressing, so now more parts are in leadless and even chip scale packaging, but those are still more challenging/expensive to assembly, so there's still plenty of market for the SSOPs.  Or if you're wave soldering, you still probably want SOIC rather than SSOP, but you probably wouldn't be wave soldering anything complex enough to need an ADC. 

Would it be fair to say, first SOIC and then SSOP packages will disappear or will be very rare when assembling leadless packages becomes as simple as assembling SOIC etc. ? or would there be other reasons to keep leaded packages in the market ?

Do you know at what point the electrical performance of the package becomes a concern and makes for example DFN a must (compared to SOIC/SSOP etc.) ?

[ajb]

Would it be fair to say, first SOIC and then SSOP packages will disappear or will be very rare when assembling leadless packages becomes as simple as assembling SOIC etc. ? or would there be other reasons to keep leaded packages in the market ?

Things tend to stay on the market as long as someone is willing to buy it for more than it costs to make and sell.  No one wants to respin a board just because of an IC package change, which is why you can still buy ICs in DIP packages, and parts that are already in SOIC or SOP will be around for a long time to come.  Introducing a new package is much more expensive (qualification etc) than continuing to produce one that's already on the market, so you'll just continue seeing new parts only introduced in smaller sizes, and SSOP/SOIC will stay on the market as long as those older designs do. 

Do you know at what point the electrical performance of the package becomes a concern and makes for example DFN a must (compared to SOIC/SSOP etc.) ?

When the wavelengths of concern approach the dimensions of the leads, so quite high frequencies. 

[PCB.Wiz]

Would it be fair to say, first SOIC and then SSOP packages will disappear or will be very rare when assembling leadless packages becomes as simple as assembling SOIC etc. ? or would there be other reasons to keep leaded packages in the market ?

Do you know at what point the electrical performance of the package becomes a concern and makes for example DFN a must (compared to SOIC/SSOP etc.) ?

DIP have certainly largely gone.

gull wing parts will never go away, as they have thermal and inspection benefits, and many can wave solder.

There are gull wing parts with under package GND PADS, which then get close to DFN parts thermally and electrically.
I have heard chatter that some QFN packages are cheaper, but not all vendors show that in their prices.

[kimballa]

Various part datasheets also quote very different values for T_jA for the SOIC vs TSSOP packaging of the same chip. Whereas SOIC is usually somewhat comparable to PDIP.

For applications where the circuit is going to build up heat that needs to be evacuated, there's only so much that can happen on tiny parts. In those applications I would think larger pitch packages like SOIC or D2PAK are going to stick around longer.

(Although PowerPad packages with a large thermal pad under the chip can help with package shrink in the most heat-intensive cases, so maybe there's still enough area under SSOP to make board contact instead of cooling by radiating heat to the air?)