Hi,

Yes, you're 'un-expensing' so it's a credit to the Assembly Cost account.  It's a negative expense, since an Assembly transaction 'moves' the assembly costs from expense into assets (inventory value).

Those inventory values will (hopefully) later become part of cost-of-goods sold

You could also use the Assembly Cost account to keep track of manufacturing overhead as well, it requires more adjustments for variance if your manufacturing volume changes from period to period.  It's probably best to use an intermediate clearing account, but it would also work to 'expense' overhead such as utilities to Assembly Costs, and apply the total of direct cost and overhead to the items as you build them.

A fair bit of manual work will be required to keep it all straight, but it can be done. 

I hope that helps, please post back.