G,
You'll find at least one old thread in this Forum on dual-reflow if you do a search. I found one dated April 7, '98 (searched on "reflow"); this particular thread includes a discussion about the maximum weight-to-surface_area for successful dual reflow (30 g/in^2). In this same thread, somebody stated that SMTA would soon publish a paper on this subject...anybody know if this actually happened?
Also: when applying this 30 g/in^2 rule, it's not clear to me exactly how you need to calculate the surface area. The earlier thread calls it the "surface PAD area", but I imagine that the pad area is less relevant than the lead "foot" area. For example, if a QFP lead "foot" is 0.009" wide x 0.030" long, the foot area is only 0.00027 in^2. Yet the pad area might be 0.012" x 0.080", or 0.00096 in^2, which is 3.5 times the foot area. Can someone clarify this?
I tested a heavy PQFP208 (with integral heat "slug") with a mass of 10g, lead width of 0.009", and foot length of about (?) 0.030", for a Ratio of (about) 178. This part fell off.
| Has any body tried reflowing QFP's on both sides of a board. |
reply »