The companies that physically build AI infrastructure — the switch chassis, the full server racks, the busways and power modules hyperscalers bolt into data centers — spent late July reporting accelerating growth and raising guidance. Over four sessions from 17 to 21 August, the six largest listed ones fell double digits together, an average of 14.3%.

The trigger was not in their order books. On 18 August the 30-year Treasury yield touched 5.323%, its highest since 2007, on stalled talks to end the war with Iran, persistent inflation and uncertainty over policy under new Federal Reserve chair Kevin Warsh. That day Celestica fell 8.8%, Jabil 8.9%, Sanmina 8.5%, Benchmark 8.1%, Plexus 8.0% and Flex 7.6% — a range so narrow it names its own cause, since none of the six released anything. The following session AI cloud infrastructure names sold off again as a tally of roughly $3 trillion in off-balance-sheet AI commitments across nine large technology companies circulated. Long yields set the discount rate on earnings that sit in 2027 order books, and they price data-center project finance.

The control that wasn't

The cleanest test is Benchmark Electronics, a Tempe, Arizona builder of circuit boards and integrated systems for aerospace, defense, medical and semiconductor-equipment customers — the name here with the least compute exposure. It fell 13.3% across those four sessions, matching Celestica's 13.5%, and is down 14.5% over 30 days against Celestica's 13.2%. Whatever repriced, it was not AI program risk. Benchmark is not a pure control in any case: its advanced computing and communications line grew 71% year over year last quarter on an AI ramp, while aerospace and defense fell 12% on program timing. Group revenue rose 17.7% to $756m and the full year was raised to a record $3.0bn.

Celestica: the business went one way, the multiple the other

Celestica, the Toronto contract manufacturer that has moved up the value chain into designing its own 800-gigabit and 1.6-terabit Ethernet switches and full AI racks, is the sharpest divergence. Revenue growth has accelerated four straight quarters — 26.4%, 45.8%, 52.8% and 62.4% — to $4.699bn. Its Connectivity and Cloud Solutions arm grew 84% to $3.81bn with segment margin up to 8.7%, while the Advanced Technology Solutions side, serving aerospace, industrial and health customers, grew 8%. Hardware Platform Solutions, the own-design line, reached about $1.9bn. Operating margin widened to 9.75%. On 28 July management raised 2026 guidance to $20.5bn of revenue and $11.30 of adjusted earnings per share, and named custom racks for OpenAI's Jalapeno accelerator and a design role on AMD's Helios interconnect as 2027 programs.

That matters because hyperscalers increasingly bypass traditional server brands and contract directly with builders like Celestica, Foxconn and Quanta for whole racks — value capture moving upstream from the OEM to the assembler that owns the design.

The shares are flat over six months and down 16.9% over three. Because pass-through memory and component inflation — server memory contract prices rose 90–95% in one quarter — bloats the revenue line of a thin-margin conversion business at near-zero profit, price against gross profit is the honest lens. Celestica trades at 18.84x trailing gross profit and 14.07x forward, against 26.26x in mid-May.

The reasons for hesitation are about cash, not demand. Three customers were 32%, 17% and 14% of revenue, roughly 63% combined. Inventory reached $3.4bn, up $1.5bn year over year. Capital spending is guided at $1bn this year with a $1.5bn placeholder for 2027, against a trailing free-cash-flow yield of 1.52%. And on 6 August the company priced 9,677,419 shares at $310, about 15% below the prior close, raising $3bn and diluting holders by roughly 8.4%.

Flex: the margin sits in the part being spun out

Flex, the Singapore-founded manufacturer whose products run from automotive modules and medical devices to switchgear, busway and power distribution units, has also accelerated four quarters running, from 4.0% growth to 20.6% on $7.93bn of June-quarter revenue. Its Cloud and Power Infrastructure segment did $2.2bn, up 35%, at a 9.7% operating margin — well above the 6.7% adjusted corporate rate — with power up more than 70% and over 90% of the next three quarters booked. The other two segments are bigger and thinner: $3.1bn at 5.2% and $2.7bn at 6.6%. Flex guides to 23% revenue growth this fiscal year, and diluted shares have fallen from 400m to 374m. It has named the leadership for a spin-off of the cloud and power business in the first quarter of 2027. Flex trades at 14.70x trailing gross profit, down from 21.42x in May.

The other three followed rather than led. Jabil had gained about 16% from 21 July before surrendering all of it, leaving it down 1.85% over 30 days. Sanmina is the worst over three months at -20.6%. Plexus, the least AI-exposed by revenue mix, fell least at -6.5%. Benchmark, at 9.05x trailing gross profit with a 4.83% free-cash-flow yield, is the cheapest of the six on this measure.

One technical point does argumentative work: Celestica is the only one of the six whose 50-day average has fallen below its 200-day, and it crossed on 12 August — after the offering, before the yield spike. That is share-count and price mechanics, not results.

The setup

Where it stands — Six contract builders repriced together on a rates shock while five of six raised guidance. Would confirm — Celestica's Q3 revenue lands inside the $5.25–5.55bn guide with CCS margin at or above 8.7%. Would invalidate — A hyperscaler program slips, or Celestica's inventory rises again without matching revenue. Watch next — Celestica's third-quarter results in late October; Flex's spin-off targeted for the first quarter of 2027. Valuation — Celestica 18.84x trailing gross profit and 14.07x forward, against 26.26x in May; Flex 14.70x and 12.43x.