Important Message From Rav Shlomo Zalman Auerbach - Say Borchu And You Could Be Out!
Rav Shlomo Zalman Auerbach (Halichos Shlomo Tefila (6:13 footnote Chuf) says that Borchu after Yishtabach is considered part of Birchas Krias Shema. Therefore if you are in middle of Pisukei DiZimra and answer Borchu with the intention that you are saying Borchu before Yotzer Ohr, it is as if you have started Birchas Krias Shema. In that case you would not be allowed to go back and finish Pisukei DiZimra or even say Yishtabach and you will have lost out on the bracha.
What you should do in that case, says Rav Shlomo Zalman, is answer Borchu with the intention that you are answering Borchu because the shul is saying but not because it is part of your davening.
Rav Shlomo Zalman said many times that this should be publicized because most people don't know it. The sad side effect is that even if we do it as Rav Shlomo Zalman says, then our tefila is not complete as we have lost out on the Borchu before Yotzer Ohr.
Important Note: We bring this tshuvah as a starting point for discussion and not to convey any halacha. We try to convey the Tshuva to the best of our ability. We admit that our understanding may not be accurate. One should learn the tshuva to verify the accuracy of our interpretation. Please understand that this Tshuva may not be the final word on this topic. One should consult a Rav before drawing any conclusions.