Box Joint Help Needed

Looks like you're using a very soft wood that's subject to tearout.  A harder wood would give better results.
Also, your router bit may be a little dull.  Is it a bit you've used a lot before?

A straight bit also gives more tearout than a spiral.  Try using a spiral upcut bit.

banatello wrote:

I bought a jig rather than trying to build my own

Curious what jig you bought. Do you have a photo or link to where you got it?

I provided our Ask Woody AI assistant a photo of your tearout, and it recommends using a backer board.

https://www.woodcentral.com/forums/public/upload/2609/1/e591ba8b-085a-4c01-8112-c9b3a1adf43a_1134.jpg

Woody:
Analyzing your blowout photo

Thanks for uploading the picture. The tear-out is classic: you're getting heavy splintering on the exit corners of the fingers, especially where the bit leaves the wood on the far side of each cut. This is almost always caused by unsupported fibers when the router bit exits the workpiece.

Primary fix: Add backer support immediately

The fastest way to stop this is to give the bit something to cut into on the exit face.

Clamp a flat piece of scrap (½" MDF or plywood works great) tightly against the back (exit) face of your workpiece before every cut.
The bit should just kiss or slightly enter the backer board. This supports the fibers so they don't splinter outward.
For box joints you often need backer support on both the front and back of the board because the bit exits on multiple faces as the jig indexes.

If your jig allows, hot-glue or double-stick-tape a thin zero-clearance strip right against the jig's indexing pins/fingers. That creates a permanent backer that you replace when it gets chewed up.

Additional steps that will help with your Skil router and 1/4" bit

Multiple shallow passes: Don't try to cut the full finger depth in one go. Set the bit height so you're removing only about ⅛" per pass. Make 3–4 light passes, lowering the bit slightly each time. This reduces heat, chatter, and blowout dramatically.
Feed direction and speed: Always feed so the router bit is cutting with the rotation (conventional cut, not climb cut) and go slowly. If you're using a handheld router on a table, make sure the workpiece is moving steadily left-to-right or right-to-left depending on bit rotation.
Sharp bit: A dull straight bit will crush pine fibers instead of slicing them. If the bit is even slightly dull, replace it. An upcut spiral bit (if you can get one in 1/4") will give noticeably cleaner results than a straight bit.
Better test wood: Pine is one of the worst woods for router tear-out because of its wild grain. Switch to poplar, maple, or even clear straight-grained fir for practice. Once the technique is solid you can return to pine.

Quick jig tweak

If your commercial jig has any gap between the indexing fingers and the workpiece, add a thin auxiliary fence (a strip of ⅛" hardboard or plywood) so the bit has almost zero clearance when it exits the side of the finger. Screw or clamp it in place and then trim it flush with a straight bit first.

Would you like step-by-step instructions for making a simple clamp-on backer board that works with most box-joint jigs? Or do you want tips on adjusting the depth and indexing on your particular jig?

You're very close — this is a common first-time router box joint problem and it's usually fixed with better support and lighter cuts. Post another photo after you add a backer and we'll refine from there. You've got this!