How to Audit an EMS Partner for Rack Power

Rack power is not a catalogue purchase. The hardware carries 142 kW through it, sits behind a billing meter, and in the lithium lines travels under dangerous-goods rules. Choosing the factory that builds it deserves more than a price comparison. This is the audit we would want a buyer to run on us — capability questions, evidence requests, and the red flags that show up in the answers.

7 SMT lines100% AOI and burn-in coverageIPC-A-610 Class III criteriaISO 9001 quality system, certificate under NDA

STEP ONE

Ask for the process route, not the brochure.

A route you can walk

Any EMS partner should be able to draw the route their product takes: incoming material inspection, solder paste printing with in-line 3D SPI, placement, lead-free reflow under a controlled profile, AOI after reflow, X-Ray where joints cannot be seen, box build, hi-pot and ground bond, functional test on a programmed fixture, burn-in at full load, then pack-out. A factory answers this from memory and takes ten minutes to draw it. A trading desk asks you to repeat the question.

Numbers, not adjectives

The useful version of that route comes with counts and platforms attached. Our own floor runs seven SMT lines with Yamaha placement and GKG stencil printers, a MENTO MT2000 AOI gate on every line, an X-Ray station, a lead-free reflow aisle where the recipe is locked to the assembly revision, and a 100% burn-in aging room — 63 equipment positions on one production floor. Whatever the partner's version is, it should be equally specific.

STEP TWO

SMT and inspection: questions that produce numbers.

Capability questions are only useful when they force a specific answer. Each of these has a weak reply and a substantive one — and the substantive one usually comes with a machine name and a sampling rule.

Question	Weak answer	Answer with substance
How many SMT lines, and on what placement platform?	"Several lines"	A count, the platform, and how many are dedicated to high-current power assemblies
How is solder paste volume verified?	"Visual inspection"	In-line 3D SPI on 100% of deposits, with volume, area and height against stencil spec
What AOI criteria govern the verdict?	"We check the boards"	A published workmanship class, with polarity, offset and joint libraries, and defect images archived per lot
When do you X-Ray, and to what acceptance limit?	"When the customer asks"	A sampling rule by lot, plus 100% on battery welds, with voiding limits referenced to an IPC standard
How is the reflow profile controlled?	"Our engineers set the oven"	Profile locked to the assembly revision, called by program rather than by feel, archived against the unit serial
How are high-current and battery joints controlled?	"Experienced operators"	Automated laser welding with pull-force or weld-resistance data logged joint by joint
What happens at a changeover?	"We follow the checklist"	Stencil registration and tension verified, first-article report raised per lot before the batch runs

STEP THREE

Test coverage line by line.

Every rack power line has a different failure physics, so a credible partner tests each one differently. If the answer is the same test for everything, the test is a formality.

Product line	Coverage that matters	The claim it protects
Rack PDU	100% hi-pot and ground bond, programmed functional test, metering accuracy verification against a reference	±1% billing-grade metering that survives a tenant audit
CRPS server PSU	In-circuit and functional stations logging every serial, then 100% full-load burn-in at elevated temperature	96% Titanium efficiency and a nameplate that holds under load
BBU battery modules	100% X-Ray on battery welds, formation cycling, capacity grading and matched grouping, module aging	Module-to-module balance and a bridging claim that works on the first dip
Supercapacitor modules	Cell-level capacity and ESR grading, string matching, module aging under temperature	Balancing circuits that start centred instead of fighting cell spread
Data center busway	100% hi-pot and resistance check on every tap-off box, joint-by-joint millivolt-drop verification, temperature-rise type test on sample runs	Rated current at the section class, and live tap-off insertion that behaves

The fixture rule

Ask how a unit gets to the aging room. The right answer is mechanical, not procedural: the test fixture issues the pass verdict that unlocks the burn-in slot, so an untested unit cannot physically be aged. If the sequence depends on an operator remembering to run a test, it will eventually be skipped on a busy night — and nobody will know which serial it was.

STEP FOUR

Traceability: can they find one serial's history?

The live test

Take a serial number off a unit in a finished crate and ask for its complete history: the reflow recipe revision it ran under, the SPI result, the AOI verdict with defect images, the X-Ray record, torque and weld data where applicable, the hi-pot and ground-bond readings, the functional test log, and the burn-in trace with chamber setpoints. A factory with a real serial record produces this in minutes. A factory without one offers a lot certificate and a delivery note.

Why it matters commercially

Traceability is what turns a field failure from an argument into an answer. With a serial record you can see whether the failure came from a component lot, a process change or a specific chamber slot — and you can scope the problem to the units actually at risk instead of recalling a quarter's production. Ask also how long records are retained, and whether lot-level failure Paretos are produced as a matter of routine or only on request.

STEP FIVE

Compliance and export: the file, not the logo.

Certification is the area where buyers are most often misled, because a logo on a slide is free. Ask for the document, its scope and its status — and treat an honest "certification path with an evidence package" as a better answer than a certificate the vendor cannot produce.

Item	What to ask for	Red flag
Quality system	ISO 9001 system certificate covering SMT, assembly, test and burn-in, shared under NDA	A certificate whose scope excludes the processes you are buying
Safety certification	Status by type — for PDU and PSU lines, for busway — stated as a path with compliance-ready evidence, or as a completed listing with the file reference	A listing claim with no file reference and no test house named
CE and RoHS	Declarations of conformity for the specific lines shipped to your market	A single generic declaration covering everything the company sells
Lithium transport	UN38.3 test summary and MSDS per shipment, packed to PI965/PI966 instructions	Willingness to ship battery cartons inside general freight without the documentation set
Customs paperwork	Commercial invoice and packing list reconciling to the PO, consistent HS classification across shipments	Classification that changes between shipments of the same goods
Third-party inspection	Right to appoint an agency at pack-out, and a live video walk-through of the burn-in exit for remote programs	Reluctance to allow inspection, or inspection offered only after packing is sealed
Trade terms	FOB, CIF or DDP stated on the quotation with the port named, and payment terms in the same sheet	Terms that appear for the first time on the order acknowledgement

STEP SIX

The factory visit, in order.

Sign the NDA, then walk the floor with a plan. This sequence covers the whole line in about half a day and produces more information than any questionnaire.

Start at incoming inspection. Ask how critical components and cell lots are graded, and what happens to out-of-band material. An IQC station that only checks paperwork is a receiving desk.
Stand at the printer. Watch the stencil align and ask for the SPI result of the panel currently on the line — not a sample from last month.
Follow a panel to AOI. Ask to see yesterday's defect images and the disposition of each one. Then walk to X-Ray and ask which units get it and to what voiding limit.
Stop at the battery or capacitor line, if you are buying either. Weld data logging, formation cycling, grading and matching should all be visible as process steps with records, not as claims in a presentation.
Watch a hi-pot and functional test run. Confirm the fixture sequence, the pass criterion, and where the readings are stored.
Walk into the aging room. Ask what unlocks the door, ask which units skip it, and ask to see the last week of chamber traces for one slot.
Run the serial test from step four on any unit currently in the room.
Finish at pack-out. Check the lithium packing lane, the carton labelling scheme against a rack number, and who signs the QC dossier.
Then talk capacity. Line count, shifts per day, current loading and lead time by product line — 15–30 days for rack PDU, 30–45 days for CRPS, BBU and supercapacitor lines, 45–60 days for busway, counted from order confirmation and deposit.

SCORING IT

Pass evidence versus fail signal.

Audit area	Pass evidence	Fail signal
Process control	Recipe and profile locked to the assembly revision, with change control	Recipes adjusted at the machine by whoever is on shift
Inspection coverage	Stated sampling rule and acceptance criteria by defect class	"100% quality" with no criteria attached
Test coverage	100% burn-in with a zero-failure exit criterion, enforced by the fixture	Burn-in offered as an option or applied by lot
Traceability	Serial record retrievable during the visit	Records promised after the audit
Compliance	File or path stated per product type, with scope and status	Certification logos with no file reference
Accountability	One QA file, one warranty boundary and one escalation path named in the order	Responsibility split across three suppliers with no owner

Run the same audit on us

Everything in this checklist is a question we expect from a serious buyer, and the answers are on our floor rather than in a slide: seven SMT lines with SPI, AOI and X-Ray gates, a burning room no unit skips, serial-keyed QC dossiers, and an ISO 9001 system certificate shared under NDA. Book a visit, sign the NDA, and pick a serial at random — that is the fastest way to judge a rack power partner, including this one.