If there are different packages for the same component, which one you should consider?

Question

I am designing a small PCB for mass production, and I am trying to keep costs low. One of the components is available in several different packages: 24QFN, 32QFN and LP (TSSOP 24 Pin). There is a significant difference in price and size.

So, what should I consider for this? I guess that some are harder to mount than others. What I found is that most of PCB assemblers will tell you "Yes, we can do it!", but later, we will see if the board comes with the component well connected or not.

I am also concerned about temperature, it is a stepper driver (the Allegro Micro A4984), and it can get really hot. I am sure that bigger ones are better for dissipation, but also more expensive.

Ideas?

Answer 1

1. Cost. Some packages cost more.
2. Needs. Packages with higher pins probably have more features.
3. Higher pin count packages mean more physical space and routing. Smaller packages with less pins means they are easier to place and route. This means smaller PCBs, which often mean smaller costs.
4. Different packages have different heating dissipation ratings. It's not always the bigger one. But bigger ones can be easier to add heat sinking to.
5. Leadless packages tend to cause more issues in manufacturing, and can require extra testing. BGA for example, needs xraying to see if the pins (balls) have properly reflowed. High pin count packages could require extra layers and vias, raising manufacturer costs, and even need test points added, taking up pcb space and requiring expensive testing.
6. Availability. Some packages are easier to get, and in bulk, than others. Unless this is a one off production where it's easy to get either package once, you should always consider future runs.
7. Pin-for-Pin replacement parts from other manufacturers. Again, for future runs.

In your specific case, the smaller the package (24 QFN), the worse the thermal dissipation. But the smaller, the cheaper. But not by much. Considering that at Digikey's pricing, at 500 unit pricing, you are talking about under a hundred dollars in difference. Significant difference in pricing, is a very subjective idea, given the tradeoffs. TSSOP is hard to mess up for even most assemblers, it is a lead package. Size difference is also small, 4mm x 4mm, 5mm x 5mm, or 7mmx6mm. You have slightly higher costs with the TSSOP (part cost and pcb space), but routing is easier due to the pin spacing, and better thermal performance. It's a toss up really. You could get two prototypes made, one with the cheaper 24qfn and one with the TSSOP, and then make your final decision based on which one performs better.

Answer 2

Regardless of which specific part number is under consideration, here are some generic rules of thumb I've found useful:

1. Things to check with the assembler:

   • Do they charge differently for different pin pitches

     One set-up I deal with charges per solder point, and almost thrice as much per point for 0.5mm as for 0.8mm pitch

   • Do they require additional turn-around time for smaller pitch work

     The one I use does, because they share time on an automated setup for boards with smaller parts

   • Does the assembler provide a board test guarantee?

   • Do they charge a premium for through-hole parts in an otherwise SMD board

     I've found prices being doubled simply due to addition of a through-hole terminal strip on an SMD board - independent of BOM cost

2. When contracting work to manual assembly setups

   • Avoid leadless packages / BGA like the plague

     The assembler finds ways of messing it up.

   • Avoid packages with lead pitch lower than 0.5 mm

     Manual assembly might short some pads, it is a pain to debug

3. When hand-soldering by yourself, use the biggest leaded package available

   • Avoid through-hole packages, though, if you would need to drill the PCB by hand

4. For parts which may need to dissipate some heat:

   • A package with a big thermal pad is preferable. This may mean a larger package than you would like.

   • Check the datasheet:

     In some cases, a DIP might be best for greater thermal capacity and better heat dissipation

     Others might actually have better dissipation or lower heat generation in the smaller package, because the smaller package is sometimes an updated internal design

5. For parts with different pin-count packages, the larger pin-count option may expose additional pins/functions

   • Evaluate whether those functions are useful, else go with the lower lead count

6. While staying within the lead pitch and pin count recommendations above, smaller is better

   • The smaller the package, the lower the PCB area and thus cost of PCB manufacture

7. Don't forget to check if any of the packages are on life-buy / to-be-discontinued status

   • This is often the case with DIP parts and sometimes SOIC as well. Avoid those packages.

Answer 3

The A4984 has a thermal relief pad underneath the part to help alleviate heat issues. If you use the recommended land pattern and follow the datasheet's layout instructions you should be fine.

Answer 4

From PCB layout view, some package has a better pin distribution than others. For example:

• All pins from same port together
• Vcc and GND pin together for decoupling.
• Digital pins and analog pins in different sides

All these points will help you with the layout. And in my opinion you can consider them when you choose a package. Obviously, it not the main point.