Is the Summa Theologia infallible to Catholics or not?

Vatican I defined Ex Cathedra statements are those which meet the following conditions, The Roman Pontiff speaks on a matter of faith or morals to the Universal Church. St. Thomas Aquinas was never a Pope, so he never could meet these conditions.

Therefore, faithfully adhering to the tradition received from the beginning of the Christian faith, to the glory of God our savior, for the exaltation of the Catholic religion and for the salvation of the Christian people, with the approval of the Sacred Council, we teach and define as a divinely revealed dogma that when the Roman Pontiff speaks EX CATHEDRA, that is, when, in the exercise of his office as shepherd and teacher of all Christians, in virtue of his supreme apostolic authority, he defines a doctrine concerning faith or morals to be held by the whole Church, he possesses, by the divine assistance promised to him in blessed Peter, that infallibility which the divine Redeemer willed his Church to enjoy in defining doctrine concerning faith or morals. Therefore, such definitions of the Roman Pontiff are of themselves, and not by the consent of the Church, irreformable.

So then, should anyone, which God forbid, have the temerity to reject this definition of ours: let him be anathema.

The Summa Theologica (and all other documents of this nature) are not considered infallible. The reason for supposing they are indicates a misunderstanding of the Catholic concept of the Magisterium of the church, of how it is exercised, and of its specific degrees (whether "ordinary" or "apostolic").

So the short answer is that the underlying premise that everything the Pope teaches is infallible is simply a flawed premise.

More detail can be found on this answer.

No, the Summa Theologica/Theologiae is not infallible. It was written by St. Thomas Aquinas, who though he was very good, holy, and learned, was not infallible.

On those occasions on which the Pope is considered infallible (that is, when he addresses the whole Church as its teacher and pastor, and defines a doctrine which must be held by the whole Church), or for that matter on occasions when any group (e.g. an ecumenical council) is held infallible, it is not the Summa or any similar work that is ultimately the source of this infallible understanding; it is the Word of God Himself. That is the only possible source of infallible knowledge:

At the divine command and with the help of the Holy Spirit, [the Magisterium] listens to [the Word of God] devotedly, guards it with dedication, and expounds it faithfully. All that it proposes for belief as being divinely revealed is drawn from this single deposit of faith.

(Dei Verbum Section 10 Part 2, quoted in Catechism of the Catholic Church paragraph 86)

One example in which we see this quite clearly is in the doctrine of the Immaculate Conception. This is a belief of the Church from ancient times, proposed as a dogma in 1854 by Pope Pius IX. Yet Aquinas disagreed with the idea! In his discussion of the question "Whether the Blessed Virgin was sanctified before animation?", that is, before the infusion of the rational soul, which was understood at the time to take place after conception, Aquinas says:

In whatever manner the Blessed Virgin would have been sanctified before animation, she could never have incurred the stain of original sin: and thus she would not have needed redemption and salvation which is by Christ, of whom it is written (Mat. 1:21): "He shall save His people from their sins." But this is unfitting, through implying that Christ is not the "Saviour of all men," as He is called (1 Tim. 4:10). It remains, therefore, that the Blessed Virgin was sanctified after animation.

(Summa Theologica, Third Part, Question 27, Article 2)

Obviously St Thomas, being wrong in this respect, could not have been infallible. It was quite some time before the theology of the Church's understanding caught up to the belief itself.