Screaming Circuits: Let's Talk About HAL - And Another Thing


Let's Talk About HAL - And Another Thing

BGA on HASL closeA few days ago, I wrote about HASL PC board surfaces, explaining that it's not an appropriate choice for small parts.

Look at the same PCB image as I used yesterday. You might not recognize it because yesterday, it was on the right, and today it's on the left. Getting past the fact that I just insulted everyone's intelligence, there is something else about this board that we don't recommend.

I'll give you 30 seconds to figure it out. I don't have a stop watch, so the 30 seconds is on the honor system.

This a a land for a 0.5 mm pitch BGA. As I wrote yesterday, HASL is not the right choice for BGAs, especially for those of the smaller pitch variety. The other problem with this board is in the pad layout.

These are solder mask define (SMD) pads - the soldermask covers the outer part of the pad, so the solderable copper surface is determined by the size of the opening in soldermask, not by the area of the copper pad.

For BGAs 0.5 mm pitch and larger, we (and pretty much everyone else) recommend non solder mask defined (NSMD). With a NSMD pad, the solder mask opening is larger than the pad. This leaves more copper area to adhere to, including the sides of the copper pad. It tends to be much more reliable.

The image on the right illustrates the difference. NSMD vs SMD BGA pads

The left most pad in the image illustrates an SMD pad, while on the right is an NSMD pad. The NSMD pad leaves a lot more surface area of the copper pad for the solder ball to grip on, including the sides. 

0.4 mm pitch BGAs might need either SMD or NSMD pads, depending on a number of circumstances. Read this blog information for a bit more on 0.4 mm. When in doubt, look in the back of the datasheet.

Duane Benson
Question for physicists and mathematicians:
Should the last recursion in the Mandelbrot set land on Plank's constant?
Show your work.



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