When it applies
Form 5472 applies whenever a reporting corporation has had a reportable transaction with a related party in the tax year, and either (a) the corporation has a 25%-or-more foreign shareholder or (b) the corporation is a foreign person engaged in a US trade or business. For non-US founders of US LLCs, the common case is (a): a single-member LLC taxed as a disregarded entity, 100%-owned by a foreign person, with any transactions between that owner and the LLC during the year.
What makes a single-member LLC file it
Starting with tax year 2017, treasury regulations (TD 9796) treat a US disregarded entity that is wholly owned by a foreign person as a corporation for Form 5472 purposes. That means the LLC files Form 5472 attached to a nearly-blank Form 1120 — the “pro-forma 1120” — carrying the header “Foreign-owned U.S. DE”.
When it’s due
The 15th day of the 4th month after the LLC’s tax year end — 15 April for a calendar year — extendable to 15 October with Form 7004.
Penalty
$25,000 per form, per year under 26 U.S.C. §6038A(d). Additional $25,000 penalties can be assessed for each 30-day period of continuing non-compliance after the IRS notifies you. The IRS may also hold the full 1120 statute of limitations open indefinitely while a 5472 is outstanding.
See also: pro-forma 1120, related-party transaction, 25% foreign ownership.