Boston — For years, greeting cards contained only some large corporation's idea of poetry.
But now the card industry has decided to join the electronics revolution by marrying semiconductors and sentiment.
The first fruit of this union is a Christmas card that, when it is opened, plays ''Silent Night'' and ''We Wish You a Merry Christmas.'' American Express Company offered this musical missive to cardholders in bill-stuffing inserts mailed earlier this month.
''This is the beginning of a new technology,'' exclaims Robert Meyers, an American Express senior vice-president. The built-in musical equipment is so small that electronic cards cannot be readily distinguished from their silent competitors.
While American Express is first on the block with electronic cards, others in the greeting-card industry are planning a chorus line of playing - and talking - cards.
Early next year, for example, American Greeting Corporation will unveil 12 electronic cards. Six birthday cards will play ''Happy Birthday.'' The company's five electronic friendship cards will strike up ''These Are a Few of My Favorite Things.'' And one card will try to warm the recipient's heart with ''Let Me Call You Sweetheart.''
Already companies are laying plans to move beyond musical cards. One possibility is a talking card, say a Valentine's Day greeting that promises the sender's undying love in an electronically synthesized voice. All Mr. Meyers of American Express will say on the subject is that during the winter months the company ''might have one that talks.''
If card buyers like electronically expressed sentiments, miniturized electronic components will also make it possible to express their feelings in other high-tech ways.
''We are prepared to go ahead with blinking lights and anything else the market wants,'' says a spokeswoman James M. Semon, American Greeting assistant vice-president.
Not all card companies are moving aggressively on electronic cards. Hallmark Cards Inc. has been examining the technology, but has ''not made any decision as to producing,'' a spokesman says. The company is concerned that the novelty of electronic cards may wear off. Earlier, nonelectronic attempts to add sound to cards were bulky and unsuccessful. In addition, 'there is some question about the price,'' the company spokesman adds.
It is too soon to tell how the general public will react to electronic cards, which American Greetings expects to sell for $6.00 retail.
''It is really early, but (sales) are doing better than we expected'' at $35 for six cards to American Express Card holders, Mr. Meyers says. The cardholders tend to have relatively high incomes.
Merchants think electronic cards may catch on. ''It boils down to an attempt to take greeting cards into the gift catagory,'' says Jane Scott, stationery buyer for Bloomingdales in New York City. ''It is an expensive card, but not an expensive gift.''
The Bloomingdales official notes that gift-buying practices are changing. ''Most people have less disposable income than they once had. So while people once may have bought a card for 80 cents and then attached it to a gift, now they are spending more on a card and not buying the gift.''
The new electronic cards are one way to expand an already huge market. The National Association of Greeting Card Publishers estimates that 7 billion cards are sold each year in the US, about 3 billion of them at Christmas.
The music system in the American Express cards is a spinoff of equipment used in electronic watches. It is built around a plastic circuit board that is only 1 -inch square and 1/32 of an inch thick. The board contains a semiconductor memory chip that stores songs in digital form. A contact switch built into the card turns the music system on when the card is opened. Then the music is ''played'' on a thin piezo-electric transducer, whose sound is amplified by its paperboard enclosure. Powering the whole system is a 1/32-inch-thick watch battery.
Indiviauls with a strong desire to hear 15-second versions of two Christmas carols could listen to them for six to eight hours before the replaceable battery wears out.
Neither American Express nor American Greetings will say who makes their sound devices.
Although the new cards are designed to camouflage their electronic contents, manufacturers expect recipients to be curious. Hidden in the card American Express sells are a diagram and text explaining the mechanism. ''You can rip open the card,'' a company official explains, ''and have everything explained.''