How to set the boost of terms.

How can I boost different terms in a search text differently in Solr? For example I search for "red pen", however the results are showing up products from brand "red tape". So to overcome this problem I decided to go for boosting. I'll set the boost of "pen" to be much higher than that of "red". How can I achieve that? I looked into the solr.impex file but it boosts fields and not search terms. Anyone got an idea?