They are called third-party modules because the manufacturers are not necessarily directly involved in transactions with the chip manufacturers. (One of two parties for some reason becomes "third-party")
Anycase, the long and short of it is that there are many good third-party manufacturers out there, but there are a few that aren't. Knowing your source is the best way to protect yourself from a vendor selling low-quality or bad modules.
Kingston and Crucial for example are reputable sources.
Third party modules, for various reasons, have a higher percentage of returns than major brand modules manufactured by the maker of the chips- these modules are called major original. For this reason, major original modules are more valuable than third-party modules.
An interesting observation:
If a major-original module is not working in a particular system, most people blame the system.
If a third-party module is not working in a particular system, most people blame the module.
Things to look out for when purchasing third party modules:
1. Quality PCB design
2. No more than two different date codes per module.
Different date-codes and different die-revisions, even the same brand, are different chips.
Third-party manufacturers do not always follow this. Major manufacturers, it's pretty much a rule.
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