Hallo Brett,

in a similar thread I wrote:

"It's not wise to insist on DIP40 package, today. Modern '51 derivatives are rather fast and have additional features like PCA which highly benefit from minimizing package size. Compare length of internal connection from Vcc pin and Gnd pin to die: Use of DIP40 package results in 5 times the inductivity compared to modern package styles! This inductivity erodes the function of power supply decoupling and leads to increased ground bounce."

Have you ever had a look at internal wiring of a chip? In the center of package there's the die, the actual chip. And from it's bonding pads there's a certain wiring to terminals of package. When you read in application notes that decoupling capacitor is to be connected as close as possible to Vcc pin and ground pin of chip, this seems to be rather ridiculous: Manufacturer wastes so much volume and at the same time he is recommending to shorten distance to decoupling capacitor.
Solution is quite easy: DIP40 is a very old package style, borne at a time when chips weren't so fast as today. But with increasing speed old package style begun to make problems, which could only be overcome by connecting decoupling capacitor as close as possible to chip.
Today you should not use such old DIP40 packages. This is especially true for the faster mcu. Try to use at least PLCC44 package style, which still can be handled very easily: Just plug into a socket, which is soldered on PCB.

Today even more volume saving package styles can be used, when using 'in circuit programing' feature. Good effect: Power supply decoupling becomes even more effective.

Soldering of SMD packages is not so difficult. For high volume productions mounting roboters are used and soldering is done by highly sophisticated machines, leading to very reliable results. Even soldering by hand is possible, when certain handling rules are carefully obeyed: Have some preheating and use specialized soldering tools. Just SMD discretes suffer from high temperatur shocks, which must be avoided. SMD chip packages, on the other hand, are much easier to solder, because sensitive die is farer away from soldering point than compared to SMD discretes, like capacitors and resistors.

Compared with common through hole packages you will save an enormous volume/area, which results in very small, nice looking PCB, showing improved EMI and reliability.

Good luck,
Kai