CIUDAD ACUNA, Mexico -- By day, Sergio Martinez labors in a modern air-conditioned factory a few miles from the Texas border, a human cog in the global supply chain that helps build pickups and tractor-trailer cabs. He wears a smart uniform at work.

At night, he comes home to a dirt-floor shack with a bare light bulb and no indoor plumbing. Mosquitoes buzz incessantly. He and his family live like poor dirt farmers.

His salary of $7.50 a day is enough to provide for the family dinner table, the cost of bootleg water and electricity, and an occasional article of discarded clothing for his wife or two girls, but rarely anything else.

Martinez, 35, is emblematic of the industrial sector of Mexico, a magnet for foreign investment hitched to a strong U.S. locomotive. Factories in Mexico pump out plasma TVs, BlackBerry smartphones, kitchen blenders, airplane components and automobiles. Yet millions of workers, like Martinez, can only dream of climbing from the lower class to buy the appliances, smartphones and cars they help manufacture.

Some four decades after welcoming foreign assembly plants and factories, known as maquiladoras, Mexico has seen only a trickle of its industrial and factory workers join the ranks of those who even slightly resemble a middle class. Instead, the poverty trap clutches them tightly. Some have earned the same wages for years. The government subsidizes credit, allowing purchases of appliances and even simple houses. But the credit sinks them into debt they can never hope to repay. Their teenage children, rather than staying in school, rush to factories themselves or join criminal gangs.

Without deep political and social reforms, experts say, the thousands of maquiladora plants that cluster at the U.S. border and around cities in the interior will remain a fixture for decades to come, and Mexico won’t build a middle class that’s big enough to fuel faster economic growth.

Mexico does all it can to ensure that workers don’t unionize, or if they do that they join so-called “protection unions” designed to assure the interests of plant owners and keep wages low.

For nine years, Martinez has been a constant presence on a fast-moving assembly line. He unspools and tapes electrical wiring systems for Ford pickups, Harley-Davidson motorcycles, Volvo and Scania trucks and other vehicles.

He and his wife, Elba, are from a rural area of Veracruz state on the Gulf of Mexico, part of a large community from that state that’s moved to Mexico’s northern border. Once a municipal policeman, Martinez heard of good-paying jobs so he migrated, later bringing his wife and starting a family.

They live in what Mexicans call a “jacal,” a homemade shanty of scrap wood and tarpaper. Boulders keep the corrugated tin roofing on in case of high wind. An outhouse is a few feet away. Next to it is a washing machine set on pallets in the open air. A broken stove also lies outside, hollowed out and jerry-rigged to serve as a barbecue. A makeshift electrical line brings power from a neighbor’s house. A homemade pipe brings water from a different direction

After nearly a decade at his job, Martinez isn’t optimistic.

“We don’t have hopes for a better life here for our kids,” he said.

Yet he sticks it out, even as ownership of the plant revolves. Once a division of Alcoa, the plant was sold in 2009 to Beverly Hills, Calif.-based Platinum Equity, then sold again last October to a Finnish company, PKC Group.