Reworkable coard level underfills

Why do we need Reworkable Underfills?

If you have been pay­ing atten­tion to the prod­uct devel­op­ments over the last few years, rework­able under­fills are gain­ing more and more trac­tion over non rework­able ones.

With­out think­ing about it too much, it kin­da makes sense, but why and when would you ever need a rework­able under­fill? And what’s the dif­fer­ence between a rework­able and a non rework­able one?

To get to that let’s start with a typ­i­cal rework process. There are two steps to it, that should­n’t take longer than 5 min­utes in total. Remov­ing the com­po­nents and clean­ing the board. If you want to enu­mer­ate them:

Removing components
  1. Pre­heat board (~100sec)
  2. Reflow com­po­nents (~100 sec)
  3. Man­u­al­ly debond components
Cleaning the board
  1. Apply tacky flux
  2. Scrape under­fill residue
  3. Pick up remain­ing residue with flux sat­u­rat­ed cot­ton swab
  4. Cool down board and wipe with solvent

Marked in red are the steps where things can go hor­ri­bly wrong and where rework­able under­fills shine over clas­sic ones.

Non reworkable underfills

Remov­ing a com­po­nent and scrap­ing an under­fill that was­n’t designed with rework­ing in mind can be quite dam­ag­ing for the board. Non rework­able under­fills tend to have high adhe­sion to sol­der masks. As a result, scrap­ing them can com­plete­ly destroy the pads and traces and heav­i­ly dam­age the sol­der mask.

By using a clas­sic prod­uct, the board sur­face will have more than 50% under­fill residue after debond­ing and it will most like­ly dam­age the sur­face. Even after try­ing to clean it there will be 30–40% residue and at least 10% of the sol­der mask will be damaged.

Non Rework­able Under­fill — Before and after cleaning

Reworkable underfills

With a rework­able under­fill, there will be min­i­mal residue after debond­ing. Rework­able under­fills have low adhe­sion to sol­der mask so mechan­i­cal scrap­ping is easy. This means that after the clean­ing step the board sur­face will be undam­aged and residue free.

So when you are deal­ing with expen­sive pro­duc­tion moth­er­boards that might need to be repaired or when you are towards the end of the assem­bly process (fin­ished part) and some­thing needs fix­ing, a rework­able under­fill is what you need and what will save the day.

Rework­able Under­fill — Before and after cleaning

More­over, you are in for a treat since many cur­rent gen­er­a­tion epoxy under­fills such as LOCTITE ECCOBOND UF3811 or UF3812 are already rework­able. Maybe you just dis­cov­ered a new fea­ture for some­thing you’ve already qualified.

Does this make non rework­able under­fills bad? Well, no, obvi­ous­ly not. There are many appli­ca­tions where rework­ing is not even a viable option either from a finan­cial or from a process stand­point so you would nev­er have to wor­ry about rework­ing your board. 

Maybe you just want a mate­r­i­al that has been for­mu­lat­ed with­out any reportable REACH SVHCs and is not CMR clas­si­fied. If so, LOCTITE ECCOBOND UF1173 is the right prod­uct for you. 

Or maybe you just want to know more about under­fills or have a ques­tion about your very cus­tom, very tricky and very demand­ing pack­age. In that case we will be very hap­py to help you if you Con­tact us or if you leave a com­ment below.

About George Kountardas

George is a Jack of all trades with an unappeasable inquiring mind. Obsessed with new products and technologies, he is always pushing forward for better, faster and more efficient applications. Always learning something new.

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