The chip that I want to attach a heat sink is a WiFi chip Ralink RT3070 and its separate RF amp chip YP243433 both have 85C Tjunc. The amp chip barely gets hot. The main chip gets hot but still can be touched (maybe 50-55C). Will the RF performance affected by heat sink? There's no RF can/lid/shield on both chip.
The 60-page data sheet is available for download. See the link "Download PDF File" near the bottom of this web page: http://www.datasheet4u.com/datasheet/R/T/3/RT3070_Ralink.pdf.html
Interesting, on page 7 of the data file: while it mentions nothing about the use of a heatsink, it does clarify that the AMBIENT temperature in which the chip should be used is 85 degrees C. However it states that the maximum case temperature for operation is 125 degrees C. You could instantly burn your finger by touching it and it could still be working within thermal spec. I know this doesn't answer your question, but perhaps it does, albeit obliquely.
Another interesting part of the data sheet, still page 7, says you must bake the chip if you unwrap it from its package and then do not install it within 168 hours. The baking regime is 8 hours at 125 degrees C.
All this is highly suggestive that one does not need a heat sink for this obviously thermally rugged chip.
I did retire before I got into this general level of chip, but having learned over years of personal experience in reading - and trusting - data sheets, I would suggest using this chip without a heatsink if indeed you have found that you can rest your finger on it in use without burning your finger.
I hope this helps although I am aware I still have not directly answered your question.