请教：如何谈SIGN ON Bonus

My hu*****and got an offer from his previous company (called company A): 12W base + 15% bonus,with NO sign-on bonus. His salary increase is 18% increase from his current base with the adjustment for the potential annual increase expected by the end of month from his current company (called company B). My hu*****and and I would like to negotiate the sign-on bonus. Here are some facts.

HR in company A said that sign-on bonuses are implemented when someone is walking away from the bonus they earned through the previous year, OR when the company cannot offer a robust base salary and need to "sweeten the offer". Also, HR said that this offer went beyond the range for this position to present a nice base salary offer. I work for the company A. We met there and got married. Since we were in the same team, he decided to go to the company B, his current employer. Both of us know normally there is sign-on bonus. At least, both of us got sign-on when we joined company A. Also, his base plus bonus is in the middle range of the market survey based on his experience etc..

Please advice the best strategy to negotiate the sign-on bonus. There are some good links to share salary negotiation. Please share with me as well. Thanks a lot. I will report the result to return your kindness.